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What is decimal, American and fractional odds?

Decimal, American and fractional odds explained in plain English: a direct definition, a worked example, the formula and related terms, with sources.

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Last updated: · Written and reviewed by Ken Lin

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Quick answer

Decimal, American and fractional odds: Decimal, American and fractional odds are three ways of writing the same price: decimal shows the total return per 1 staked, American shows profit on 100 (or stake needed to win 100), and fractional shows profit relative to stake.

Also called: odds formats, moneyline odds, UK odds

Explained, with an example

The same price written three ways: decimal 2.50 = fractional 3/2 = American +150, all implying 40% [1]. Decimal 1.67 ≈ American −150 [2]. Decimal is easiest for maths, which is why most APIs (including Odds-API.io) return decimal strings.

Formula

decimal ≥ 2: American = +(decimal − 1) × 100
decimal < 2: American = −100 / (decimal − 1)
American +A → decimal = A / 100 + 1
American −A → decimal = 100 / A + 1
fractional a/b → decimal = a / b + 1

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Frequently asked questions

What is decimal, American and fractional odds?

Decimal, American and fractional odds are three ways of writing the same price: decimal shows the total return per 1 staked, American shows profit on 100 (or stake needed to win 100), and fractional shows profit relative to stake.

About this guide

Pred-Data publishes independent, source-linked explainers about prediction markets and the AI tools used around them. Every factual claim on this page links to a primary or reputable secondary source, checked on the date shown above. We do not run a trading venue, take bets, or give personal advice.

Author: Ken Lin · Corrections: if you spot an error, we review it and date-stamp the fix.

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Sources

  1. Odds Converter: Decimal, Fractional, American (conversion formulas) — Odds-API.io tools, accessed 2 Nov 2026
  2. What Is a Moneyline Bet? Definition & Example — Odds-API.io glossary, accessed 2 Nov 2026