Fractional Odds Calculator

Enter fractional odds (like 5/1 or 11/4) and your stake to convert them into implied probability, decimal odds, profit, and total return.

Quick Facts

Method
Decimal = a/b + 1; implied probability = b/(a+b)
Fractional odds a/b state profit (a) per stake (b). Profit = stake × a/b.

Your Results

Calculated
Implied probability
-
Break-even win chance b/(a+b)
Decimal odds
-
a/b + 1
Profit if it wins
-
Stake × a/b
Total return
-
Stake plus profit

Ready

Enter odds and a stake, then press Calculate.

What fractional odds mean

Fractional odds are the traditional way UK and Irish bookmakers quote a bet. Odds written as a/b (said "a to b") tell you the profit you win relative to your stake: for every b units you stake, you win a units of profit if the bet succeeds. So 5/1 ("five to one") returns 5 units of profit for every 1 unit staked, and 11/4 returns 11 units of profit for every 4 staked. Your original stake is returned on top of the profit whenever the bet wins.

The formulas this calculator uses

  • Profit ratio: a / b — the profit per unit staked.
  • Decimal odds: a/b + 1 — total return per unit staked, the format used by most exchanges and European books.
  • Implied probability: b / (a + b) — the break-even win probability baked into the price, expressed as a percentage.
  • Profit on a stake S: S × (a / b).
  • Total return on a stake S: S × (a/b + 1) = stake + profit.

Why implied probability matters

The implied probability is the win rate at which a bet breaks even over the long run. If 5/1 implies 16.67% and you believe the true chance is higher than that, the bet has positive expected value. Bookmakers deliberately set the sum of implied probabilities across all outcomes above 100% — that excess is the "overround" or "vig," their built-in margin. Comparing your own probability estimate against the implied probability is the core of value betting.

Common reference points

  • 1/1 (evens): implied probability 50%, decimal 2.00 — double your money on a win.
  • 1/2 (odds-on): implied probability 66.67%, decimal 1.50 — you risk 2 to win 1.
  • 2/1: implied probability 33.33%, decimal 3.00.
  • 5/1: implied probability 16.67%, decimal 6.00.
  • 10/1: implied probability 9.09%, decimal 11.00.
  • 100/1: implied probability 0.99%, decimal 101.00 — a long-shot.

Frequently Asked Questions

How do I convert fractional odds to decimal odds?
Divide the numerator by the denominator and add 1: decimal = a/b + 1. For 5/1 that is 5 + 1 = 6.00; for 11/4 it is 2.75 + 1 = 3.75; for 1/2 it is 0.5 + 1 = 1.50. Decimal odds already include your returned stake, so total return = stake × decimal odds.
What does "odds-on" mean?
"Odds-on" describes a favourite priced at less than evens, where the numerator is smaller than the denominator (for example 1/2 or 4/6). The implied probability is above 50%, and you have to stake more than you stand to win in profit. "Odds-against" is the reverse (for example 3/1), where profit exceeds the stake.
How much do I win from a £10 bet at 7/2?
Profit = stake × a/b = 10 × 7/2 = 10 × 3.5 = £35. Your total return is the £35 profit plus your £10 stake back = £45. The implied probability of 7/2 is 2/(7+2) = 22.22%.

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