Direct Answer
The half-spread is half the quoted bid-ask spread: the distance from the midpoint to either the bid or the ask. It approximates the cost of one market-order trade if the midpoint is fair value. It is the expected price concession of a single market order against the midpoint, so a round trip costs about two half-spreads.
Half-Spread: Formula, Meaning, Signals, Examples and How to Use It
How Is Half-Spread Calculated?
The half-spread is half the quoted bid-ask spread: the distance from the midpoint to either the bid or the ask. It approximates the cost of one market-order trade if the midpoint is fair value.
Formula: Half-Spread = (Ask - Bid) / 2
Inputs:
- Best bid price
- Best ask price
Worked example. A quote of 100.00 bid and 100.04 ask has a half-spread of 0.02 per share. The inputs are illustrative numbers, not market data.
Reading it. It is the expected price concession of a single market order against the midpoint, so a round trip costs about two half-spreads. Smaller values mean cheaper immediate execution.
Limits. It uses the quote at one moment and assumes the order is small enough to fill at the best price, so larger orders pay more. It ignores price improvement, fees and rebates, and the midpoint is not always the true value.
What is Half-Spread?
The half-spread is half the quoted bid-ask spread: the distance from the midpoint to either the bid or the ask. It approximates the cost of one market-order trade if the midpoint is fair value.
How to use Half-Spread
It is the expected price concession of a single market order against the midpoint, so a round trip costs about two half-spreads. Smaller values mean cheaper immediate execution. A reading alone is never a buy or sell signal; it needs context from price structure, volume, volatility and the broader market regime.
Strengths
- It takes only the best bid and ask, so it is the simplest direct statement of the cost of one small marketable order.
- Expressed in price units or as a percent of midpoint, it converts straight into a per-share or per-dollar cost estimate.
- Reproducible when the formula and data convention are fixed.
Weaknesses and limitations
- In stocks quoting at one tick the half-spread is a constant half tick, so differences in liquidity between them do not show up.
- The quote at one instant can be stale or crossed in a fast market, so the half-spread can read negative or implausibly large between updates.
- Correlated indicators can create false confidence by repeating the same underlying information.
Editorial and risk note
This page is educational content, not individualized investment advice. It should not imply guaranteed prediction, accuracy or outperformance.
Frequently Asked Questions
Is Half-Spread a buy or sell signal?
No. It is the expected price concession of a single market order against the midpoint, so a round trip costs about two half-spreads. It uses the quote at one moment and assumes the order is small enough to fill at the best price, so larger orders pay more. A trade built on it still needs a trigger, an invalidation rule, a position size and realistic execution assumptions.
What is the best setting for Half-Spread?
Half-Spread is computed from best bid price; best ask price, so the choices that matter are the data convention and the window it is read over. Test those across instruments, regimes and out-of-sample periods instead of fitting one historical winner.
Can Half-Spread be used by itself?
It uses the quote at one moment and assumes the order is small enough to fill at the best price, so larger orders pay more. It ignores price improvement, fees and rebates, and the midpoint is not always the true value. For that reason it is read next to measures of direction, participation, valuation, liquidity and risk, not alone.
Does Half-Spread work on every timeframe?
The calculation (Half-Spread = (Ask - Bid) / 2) can be run on any bar size, but behavior is not portable: a reading on a five-minute chart describes a very different market window from the same reading on a daily chart. Validate the exact timeframe before relying on it.
Why does Half-Spread give false signals?
In stocks quoting at one tick the half-spread is a constant half tick, so differences in liquidity between them do not show up. The quote at one instant can be stale or crossed in a fast market, so the half-spread can read negative or implausibly large between updates. Lag, noise and regime changes add to this, because market participants react to new information after the reading is calculated.
How should Half-Spread be backtested?
Rebuild Half-Spread from best bid price using point-in-time data, write the rules before testing, include delisted securities where relevant, model realistic fills and costs, reserve a validation sample and compare with a simpler baseline.