Each-Way AtlasGlossary: each-way, odds and racing terms
Overround
Overround is the amount by which the implied probabilities of every outcome in a market add up to more than 100 per cent, and it is the source of a bookmaker's margin. It is worked out as the sum of 1 divided by each decimal price, minus 1. The five-runner calculation is on the bookmaker margin claim.
Editorial team of Each-Way Atlas · Checked
What overround means
A fair market on every outcome would add up to exactly 100 per cent. A bookmaker's prices add up to more, and the extra is the overround.
Think of it as the built-in margin in a set of odds. The bigger the extra, the less the prices pay in total compared with the true chances.
Why it matters: you cannot read a single price as a pure chance of winning, because the margin is spread across the prices.
The short version: overround is the percentage above 100 per cent in a market.
The formula, step by step
Each step uses implied probability. Prices should be in decimal first; see how to read odds.
| Step | What to do |
|---|---|
| 1 | Write every price in the market as a decimal price |
| 2 | Divide 1 by each decimal price to get its implied probability |
| 3 | Add the implied probabilities of every outcome |
| 4 | Subtract 1 (or 100 per cent) from the total: the result is the overround |
We run no numbers here. The calculation on a five-horse market is on the bookmaker margin claim.
The short version: total the implied probabilities and subtract 100 per cent.
What it means for each-way bettors
An each-way bet adds the place part, which pays at a fraction of the win odds. That is a separate question from the margin; see places and fractions.
The margin is a reason to compare terms across bookmakers rather than to assume any price is fair.
The short version: the margin sits in the prices, so read the odds, places and fraction together.
What we have and have not checked
This page has no calculation of its own, to keep the working in one place. The definition source is still to be read.
| Fact to read | Where to read it | Status |
|---|---|---|
| A definition of margin from a regulator, organiser or operator | Gambling Commission or operator help pages, dated | Not verified |
| The formula as published by a primary source | Primary source, with the date read | Not verified |
| A market overround we have calculated | Page on the bookmaker margin claim, with the date | Not verified |
The short version: definition and formula are shown; sources are marked Not verified.
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