Bid-Ask Spread Cost Calculator
From bid/ask quotes, compute the cost of crossing the spread: one side carries half a spread, a full round trip carries one full spread.
Results
Fill in the inputs and click Calculate.
- Spread per Unit
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- Spread (Basis Pts)
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- One-Side Cost
- –
- Round-Trip Cost
- –
- Round-Trip Cost %
- –
Risk disclaimer
Quotes change with the market; actual fills follow the order book — the page only illustrates the math.
When does spread cost occur?
To fill immediately you cross the book: buy at the ask and sell at the bid — the gap between them is the crossing cost.
The spread tracks liquidity; the wider the spread or the larger the size, the more a round trip costs.
How to use
- Enter the current bid and ask prices.
- Enter the quantity.
- Click Calculate for the one-side, round-trip costs and their share of notional.
Formula
- Mid = (bid + ask) ÷ 2
- Spread = ask − bid; basis points = spread ÷ mid × 10,000
- One side = spread ÷ 2 × quantity; round trip = spread × quantity
Example
Assume bid 100, ask 100.5, buy 10 → round-trip spread cost = 0.5 × 10 = 5, roughly 0.5% of notional.
Narrower spreads or smaller sizes lower the cost share.
FAQ
Relation to slippage cost?
Slippage cost estimates from a slippage % times notional; this page uses real bid/ask quotes — the two complement each other.
Do limit orders pay this?
No. Passive fills take the far best price and usually avoid the spread, but may wait longer or go unfilled.