This odds calculator is also an odds converter, for when a UK site shows 3/2, a US site shows +150, and you think in decimals. They’re all the same price: 2.50. Bet £10, get £25 back, £15 of it profit.
Depending on the site you’ll see 2.50, 3/2 or +150, and most let you switch in the settings. Same bet, three ways of writing it.
Type the odds above in any format, 6/4, 2.5, evens or −110, and you get the other two formats, the implied probability and what your stake pays.
Odds calculator and converter cheat sheet
| Decimal | Fractional | American | Implied chance |
|---|---|---|---|
| 1.50 | 1/2 | −200 | 66.7% |
| 1.91 | 10/11 | −110 | 52.4% |
| 2.00 | 1/1 (evens) | +100 | 50.0% |
| 2.50 | 3/2 | +150 | 40.0% |
| 3.00 | 2/1 | +200 | 33.3% |
| 5.00 | 4/1 | +400 | 20.0% |
| 11.00 | 10/1 | +1000 | 9.1% |
The conversions
Fractional = decimal − 1, written as a fraction American = (decimal − 1) × 100, if decimal is 2.00 or more American = −100 ÷ (decimal − 1), if it's under 2.00 Implied % = 100 ÷ decimal
American odds confuse people because the sign flips the meaning. +150 is what you win on a 100 bet. −200 is what you have to bet to win 100.
Implied chance isn’t the real chance
Look at a match with both sides at 1.91. That’s 52.4% each, 104.7% in total. The extra 4.7% is the bookmaker’s margin, built into the prices. So when a price says 40%, the bookie actually thinks it’s a bit less likely than that.
That’s the useful part. If you reckon something happens more often than its implied chance, that’s a bet with value. If you don’t, the price is just telling you what it costs.
Working out what a bet pays
Fractional odds tell you the profit, not the return. 11/8 means you win £11 for every £8 staked. So £10 at 11/8 is £13.75 profit, plus your £10 back, £23.75 in all. In decimal that’s 2.375, and stake × odds gives you the same £23.75 in one go. That’s why most betting maths is done in decimal, even in the UK.
The stake box above does this for you. It’s handy for the awkward prices: 13/8, 15/8, 8/13. Nobody can do those in their head halfway through a Saturday.
Spotting the margin
Put both sides of a market through the odds calculator and add up the implied chances. A tennis match at 1.80 and 2.05 comes to 55.6% + 48.8% = 104.4%. That 4.4% is what the bookie keeps on average. Under 3% is a sharp price, 5% is normal, and some markets go well above 10%. Shopping around for a better price is the one edge everyone can get.