Bid-Ask Spread Calculator
Calculate the bid-ask spread in dollar terms and percentage, find the midpoint price, and see the effective cost of a round-trip trade.
Did this tool work for you?
How to use this calculator
The spread is the difference between the lowest ask and highest bid price. Spread % is typically expressed relative to the ask. A round-trip trade costs the full spread.
- 1
Enter the current bid price (the highest price a buyer will pay).
- 2
Enter the current ask price (the lowest price a seller will accept).
- 3
The calculator shows the absolute spread, percentage spread, midpoint, and round-trip trading cost.
- 4
Read the spread band to see where the percentage falls among the built-in cut-offs.
Frequently asked questions
Which formulas does this tool use?
This tool uses Spread = Ask − Bid, Spread % = Spread ÷ Ask × 100, Spread in basis points = Spread % × 100, and Midpoint = (Ask + Bid) ÷ 2. It also shows Spread (% of Midpoint) = Spread ÷ Midpoint × 100 as a second way to express the same gap.
Why are there two percentage figures?
Spreads can be quoted relative to the ask or to the midpoint. They differ slightly because the denominators differ. For a 49.75 bid and 50.00 ask they are 0.5000% and 0.5013%, and for 19.80 and 20.20 they are 1.9802% and 2.0000%.
What does Round-Trip Cost (per unit) mean?
It equals the spread. If you buy one unit at the ask and immediately sell it at the bid with no change in either quote, the difference is the spread. The tool shows it per unit, so multiply by your quantity yourself. Commissions, fees and price movement during the trade are not included.
What is a basis point?
One basis point is 0.01%. A spread of 0.0987% is 9.87 bps. The tool shows basis points to two decimals so very small spreads are still readable.
What inputs are rejected?
An ask of zero or less, a negative bid, and a bid above the ask are rejected with a message, because a bid above the ask is a crossed quote. A bid equal to the ask gives a spread of 0.00. A bid of 0 with a positive ask is accepted: 0 and 2 gives a spread of 2.00, which is 100.0000% of the ask and 200.0000% of the midpoint.
What is the Spread Band?
It states which built-in range the spread falls into, measured as a percentage of the ask: under 0.01%, 0.01% to under 0.1%, 0.1% to under 0.5%, 0.5% to under 2%, or 2% and above. The cut-offs are the figures built into this tool and the band is a label, not a judgment about any market.
How are the numbers rounded?
Currency amounts are rounded to two decimals for display and percentages to four. A spread smaller than half a cent will therefore display as 0.00 in the absolute field, so use the percentage and basis point fields for very small differences.
What does the tool leave out?
Live quotes, order size, depth of book, volume, commissions and slippage. It converts two prices you type into spread arithmetic and nothing more.
Bid-ask spread arithmetic: from two quotes to one gap
From two quotes to one gap
The bid is the highest price a buyer is quoting and the ask is the lowest price a seller is quoting. The gap between them is the spread, in currency per unit. Dividing by the ask gives a percentage, multiplying that by 100 gives basis points, and averaging the two quotes gives the midpoint. Every figure on the result card is one of those few operations applied to your two inputs.
Worked example: the built-in quote
The figures built into this tool are a bid of 49.75 and an ask of 50.00. Running them gives a spread of $0.25, which is 0.5000% of the ask, 50.00 bps, with a midpoint of $49.88 and a round-trip cost of $0.25 per unit. Measured against the midpoint instead, the same gap is 0.5013%. A tighter quote, 101.20 and 101.30, gives a spread of $0.10, 0.0987%, 9.87 bps and a midpoint of $101.25, in the 0.01% to under 0.1% band.
Four quotes compared
The table runs four bid and ask pairs through the tool. Notice that the 101.20 and 101.30 quote has a smaller percentage than the 49.75 and 50.00 quote even though both gaps are small in currency terms, and that the 3.50 and 4.00 quote has a modest-looking gap of $0.50 that is a very large share of the ask.
| Bid | Ask | Spread | % of ask | Basis points | Midpoint |
|---|---|---|---|---|---|
| 49.75 | 50.00 | $0.25 | 0.5000% | 50.00 | $49.88 |
| 19.80 | 20.20 | $0.40 | 1.9802% | 198.02 | $20.00 |
| 101.20 | 101.30 | $0.10 | 0.0987% | 9.87 | $101.25 |
| 3.50 | 4.00 | $0.50 | 12.5000% | 1,250.00 | $3.75 |
Why the percentage matters more than the gap
Two quotes with the same dollar spread can carry very different relative costs. A $0.10 gap on a price near 101 is under 10 basis points, while a $0.50 gap on a price near 4 is 1,250. That is why the tool puts the percentage and basis point fields next to the absolute spread, and why the band is assigned from the percentage, not from the currency amount.
Edge cases and limits
A bid equal to the ask gives a spread of $0.00 and falls in the lowest band. A crossed quote, with a bid above the ask, is reported as an error instead of a negative spread. The calculation is a snapshot of two numbers, so it says nothing about how quotes move, how much can be traded at each price, or what a broker charges.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →