A trader looking at a share priced around £50 may see one price on a chart and two prices in the order window. Buyers can currently purchase a small quantity at £50.05, while sellers can receive £49.95. The chart may display the last completed transaction or a price halfway between those quotes. Neither necessarily represents the price at which the next order will execute.
Those two executable prices introduce the spread. The bid is the highest displayed buying price in the relevant market, and the ask, also called the offer, is the lowest displayed selling price. Their difference is the bid and ask spread. In this example, it is £0.10 per share. The midpoint is £50, giving a spread of 0.20% of the midpoint, or 20 basis points. A basis point is one hundredth of a percentage point.
The quote also has a quantity attached to it. An ask of £50.05 for 100 shares does not establish that 10,000 shares can be bought at that price. The displayed spread describes the gap between the best available prices for the quantities quoted. It does not describe every price that a larger order might encounter.
A purchase at the ask and an immediate sale at the unchanged bid would lose £0.10 per share, before commissions and other charges. That loss does not require a fall in the market’s midpoint. It arises from accepting the price available to an immediate buyer and then the price available to an immediate seller. The same mechanism applies in reverse to a trader who sells first and later buys back.
This matters when interpreting apparent opportunities. A movement on a chart is a movement in the displayed reference price. A trading profit requires a favourable difference between actual entry and exit prices after costs. The spread helps explain why an account can show a loss immediately after opening a position even when the reference market has barely moved.
An investor buying the same share encounters the same quotes. There is no separate spread reserved for people who call themselves traders. The distinction develops from the intended use of the position. A trader may expect a small movement within minutes or hours and repeat the transaction frequently. An investor may hold through years of company earnings, dividend payments and changes in valuation. To compare their exposure to spread properly, the next step is to follow the transaction through entry and exit.
How spread enters the cost of a completed trade
Using the £49.95 bid and £50.05 ask, suppose a trader buys 200 shares immediately at the ask. The purchase costs £10,010 before fees. An immediate sale of all 200 shares at the unchanged bid returns £9,990. The difference is £20, which equals the 200 shares multiplied by the £0.10 spread.
The trader crosses the spread on both transactions, but this does not mean the completed trade costs two full spreads relative to an unchanged midpoint. Buying at £50.05 costs £0.05 above the £50 midpoint. Selling at £49.95 gives up £0.05 below it. The two half spreads add to one full spread. Calling each transaction a full spread cost would count the same gap twice in this example.
If the spread changes between entry and exit, the arithmetic changes. Relative to the midpoint at each transaction, a purchase at the ask incurs half the entry spread and a sale at the bid incurs half the exit spread. Their sum estimates the spread component of the completed trade. This assumes execution at those quotes and keeps market movement separate from transaction cost.
Suppose the initial midpoint is £50 with the same £0.10 spread, but the exit midpoint later rises to £50.20. If the exit spread remains £0.10, the bid becomes £50.15. Buying at £50.05 and selling at £50.15 produces a £0.10 profit per share. The midpoint increased by £0.20, but the trader retained half that movement after crossing the spread at entry and exit. Commissions would reduce the result further.
With equal entry and exit spreads, the midpoint must rise by one full spread for this immediate long trade to break even before other costs. Here that means a rise from £50 to £50.10, producing an exit bid of £50.05. The necessary movement is different if execution improves on the quotes, the spread changes, or the position receives income while it is held.
The spread is one component of total cost
Commission is a separate charge and can apply even where spreads are narrow. Conversely, a platform advertising no commission can still quote a spread or route orders into a market with a spread. A comparison of providers needs to include both rather than treating a zero commission label as evidence of costless trading.
Slippage describes a difference between the execution price and an identified expected price. Its measurement depends on the benchmark used. Buying above the ask recorded when the order was submitted might represent adverse slippage beyond the quoted spread. Measuring execution against the midpoint includes the half spread within the measured shortfall. Adding that measurement to a separate half spread charge would double count part of the cost.
