- Fact-checked by Josh Miller
- Edited by Diana Westfield
How betting odds work, or how do odds work in betting if you’d rather ask it that way, trips up more punters than it should, mostly because nobody ever sits you down and explains it. The price on a selection says two things: what you’d collect if it wins, and how likely the bookie thinks that is. The free CasinoCalcs odds converter does the swapping for you. This page shows what’s going on underneath, with as few paragraphs as we could get away with.
How betting odds work: one bet, three formats
Our example is a grey at Kempton. You put £10 on, and it wins.
| Format | Price shown | How to read it | £10 bet returns |
|---|---|---|---|
| Fractional | 5/2 | win 5 for every 2 staked | £35 (£25 profit) |
| Decimal | 3.50 | total back per £1 | £35 (£25 profit) |
| American | +250 | profit on a 100 stake | £35 (£25 profit) |
Pick whichever format your site shows; the money doesn’t change.
Fractional odds
| Price | £10 returns | Worth knowing |
|---|---|---|
| 1/2 | £15 | odds-on: risk £2 to win £1 |
| Evens (1/1) | £20 | doubles your money |
| 11/10 | £21 | a shade over evens |
| 6/4 | £25 | a racecourse regular |
| 5/2 | £35 | our grey |
| 100/30 | £43.33 | 10/3, from the old ladder of prices |
Our honest take on odds-on: £10 to make a fiver is a lot of sweat. Short doesn’t mean safe, and anyone who’s bet for a while has a story about a 1/5 shot getting turned over.
Decimal odds
Our favourite format, and not only because we’re lazy.
Twenty quid on at 2.75 comes back as £55, so £35 of it is yours to keep. Got a fraction instead? Do the division and then add one, so 5/2 (that’s 2.5) ends up as 3.50 on an exchange.
Fine for round prices, miserable for 8/13. Let the converter have those.
American odds
The plus number is what a 100 stake would win, so +250 is our grey again (5/2, or 3.50). The minus number is what you’d have to lay out to win 100, which makes −200 the same favourite as 1/2. And −110, which you’ll find all over point spreads and totals, asks for 110 to win 100: 10/11 in old money, 1.91 in decimal.
Seen −110 on both sides of a market? Hold that thought, because the margin section below explains why it’s less fair than it looks. Regular North American bettors can lean on the moneyline calculator.
How to interpret betting odds as a probability
The sum is one over the decimal price. Do it for our grey (1 ÷ 3.50) and you land on 28.6%; evens is a straight 50, and a 1.50 favourite comes out a touch under 67.
This is the bit we’d make compulsory. Rate a team at 40% when the price implies 28.6% and you’ve found value. Rate them at 20% and that juicy price is still a bad bet. The implied probability calculator works it out for any format.
How the bookmaker builds in its margin
Add up the implied probabilities across a market and you’d expect 100%. You won’t get it. Take a fairly ordinary Premier League game with the home side at 2.10, the draw at 3.40 and the visitors at 3.60. Run those through the sum and you get 47.6, 29.4 and 27.8, which totals 104.8 rather than a tidy hundred.
The extra 4.8 points are the bookie’s cut: margin, overround or vig, take your pick. Those twin −110 prices do the same thing, 52.4% plus 52.4%.
| Market | Typical margin |
|---|---|
| Big televised football match | 3–5% |
| Two-way market at −110 / −110 | about 4.8% |
| 20-runner handicap | 20% or more |
| First goalscorer, darts outright | 20% or more |
Check any market with the bookmaker margin calculator, or strip the cut out with the no-vig calculator.
So which should you use?
Decimals, for us. If you follow the horses it’s still worth getting 6/4, 9/4, 11/8 and 100/30 into your head, since nobody on the telly is going to translate.
Related terms in the glossary: fractional odds, decimal odds, American odds, implied probability, margin.
What does odds-on mean?
Odds-on means the price is shorter than evens, so you win less than you stake. At 4/6, £6 wins £4. The bookmaker thinks it’s more likely to happen than not, 60% in that case. Odds-against is the opposite, like 5/2, where you win more than you stake. When you hear that a favourite is “odds-on”, it’s being rated better than a coin flip.
What does evens mean?
Evens is the dividing line between odds-on and odds-against: 1/1 in fractional, 2.0 in decimal and +100 in American. Stake £10, win £10, get £20 back. A bookmaker pricing something at evens rates it slightly less than a 50% chance, because of the margin. The betting odds chart shows evens alongside every common price.
Fixed odds explained
Fixed odds means the price is locked in when you place the bet. Take 8/1 on Tuesday and it’s 8/1 you’re paid, whatever the price does by Saturday. It’s the normal way to bet with a bookmaker. The alternative is pool betting, the tote, where the dividend depends on how much everyone else put in and you only find out after the race. Starting price (SP) sits in between: you don’t know it until the off, but it’s still set by the bookmakers rather than a pool.
Dynamic odds
Dynamic odds are prices that move while the event is on. In-play football, live tennis and racing on the exchanges all work this way: a goal, a break of serve or a horse making a move changes the price within seconds. The bet you place is settled at the price shown when it was accepted, so a delay of a second or two can matter. Nothing about the maths changes; it’s still a probability with a margin on top, just recalculated constantly.