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Odds converter: decimal, American, fractional and implied probability

Convert betting odds between decimal, American (moneyline) and fractional formats and see the implied probability instantly. Free, private, works with comma or dot decimals.

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Last updated: · Written and reviewed by Ken Lin

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Quick answer

Type a price in any format and the converter shows the same price as decimal, American and fractional odds plus the Implied probability. Decimal 2.50 = American +150 = fractional 3/2 = 40% [1].

Calculator

Decimal (2.50), American (+150 / −200) or fractional (3/2); comma or dot decimals both work.
Decimal2.500
American+150
Fractional3/2
Implied probability40.00%

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How it works, with an example

All three odds formats describe the same thing: how much a winning unit returns. The converter first turns your input into decimal odds (the total return per 1 staked), then derives the others. Example: American −200 means risking 200 to win 100, so the total return per 1 staked is 1.50 decimal, fractional 1/2, and the implied probability is 1 / 1.50 = 66.7% [1] [2]. Prediction-market prices work the same way: a 0.40 Yes share is a 40% implied probability, or 2.50 in decimal before fees.

Formula

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

Feed it live odds

Converting one price is easy; comparing hundreds as they move is where an API helps. Odds-API.io Partner link returns decimal odds from 450+ bookmakers plus Polymarket and Kalshi over REST and WebSocket, so you can run these formulas on live data [3].

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Frequently asked questions

How do I convert American odds to decimal?

For positive American odds divide by 100 and add 1 (+150 → 2.50). For negative odds divide 100 by the number and add 1 (−200 → 1.50).

What does implied probability mean?

It is the chance a price implies: 1 divided by the decimal odds. A price of 2.50 implies 40%. A bookmaker's implied probabilities add up to more than 100% because of its margin.

Why does a fractional result show ≈?

Some decimal prices have no tidy fraction. The converter shows the closest fraction with a denominator up to 100 and marks it as approximate.

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.

Affiliate disclosure

Some links on this site (marked “Partner link”) may be affiliate links: if you sign up through them we may earn a commission at no extra cost to you. Partner links use rel="sponsored nofollow" and never change our assessment. Platform links in our comparison tables are plain, non-affiliate links to official sites.

Sources

  1. Odds Converter: Decimal, Fractional, American (conversion formulas) — Odds-API.io tools, accessed 2 Nov 2026
  2. What Is Implied Probability in Betting? Formula & Example — Odds-API.io glossary, accessed 2 Nov 2026
  3. Odds-API.io — homepage (coverage, endpoints, MCP, plans) — Odds-API.io, accessed 2 Nov 2026