Taxes, currency conversion and financing can also affect net returns, depending on the instrument and account. These charges should remain distinct in the calculation. Spread is particularly relevant to short trades because it appears directly in entry and exit prices, even when no separate fee is deducted from the cash balance.
Why a small spread can consume a trading edge
A trading strategy needs a positive expected result after transaction costs. A profitable chart pattern before costs does not establish that condition. Where the anticipated movement is small, even a spread that looks modest as a percentage of the asset price can absorb a large part of the expected profit.
Consider a hypothetical strategy in which successful trades gain 0.30% of position value before execution costs and unsuccessful trades lose 0.20%. Assume a 50% probability for each outcome and that these figures measure movements in a midpoint reference. The gross expected result is 0.05% per completed trade. Half of 0.30% is 0.15%, and half of the 0.20% loss is 0.10%, leaving that 0.05% difference.
If crossing the spread costs 0.08% per completed trade, the expected result becomes a loss of 0.03% before commission or slippage beyond the quotes. The strategy can identify direction correctly on half its trades, earn more on winners than it loses on losers, and still fail to produce a positive net expectation.
The spread also changes the win rate required for break even. Without costs, gains of 0.30% and losses of 0.20% require a 40% winning rate. With a fixed 0.08% cost on every trade, a winner retains 0.22% and a loser loses 0.28%. The break even winning rate rises to 56%. That calculation assumes the stated outcomes and constant costs; actual results can differ if execution costs are larger during losing trades.
This is why traders need to compare spread with the average expected gain, not just with the market price. A spread of 0.08% is eight basis points of position value. Against a gross expected result of five basis points, it is already too large under the assumptions above. Against a much larger expected movement, it occupies a smaller proportion of the potential return, although that larger movement may take longer and involve greater risk.
Spread changes the realised reward and risk
Profit targets and stop levels often appear symmetrical on a chart. Suppose a trader expects a midpoint gain of £0.20 or a midpoint loss of £0.20 per share. With a £0.10 spread at both entry and exit, purchasing at the ask and selling at the bid turns the favourable outcome into a £0.10 gain and the unfavourable outcome into a £0.30 loss. Equal chart distances have produced unequal net outcomes.
If targets and stops are instead defined using actual entry prices and executable exit quotes, some or all of this effect is already included. The trader must identify the price basis before adjusting the calculation. A backtest using midpoint data and a broker statement using actual fills cannot be compared accurately without reconciling those differences.
Tight targets leave less room for execution costs. Tight stops can also interact with spread changes, depending on which quote triggers the order. A platform might show a bid chart while a short position closes through a purchase at the ask. The relevant trigger rules belong to the instrument and provider; the visible chart alone may not explain the order’s behaviour.
The practical consequence is that spread can alter which setups qualify for a trade. A forecast that remains attractive with a narrow spread may no longer justify entry when the spread widens. The forecast itself has not changed, but the amount the trader can reasonably expect to retain has.
The estimated edge also contains uncertainty. A five basis point average profit before costs, calculated from a small sample of trades, might reflect chance rather than a repeatable advantage. If the spread estimate is four basis points, the apparent one basis point margin leaves little room for estimation error. A trader needs evidence that the opportunity survives plausible differences between the historical sample and future execution.
Increasing the profit target does not automatically solve this problem. A target further from entry may be reached less often, require a longer holding period, or expose the position to a larger adverse movement. The resulting strategy has different outcome probabilities. Its expected result must be recalculated rather than assuming that a bigger target leaves the original winning rate unchanged. Spread analysis can reveal that a strategy is too expensive to trade, but changing the strategy requires another assessment of its market behaviour.
Trading frequency turns a transaction cost into a repeated expense
Spread is incurred when a position is transacted, rather than as a daily charge simply for holding it. A person who buys an asset once and holds it for several years does not pay the quoted spread again each morning. A trader who repeatedly opens and closes positions creates repeated opportunities to incur the cost.
