Bid-Ask Spread: Definition, Calculation & Trading Costs

The bid-ask spread is the difference between the highest price a buyer will pay (the bid) and the lowest price a seller will accept (the ask). The invisible friction cost you cross on every market order. It is driven by liquidity, volatility, asset class, and time of day, and is essential for controlling transaction costs that most traders never see on a statement. If you're new to trading basics, understanding spread costs is one of the first concepts that separates profitable traders from those who leak money on every entry and exit.

What is the bid-ask spread?

The bid-ask spread is the built-in friction cost of every market transaction, and it exists because buyers and sellers rarely agree on price at the same instant. The bid price is the maximum a buyer currently offers; the ask price is the minimum a seller currently demands. The spread, the gap between them, is not a fee charged by a broker in a separate line item. It is absorbed the moment you execute a market order, because you buy at the ask and sell at the bid, instantly sitting at a loss equal to the spread width. For prop-firm traders operating under tight daily drawdown limits (the maximum loss permitted in a single session before a rule breach), even a 2-pip spread on a standard lot represents a real drag against that ceiling before the trade has moved a single tick in your favour. A "good" spread is context-dependent: for EUR/USD, anything under 1.5 pips is competitive; for a small-cap equity, a $0.10 spread on a $5 stock is a 2% round-trip cost before any price movement.

Bid price vs. ask price: Understanding the order book

The bid and ask prices are not arbitrary. They are the live output of an order book, the real-time ledger of all pending buy and sell orders at every price level. Market makers (firms that continuously quote both a bid and an ask, standing ready to trade either side) set the visible spread, but their quotes respond to order flow. When large buy orders arrive, market makers widen the ask to protect their inventory; when sell pressure dominates, the bid drops. For retail traders, the practical implication is simple: when you place a market order to buy, you pay the ask; when you sell, you receive the bid. The mid-price, the arithmetic average of bid and ask, is what charts typically display. Which means the entry price you see when reading a chart is not the price you actually trade at. That invisible gap between chart price and execution price is where spread cost hides.

How is the bid-ask spread calculated?

The formula is straightforward: Spread = Ask Price - Bid Price. Expressed as a percentage of mid-price, it becomes (Ask - Bid) / Mid-Price x 100, which allows fair comparison across instruments at different price levels. A concrete example: EUR/USD quoted at 1.08502 bid / 1.08514 ask has a spread of 0.00012, or 1.2 pips. To understand what a pip is and how it translates to dollar value, a pip is the fourth decimal place in most forex pairs, worth $10 per standard lot. Now apply the inverted question that matters for position sizing: a $0.05 spread on a $10 options contract looks trivially small in absolute terms: 0.5% of price. But if that option expires worthless 70% of the time and you trade it 20 times a month, the round-trip spread cost alone is $2 per contract x 20 trades = $40/month per contract, regardless of directional outcome. Compare that to a $1.00 spread on a $200 stock traded twice a month: $2 round-trip x 2 = $4/month. The percentage looks worse on the option, but the frequency multiplier is what determines total drag. Prop-firm traders should calculate spread cost as a share of their daily drawdown budget, not just as a share of position size. Use a pip value calculator to quantify exactly what each spread crossing costs in your account currency before committing to an instrument.

Bid-ask spreads across asset classes: Where retail traders leak money

Spreads vary by orders of magnitude across markets, and the asset classes where retail traders are most active are not always the cheapest to trade. The table below shows typical spread ranges under normal market conditions: not during earnings, macro events, or off-hours, when all figures widen materially.

The hierarchy is clear: forex majors and index futures are the cheapest markets to enter and exit. Small-cap equities and options are where retail traders leak the most money without realising it, because the spread cost is rarely displayed as a separate line in a brokerage statement. Reviewing failed prop-firm challenges, the recurring pattern is traders selecting instruments with wide spreads, particularly options and micro-caps, without accounting for the round-trip cost against their drawdown ceiling. For those getting started with forex trading, currency spreads on major pairs are among the most competitive available to retail traders.

Bank for International Settlements, 2000: A BIS working paper found a positive correlation between volatility and bid-ask spreads in foreign exchange markets, consistent with inventory cost models where market makers widen quotes to compensate for directional risk.

How does liquidity affect the bid-ask spread?

