What are decimal odds?

Decimal odds show the total payout per unit staked, including your original stake. For example, decimal odds of 2.50 mean you receive GBP 2.50 for every GBP 1 you stake, giving a profit of GBP 1.50. They are widely used in Europe, Australia, and on betting exchanges.

How do fractional odds work?

Fractional odds show profit relative to your stake. Odds of 3/2 mean you win GBP 3 for every GBP 2 staked, plus your original stake back. To convert to decimal, divide the fraction and add 1: (3/2) + 1 = 2.50. Fractional odds are traditional in UK horse racing and high street bookmakers.

What do American odds mean?

American odds use positive and negative numbers. Positive odds (e.g. +150) show profit on a GBP 100 stake. Negative odds (e.g. -200) show how much you need to stake to win GBP 100. Positive odds indicate an underdog; negative odds indicate a favourite.

What is implied probability?

Implied probability converts odds into a percentage chance of an outcome occurring, as estimated by the bookmaker. For decimal odds of 2.50, the implied probability is 1/2.50 = 40%. This helps you assess whether a bet offers value compared to your own estimate of the likelihood.

Why do bookmaker probabilities add up to more than 100%?

Bookmakers build a margin (overround) into their odds, so the sum of implied probabilities across all outcomes exceeds 100%. The difference represents the bookmaker profit margin. For example, a two-outcome market might have combined implied probabilities of 105%, meaning a 5% margin for the bookmaker.

Betting Odds Calculator

Convert between decimal, fractional, and American odds with implied probability

Published by UltraCalculatorsLast reviewed: 31 Mar 2026Report an issue

This calculator is for informational purposes only. It does not encourage gambling. Please gamble responsibly and be aware of the risks involved.

Betting odds can be displayed in three different formats: decimal, fractional, and American. Each format represents the same information in a different way, and understanding how to convert between them is essential for comparing odds across different bookmakers and betting exchanges. This calculator converts between all three formats instantly and shows the implied probability of any set of odds. Decimal odds are the most common format used in the UK for online betting and exchanges. Fractional odds are traditional in UK horse racing and high street bookmakers. American odds are standard in the United States and appear in international sports markets. Regardless of format, the underlying probability and potential payout remain the same. This calculator also computes your potential return and profit for any given stake, making it easy to evaluate whether a bet offers value before placing it. The implied probability output helps you compare the bookmaker's assessment of likelihood with your own analysis.

How to Use This Calculator

To use the betting odds calculator: 1. Select your odds format from the dropdown. Choose Decimal if you have odds like 2.50, Fractional if you have odds like 3/2, or American if you have odds like +150 or -200. 2. For decimal or American odds, enter the value in the "Odds Value" field. For fractional odds, enter the numerator and denominator separately (e.g. 3 and 2 for 3/2 odds). 3. Enter your stake amount. This is how much you plan to bet. 4. Review the results showing all three odds formats, implied probability, potential return (including your stake back), and profit (return minus stake). 5. Use the chart to visually compare your stake against the potential return and profit.

How It Works

The calculator converts all odds to decimal format first, then derives the other formats: Converting to Decimal Odds: - From fractional (a/b): Decimal = (a / b) + 1 - From American positive (+X): Decimal = (X / 100) + 1 - From American negative (-X): Decimal = (100 / X) + 1 Converting from Decimal Odds: - To fractional: Simplify (Decimal - 1) as a fraction using GCD - To American: If decimal >= 2: American = (Decimal - 1) x 100. If decimal < 2: American = -100 / (Decimal - 1) Probability and Returns: - Implied Probability = (1 / Decimal Odds) x 100% - Potential Return = Stake x Decimal Odds - Profit = Potential Return - Stake Example with decimal odds of 2.50: - Fractional: 2.50 - 1 = 1.5 = 3/2 - American: (2.50 - 1) x 100 = +150 - Implied probability: (1/2.50) x 100 = 40% - Return on GBP 10 stake: 10 x 2.50 = GBP 25.00 - Profit: GBP 25.00 - GBP 10.00 = GBP 15.00

Worked Examples

Example 1: Converting decimal odds 2.50

Inputs: Odds format: Decimal, Odds value: 2.50, Stake: GBP 10

Decimal odds = 2.50 (already in decimal format)
Fractional: 2.50 - 1 = 1.5 = 3/2
American: (2.50 - 1) x 100 = +150
Implied probability: (1 / 2.50) x 100 = 40.0%
Potential return: GBP 10 x 2.50 = GBP 25.00
Profit: GBP 25.00 - GBP 10.00 = GBP 15.00

Result: Decimal 2.50 = Fractional 3/2 = American +150 with 40% implied probability

Example 2: Converting fractional odds 1/4 (strong favourite)

Inputs: Odds format: Fractional, Numerator: 1, Denominator: 4, Stake: GBP 20

Decimal: (1/4) + 1 = 0.25 + 1 = 1.25
American: -100 / (1.25 - 1) = -100 / 0.25 = -400
Implied probability: (1 / 1.25) x 100 = 80.0%
Potential return: GBP 20 x 1.25 = GBP 25.00
Profit: GBP 25.00 - GBP 20.00 = GBP 5.00

Result: Fractional 1/4 = Decimal 1.25 = American -400 with 80% implied probability

Example 3: Converting American odds +200 (underdog)

Inputs: Odds format: American, Odds value: 200, Stake: GBP 10

Decimal: (200 / 100) + 1 = 2 + 1 = 3.00
Fractional: 3.00 - 1 = 2.0 = 2/1
American: already +200
Implied probability: (1 / 3.00) x 100 = 33.3%
Potential return: GBP 10 x 3.00 = GBP 30.00
Profit: GBP 30.00 - GBP 10.00 = GBP 20.00

Result: American +200 = Decimal 3.00 = Fractional 2/1 with 33.3% implied probability

Understanding odds conversion is important for finding value bets across different bookmakers. When comparing odds from UK high street shops (typically fractional) with online exchanges (typically decimal), converting to the same format lets you see which offers the best return. The implied probability is particularly useful because it shows what the bookmaker believes the chance of an outcome is. If your own assessment of the probability is higher than the implied probability, the bet may represent value. However, remember that bookmaker odds include a built-in margin (overround), so the sum of implied probabilities across all outcomes in a market will exceed 100%. For responsible gambling guidance and support, visit the Gambling Commission website or contact GambleAware on 0808 8020 133. If you are tracking your finances, our Percentage Calculator can help with probability comparisons, and the eBay Fees Calculator and PayPal Fee Calculator handle transaction fee calculations.