Suppose a trader completes 100 round trips during a period, each with £10,000 of position value and an estimated spread cost of 0.05%. Each completed trade costs about £5 in spread, giving £500 across the 100 trades. On a £10,000 account, that is 5% of starting equity. The estimate assumes constant position value, unchanged costs and execution across both quoted sides. It does not assume the trading profits or losses are zero.
The same £10,000 investor making one purchase incurs only the entry component at that stage. With a symmetrical 0.05% spread, that component is approximately £2.50 relative to the midpoint. A future sale incurs its own cost at whatever conditions then apply. Comparing the investor’s one purchase with the trader’s 100 completed trades requires keeping this difference in transaction count explicit.
Turnover can therefore be more informative than the labels trader and investor. A portfolio described as a long term investment may still replace its holdings frequently. A trader holding a position for several months may incur fewer spread costs than a fund that repeatedly rebalances. What matters is the amount bought and sold, the spread on those transactions and the capital supporting them.
Position value determines the cash cost
With leveraged positions, spread is normally calculated against the exposure represented by the position, according to the contract’s pricing method. It is not generally calculated only against the cash posted as margin. A hypothetical £50,000 position with a 0.04% round trip spread cost incurs £20. If it is supported by £5,000 of margin, that £20 is 0.40% of the margin amount.
Leverage has increased the cost relative to the cash committed because the position is larger. It has not changed the spread percentage applied to the exposure in this example. Gains and losses from market movement also scale with the exposure, so increasing leverage does not repair a negative expected return after costs.
Repeated costs also reduce the capital available for later trades. The simple £500 total above is useful for planning, but a changing account balance and changing position sizes can make the eventual effect different. Records should use actual traded quantities and prices. Estimating costs from the number of trades alone can miss large differences between a small exploratory order and a position several times larger.
Execution determines how much spread a trader actually pays
The quoted spread is an indication of immediately available prices, not a guarantee that every order will execute at either displayed side. Order size, queue position and movements between submission and execution affect the result. A trader considering a small expected gain needs to account for these features because each can change the amount retained after costs.
A marketable order accepts available prices for prompt execution. A buy limit order set at or above the current ask may also execute immediately, so using a limit order does not automatically avoid crossing the spread. Its price constraint can restrict how far execution moves, but it may still buy at the ask.
A nonmarketable buy limit order placed below the ask waits for an eligible seller. If it executes at the bid or inside the spread, its entry price can be better than an immediate purchase at the ask. That improvement comes with uncertainty. The order may remain unfilled, execute only partly, or lose priority to orders already waiting at the same price.
Execution can also be selective. An order to buy at the bid may fill when selling pressure is increasing, after which the market continues lower. An unfilled order may be the one that would have benefited from a rise. A strategy that assumes all desired limit orders execute can therefore overstate performance even if it models a favourable entry price.
Quote size and changing market conditions
If 100 shares are offered at £50.05 and the next 200 at £50.10, an immediate order for 300 shares cannot acquire all of them at £50.05 under that unchanged order book. Its average purchase price is about £50.0833. The extra cost reflects insufficient quantity at the best ask. A narrow displayed spread can coexist with a higher average execution cost for a larger position.
Liquidity is consequently broader than the gap between two prices. It also includes the quantity available, how quickly quotes can change and whether buying or selling materially moves the market. A trader’s relevant spread is the one available for the intended size at the intended time, together with any additional execution shortfall.
Quotes can widen when participants face greater uncertainty about value or greater difficulty hedging positions. Around an earnings announcement or another rapid repricing event, a price available seconds earlier may disappear. The wider spread can occur alongside larger price movements, making the market look more attractive on a chart while making actual entry and exit more expensive.
Trading outside an asset’s busiest session can also produce different conditions. The SEC’s investor bulletin on extended hours trading identifies lower liquidity and wider spreads as potential risks in US stocks. That observation should not become a universal claim about every instrument or time of day. A trader needs evidence from the market and session actually being used.
Waiting for a narrower spread has a cost too
A trader can sometimes reduce costs by avoiding thin periods or delaying an order until quotes stabilise. But a delay may allow the anticipated price movement to occur before entry. A cheaper execution does not necessarily produce a better trade if the remaining opportunity has shrunk by more than the saving.
An urgent exit presents the opposite problem. After unexpected news, paying a wider spread may be preferable to continuing to hold unwanted exposure. The decision compares the transaction cost with the risk of waiting. A rule that refuses every wide spread can leave a position open at precisely the time its risk has changed.
This tradeoff explains why spread analysis belongs within execution planning. It supports decisions about timing, size and order type, but cannot replace the reason for entering or leaving a position. Traders need both a market view and a realistic estimate of the price at which that view can be acted on.
Why investors usually give spread less weight
An investor normally evaluates an asset through a longer stream of expected returns. For a company share, that might involve earnings growth, cash distributions and the valuation eventually assigned to the business. For a bond, it can involve contractual payments and credit risk. The purchase spread still reduces the investor’s return, but it is often small relative to the change in value being assessed across the holding period.
Return to the share with a £50 midpoint and £0.10 spread. Buying at £50.05 rather than the midpoint costs £0.05 per share. Suppose, purely for illustration, that after several years the midpoint is £70 and the spread is still £0.10. A sale at £69.95 produces a £19.90 gain per share before any dividends or other costs. Trading at the two midpoints would have produced £20.
The £0.10 difference is the same absolute spread cost as in the earlier immediate round trip. Here it consumes 0.5% of the £20 gross price gain. In the short trade with a £0.20 midpoint gain, it consumed 50% of that gain. The longer holding period did not cancel the spread. The hypothetical accumulated gain made the unchanged transaction cost smaller in proportion to the result.
That outcome is not guaranteed by the passage of time. If the share barely changes over several years, the spread can remain a material part of the return. If the business performs poorly, the investor loses money regardless of how narrow the entry spread was. Longer holding periods offer time for a thesis to develop; they do not establish that it will succeed.
Low turnover matters more than patience alone
An investor who leaves holdings in place generally incurs fewer transaction costs than someone who repeatedly replaces them. Regular contributions add purchases, but they need not create matching sales. Conversely, switching an entire portfolio from one fund to another involves selling the old exposure and buying the new one, each at its relevant prices.
Investors can still face large spreads in less liquid securities. A hypothetical 2% round trip spread on £20,000 of exposure represents approximately £400. It deserves attention even in a five year investment plan. An intended long holding period also does not protect an investor who later needs to sell during poor market conditions.
Recurring charges create a different comparison. Suppose an investor is choosing between two funds with otherwise comparable exposure. One has an annual charge that is 0.10 percentage points lower, but acquiring and eventually selling it is expected to cost an extra 0.30% in spread relative to the alternative. On a constant £20,000 holding, the annual saving is approximately £20 and the additional transaction cost £60. A simple recovery period is three years, ignoring changes in value, compounding and other differences between the funds.
That estimate helps explain why holding period affects the importance assigned to spread. A trader planning to hold for a day receives very little benefit from the lower annual charge, while the extra transaction cost is immediate. An investor holding for ten years has more time to accumulate the annual saving. Neither should choose solely from those two figures if the funds have different exposures, tracking performance or risks. The example isolates the cost comparison rather than claiming that the lower annual fee necessarily identifies the better investment.
Educational material on Investing.co.uk can help readers compare trading and investment approaches. For the spread question, the useful comparison is between the expected source of return and the transactions needed to obtain it. The investor still needs to examine entry and exit costs, but normally gives more weight to the asset’s longer term economics than a trader pursuing a small, immediate price change.
The instrument changes the spread comparison
Shares and exchange traded funds display spreads in the markets where their shares trade. For a long term ETF investor, the spread accompanies transactions, while the fund’s ongoing expenses reduce the fund’s assets over time. A low annual expense ratio does not establish that buying and selling its shares is inexpensive. An ETF trader needs to consider both, with transaction costs taking greater weight as turnover rises.
