What the bid and ask mean
The bid is the highest quoted buying price and the ask is the lowest quoted selling price in the market being observed. Their difference is the quoted spread. Each quote relates to a quantity, and a quote on one venue need not describe every venue. Investor.gov: Bid Price/Ask Price
For a trader seeking immediate execution, a purchase generally meets an ask and a sale meets a bid. The last traded price records an earlier transaction; it is not a current offer to transact. A quote should therefore be read with its timestamp, quantity and market coverage.
Calculating the spread
Spread = ask price - bid price.
Midpoint = (ask price + bid price) / 2.
Spread in basis points = spread / midpoint × 10,000.
Hypothetical example: a stock is bid at $49.98 and offered at $50.02. The spread is $0.04, the midpoint is $50.00 and the spread is 8 basis points. Expressing the result relative to price makes comparisons more meaningful than comparing cents alone.
A second stock quoted at $4.98/$5.02 has the same four-cent spread but an 80-basis-point spread. Equal cash spreads do not imply equal proportional costs.
A round-trip cost example
Assume sufficient quantity remains available at $49.98/$50.02. Buying 300 shares at the ask costs $15,006. Selling all 300 immediately at the unchanged bid returns $14,994. The difference is $12 before explicit fees.
This round trip crosses one full spread per share. Relative to the unchanged midpoint, each side costs half the spread. Adding a full spread at entry and another full spread at exit would double-count the cost in this particular example.
Actual trades need not execute at the displayed prices. Changes in quotes, partial fills and price improvement can alter the result. Keep commissions and other explicit charges separate so that they are not hidden inside a spread estimate.
Reading an FX quotation
Consider a hypothetical EUR/USD quote of 1.08420/1.08435. Using a pip convention of 0.0001, the spread is 1.5 pips. For a purchase of EUR 10,000 followed by an immediate sale at unchanged quotes, the gross difference is USD 1.50.
The calculation is 10,000 × (1.08435 - 1.08420). If the account reports in another currency, that dollar result requires conversion. Confirm the instrument's pip convention and unit size; a platform's displayed lot quantity is not itself a universal currency amount.
A narrow spread can conceal limited depth
Assume only 100 shares are offered at $50.02 and another 400 at $50.06. A 300-share purchase that consumes those offers would cost $15,014: 100 × $50.02 plus 200 × $50.06. The average execution is approximately $50.0467.
The initial four-cent spread describes the best quotes, but the order is larger than the quantity at the best ask. This static order-book example isolates depth. In a live market, orders can arrive, disappear or execute while the instruction is being processed.
Compare spread, quantity and your intended size together. A one-share quote and a large executable quote answer different questions.
Spread and order choice
A limit order sets a price constraint but can remain unfilled. A market order does not guarantee the price shown when it is submitted. Investor.gov: Types of Orders
Placing a buy limit at the bid does not automatically earn the spread. Execution may never occur, and the value of the position may fall after a fill. Assess the resulting position as well as the entry price. See market, limit and stop orders.
Recording costs consistently
For each execution, record the instrument, venue, side, quantity, quote timestamp, bid, ask, actual fill and explicit fees. When comparing brokers or trading sessions, use comparable order sizes and products. A quoted minimum spread in an advertisement is not a measurement of your realised trading costs.
Do not add spread costs again if your profit-and-loss calculation already uses actual purchase and sale prices. Use midpoint comparisons to explain costs, rather than charging the same cost twice.
Sources and method
The references support quote definitions and conventional securities order behaviour. All quotations, quantities and calculations above are original hypothetical examples, not observed market data. FX conventions and execution arrangements must be checked for the specific product. For exposure decisions, continue to position sizing.
Sources & method
Sources support the referenced concepts. Numerical examples are hypothetical unless stated otherwise. A link does not imply endorsement.