# Implied probability

> Implied probability is the share a price expresses, worked out as 1 divided by the decimal odds. It is not a forecast. Formulas for decimal and fractional odds.

Source: https://eachwayatlas.uk/glossary/implied-probability/
Updated: 2026-11-07

Implied probability is the chance that a price expresses, worked out as 1 divided by the decimal odds. It is not a forecast of the result, only the share a price represents before any margin is taken out. Fractional prices are converted on [how to read odds](https://eachwayatlas.uk/each-way/how-to-read-odds/). In UK betting it appears when odds are compared.

## What implied probability is

Every price carries a share. A shorter price shows a bigger share, and a longer price shows a smaller one.

The number is called implied because the bookmaker does not state it. You work it out from the price.

Why it matters: it turns odds into a common scale, so you can compare a fractional and a decimal price directly.

**The short version:** implied probability is the share a price expresses.

## Two formulas

With decimal odds, divide 1 by the price. With fractional odds a/b, divide b by the sum of a and b. Both give the same share for the same price.

**Formulas, no figures · Editorial formulas · checked 7 November 2026**

| Odds format | Implied probability |
|---|---|
| Decimal price O | 1 ÷ O |
| Fractional price a/b | b ÷ (a + b) |
| Link between the two | O = 1 + a ÷ b |

See [decimal odds](https://eachwayatlas.uk/glossary/decimal-odds/) and [fractional odds](https://eachwayatlas.uk/glossary/fractional-odds/) for each format.

**The short version:** use 1 divided by the decimal price.

## What it is not

It is not a prediction. It does not say how likely a horse is to win, because it still contains the bookmaker's margin.

When the shares of all outcomes in a market are added, the total is above 100 per cent. That excess is the [overround](https://eachwayatlas.uk/glossary/overround/). The calculation on a real market is on [the bookmaker margin claim](https://eachwayatlas.uk/each-way/bookmaker-margin-claim/).

**The short version:** a price's implied share includes the bookmaker's margin.

## What we have and have not checked

The formulas are arithmetic. A primary source for the definition is still to be added.

**Implied probability sources · checked 7 November 2026**

| Fact to read | Where to read it | Status |
|---|---|---|
| A definition from a primary source | Regulator or operator help pages, dated | Not verified |
| The fractional and decimal conversion formulas as published | Primary source, with the date read | Not verified |

**The short version:** formulas are shown; sources are marked Not verified.