ETFs also draw liquidity from their underlying holdings and the arrangements used to create or redeem shares. The quantity previously traded in the ETF is therefore an incomplete guide to the cost of a new order. An ETF linked to markets that are closed can present additional pricing uncertainty. The investor or trader should examine executable quotes rather than assuming that a familiar fund name implies a narrow spread.
In currency trading, spreads are often expressed in pips. For many currency pairs, a pip is 0.0001 in the quoted exchange rate, although conventions differ for some pairs. A 0.00010 spread in EUR/USD is one pip. On an exposure of €100,000, that difference corresponds to $10 when the position is bought and immediately sold at unchanged quotes, before other charges.
Whether $10 is modest depends on the planned trade. It consumes 20% of a hypothetical five pip gross gain on that exposure. It consumes 2% of a 50 pip gross gain. Holding the currency position longer may introduce financing costs or currency conversion considerations, so the smaller spread proportion does not by itself establish the better strategy.
Derivatives require attention to the quotation method
Futures use contract sizes and tick values to translate price differences into cash amounts. A one tick spread can be cheap or expensive relative to a strategy’s expected result, depending on that tick value and the number of contracts traded. The futures price alone cannot establish the spread’s cost in money.
Options require comparison with the option premium as well as with the underlying asset. A bid of £1.00 and ask of £1.10 represents a £0.10 spread, roughly 9.52% of the £1.05 midpoint premium. Its apparent smallness relative to the underlying share price would be a poor guide to the cost of trading the option. Contract multipliers determine the cash amount.
With an over the counter derivative, the quoted prices and execution terms come from the provider’s contractual arrangements. A fixed spread, a variable spread and a separate commission can produce different costs. Financing and any additional stop charges must also be considered where they apply. Comparing products requires the same position exposure and trading pattern, rather than selecting whichever advertisement displays the lowest spread number.
Assessing spread within a trading or investment plan
A useful spread estimate begins with the intended instrument, transaction size and execution method. An advertised minimum spread may be available only under favourable conditions. It does not establish the typical cost during the trader’s session, around the strategy’s signals, or when positions are closed after adverse moves.
For a trading strategy, historical testing should distinguish reference prices from executable prices. Midpoint candles alone cannot show whether sufficient quantity existed at the bid or ask, or whether a limit order would have filled. Where detailed quote data are unavailable, the model should state its cost assumptions and examine how the result changes when those assumptions become less favourable.
Actual trade records can connect the model with execution. Recording the bid, ask and relevant quantity around order submission, where available, allows a trader to compare expected costs with average fills. Entry and exit records also reveal whether spreads systematically widen during losing trades. An average drawn from quiet periods may underestimate the cost of the strategy’s most difficult exits.
The final calculation should use realised net results without adding costs already embedded in fill prices a second time. Separate measurements remain useful for diagnosing performance, but the account has already paid the difference between its actual buying and selling prices. A cost report and a profit report need consistent benchmarks.
Average costs also need context. A strategy can have an acceptable typical spread and still suffer from a few expensive exits that remove much of its accumulated profit. Comparing costs across ordinary sessions, scheduled announcements and unexpected market disruptions helps show where that exposure lies. This does not justify assuming that the worst observed spread will apply to every trade. It establishes whether the planned position size and expected profit allow for the conditions in which an exit may actually be required. The distribution of costs matters alongside their average.
For an investor, the same information supports purchase sizing, order selection and the cost of future portfolio changes. The relevant comparison extends to recurring fund charges and the benefit expected from a switch. Saving a few basis points annually can take years to recover a larger immediate transaction cost, and that recovery period depends on how long the replacement holding remains in the portfolio.
Spread therefore carries more weight for traders when their returns depend on small price changes and repeated execution. Investors with low turnover usually incur it less often and assess it against a longer period of asset returns. Both face the same basic requirement: the expected benefit of the position must justify the costs and risks involved in obtaining it. The distinction is measured through trade economics, rather than the name given to the account.