Liquidity in trading means the ease with which an asset can be bought or sold without materially moving its price. And it is the single most powerful determinant of spread width. When many participants compete to buy and sell at similar prices, market makers face less inventory risk and tighten their quotes. When participation thins, market makers must widen spreads to compensate for the possibility of being stuck holding a position that moves against them before they can offset it. This is not theoretical: EUR/USD, the most traded currency pair in the world, routinely trades at sub-pip spreads during London-New York overlap because thousands of institutions are simultaneously quoting. A thinly traded emerging-market currency pair on the same platform might carry a 30-pip spread because the market maker's hedge is expensive and slow. For traders choosing instruments, liquidity is not just a comfort metric. It is a direct input to break-even calculation. A strategy that needs a 10-pip move to profit is structurally unviable on an instrument where the spread alone consumes 8 pips.

Why do spreads widen during volatility and low-liquidity hours?

Volatility, the rate and magnitude of price change over a given period, forces market makers to widen spreads because their core risk changes. A market maker quoting a tight spread in a calm market can hedge their inventory quickly at a predictable cost. During an earnings release, a central bank decision, or a geopolitical shock, price can gap through multiple levels before a hedge executes, turning a small inventory position into a large directional loss. Widening the spread is the market maker's self-insurance premium. The Bank for International Settlements documented a positive correlation between volatility and spreads in FX markets, consistent with inventory cost models; unexpected trading volumes and volatility are positively correlated in most cases, reinforcing that spread widening and volume spikes are linked phenomena during stress periods. The same dynamic applies during low-liquidity hours: the Asian session for EUR/USD, or the 30 minutes before and after a major equity market close, sees fewer competing quotes and wider spreads. For retail traders, understanding price action trading during these volatile periods helps you recognize when spread widening signals a shift in market structure and liquidity. Monitoring live spread width before entering a trade around a scheduled event is a practical risk filter that requires no additional tools beyond your broker's quote panel.

Bid-ask spread as a hidden transaction cost: The compounding erosion model

The spread is a round-trip cost. You pay half on entry (buying at ask above mid) and half on exit (selling at bid below mid). What makes it insidious is not any single crossing but the compounding effect across a trading session. Consider a scalper (a trader who targets small, rapid price moves, typically 2-10 pips) trading EUR/USD 20 times per day with a 1.2-pip spread. Each round trip costs 1.2 pips; 20 round trips cost 24 pips daily in spread alone, before any commission. A strategy targeting 3 pips per trade needs a 40% win rate just to break even on spread costs. And that calculation assumes no slippage (slippage being the difference between the price you expected to trade at and the price you actually received, caused by market movement between order placement and execution). For swing traders holding positions for days, the same 1.2-pip spread is negligible against a 100-pip target. The break-even frequency model makes the case clearly: spread costs scale with trade frequency, not position size. This is why high-frequency strategies are structurally viable only on instruments with the tightest spreads. And why a prop-firm trader with a 4% daily drawdown ceiling should treat each spread crossing as a direct debit against that budget. Spread costs also directly affect your risk-reward ratio: every pip paid in spread is a pip that must be recovered before a trade contributes positively to your edge. Research by Hagstromer, published in the Journal of Financial Economics (2021), highlights that standard effective spread measures carry systematic bias in midpoint effective spread estimators, meaning even the tools used to evaluate spread costs can understate the true drag on performance.

Hagstromer, B., Journal of Financial Economics (2021): Standard midpoint effective spread estimators are subject to systematic bias, affecting trading performance evaluations and venue rankings under RegNMS Rule 605. Meaning reported spread costs in equity markets may understate actual transaction drag.

Can you profit from bid-ask spreads?

Market makers and certain algorithmic trading strategies profit from spreads by simultaneously quoting a bid and an ask, collecting the difference when both sides fill: a process called "making the spread." Retail traders are almost universally on the other side of this transaction, paying the spread rather than collecting it. The exception is limit orders: a limit order to buy placed at the bid price, if filled, means you entered at the better price rather than crossing the spread. However, limit orders carry execution risk. The market may never return to your price, or may fill only partially. Algorithmic strategies that exploit spreads include statistical arbitrage (simultaneously trading correlated instruments to capture pricing discrepancies) and latency arbitrage (using speed advantages to trade against stale quotes). Both require infrastructure and capital that retail traders do not have. The practical takeaway is not to attempt spread capture but to minimise spread payment: trade liquid instruments during peak hours, use limit orders where execution risk is acceptable, and avoid high-frequency strategies on wide-spread instruments where the math structurally works against you.

At FundedFast, prop traders under daily drawdown rules feel this most acutely. Each spread crossing is a measurable debit against the session ceiling before a single pip of directional movement occurs. If you're ready to put these principles into practice, start a funded challenge and trade with capital where every basis point of spread cost counts.