Define the concept and the measurable fields
Bid–ask spread is the difference between the highest price a buyer is willing to pay (the bid) and the lowest price a seller is willing to accept (the ask). In practice, you measure it from market quotes by using these two numeric inputs at the same moment:
- Bid: the quoted bid price for a specific instrument.
- Ask: the quoted ask price for the same instrument.
- Bid–ask spread (raw): Ask − Bid.
To make the measurement precise, you also need to specify the quote source (for example, a data feed or a trading platform quote), because the bid and ask values depend on the provider and the venue.
Unit choices that affect how you report results
Spread can be reported in different ways. Two common approaches are:
- Absolute spread (price units): Ask − Bid.
- Relative spread (percent of price): (Ask − Bid) / Mid, where Mid = (Ask + Bid) / 2.
Neither approach is “more correct” by default. What matters is that the calculation method is stated, so another person can reproduce it from the same quote data.
Measure it step-by-step with a timestamp
A bid–ask spread measurement is only meaningful when you attach it to an exact timestamp and quote snapshot. A simple measurement workflow looks like this:
- Pick the instrument (e.g., a specific currency pair) and confirm the quote is for the same contract specification.
- Choose the timestamp rule:
- Use a single provider timestamp for the snapshot, or
- Use your own capture time when you read both bid and ask from the same screen/API response.
- Extract both values (bid and ask) from that snapshot.
- Compute:
- Absolute spread = Ask − Bid
- Mid = (Ask + Bid)/2 (optional)
- Relative spread = (Ask − Bid)/Mid (optional)
- Repeat across multiple times if you want an average, median, or distribution.
Important assumption
This method assumes that bid and ask come from the same snapshot (or effectively the same moment). If they come from different moments, you risk measuring market movement rather than spread.
Example calculation and what to compare
Assume you have one quote snapshot with:
- Bid = 1.10000
- Ask = 1.10020
Then:
- Absolute spread = 1.10020 − 1.10000 = 0.00020
- Mid = (1.10000 + 1.10020)/2 = 1.10010
- Relative spread ≈ 0.00020 / 1.10010 ≈ 0.0182%
If you want to compare two providers, you can use the same formula and then compare spreads only under matching assumptions:
- same instrument definition
- same measurement rule (absolute vs relative)
- same timestamp alignment approach
Material limitation when comparing
Quote availability and formatting can differ. If one provider’s quote update timing differs from another’s, “the same time” may not represent the same market state. That limitation matters more when spreads change quickly.
Limitations, risks, and common failure modes
Even if the formula is simple, several issues limit how confidently you can interpret the results:
- Stale or mismatched timestamps: If bid and ask are captured at different times, the spread may reflect short-term price moves.
- Non-spread trading costs: Real execution can include commissions, financing components, and slippage. Bid–ask spread measurement alone does not represent the total cost you might experience.
- Dynamic market conditions: Volatility, liquidity changes, and news events can cause spreads to widen or narrow quickly. Historical averages may not describe current conditions.
- Different quote sources: A “measured spread” is tied to the quote feed or venue used. Two sources can report different bid/ask values.
One material failure mode is treating a single spread number as a stable characteristic. Spread is time-dependent; without repeated measurement and clear timestamps, conclusions can be misleading.
Verification and a next question to make measurements reproducible
To verify your measurement approach independently, do two things:
- State the exact calculation (Ask − Bid, and whether you use relative spread).
- State the snapshot rule (how bid and ask are obtained and how the timestamp is defined).
A useful next question is: How quickly did the spread change within your measurement window? If you can’t answer that, report results with a time range and avoid strong interpretations.