Three ways of writing the same number. Here's how each format works, how to convert between them, and how to figure out the implied probability — the only number that really matters.
Every odds line is saying the same thing: "if you stake £X and win, you get £Y back." The formats just differ in whether they show total return, profit ratio, or a US dollar reference.
| Event probability | Decimal | Fractional | American |
|---|---|---|---|
| 10% (10/1 shot) | 10.00 | 9/1 | +900 |
| 20% (4/1 shot) | 5.00 | 4/1 | +400 |
| 25% | 4.00 | 3/1 | +300 |
| 33.3% | 3.00 | 2/1 | +200 |
| 40% | 2.50 | 6/4 | +150 |
| 50% (coin flip) | 2.00 | 1/1 (evens) | +100 |
| 60% | 1.67 | 4/6 | −150 |
| 66.7% | 1.50 | 1/2 | −200 |
| 75% | 1.33 | 1/3 | −300 |
| 80% | 1.25 | 1/4 | −400 |
| 90% (heavy fav) | 1.11 | 1/9 | −900 |
The cleanest format — what your total return is per £1 staked, stake included.
Odds of 2.50 mean £1 returns £2.50 (£1.50 profit + £1 stake). Odds of 1.91 (standard bookmaker-margin coin flip) return £1.91 per £1 — so £0.91 profit.
It's one number, and higher always means bigger payout. Comparing 2.10 vs 2.15 is instant. Comparing 11/10 vs 23/20 requires a calculator. Decimal is the default format in most of the world and the only format pros use internally — even in the US and UK where fractional/American are public-facing.
Traditional in the UK and Ireland, especially horse racing. Shows profit as a ratio of stake, not total return.
Odds of 5/2 mean £5 profit for every £2 staked. £2 in → £7 back (£5 profit + £2 stake). Odds of 11/10 mean £11 profit on £10 staked.
5/2 = 2.5 + 1 = 3.50. 11/10 = 1.1 + 1 = 2.10.The default in US sportsbooks. Uses + and − with a $100 reference.
+200 → $200 profit on a $100 bet. −150 → bet $150 to win $100.
The dividing line is evens (2.00 decimal / 1/1 fractional). Anything more likely than 50% gets a negative number; anything less likely gets a positive number.
(American ÷ 100) + 1 — e.g. +200 → 3.00(100 ÷ |American|) + 1 — e.g. −150 → 1.67Every set of odds has an implied probability: the win-rate you'd need to break even. It's the only honest way to compare "is this bet good value?"
Odds of 2.50 → 1 ÷ 2.50 = 40%. You need to win more than 40% of these bets to profit.
Bookmakers bake a margin (the "vig" or "overround") into every line. A "coin flip" priced 1.91 / 1.91 on both sides implies 52.4% + 52.4% = 104.7% total — that 4.7% overage is the bookmaker's edge. Skilled bettors track odds movement and look for moments when the true probability is higher than the implied.
Whichever your bookmaker shows — then convert mentally to decimal to compare value across books. Most tracker apps (including Zort) store odds internally in decimal and convert to your preferred display format. That way a 3/1 bet from William Hill and a +300 bet from DraftKings are recognised as the same price.
Zort's AI bet slip scanner reads odds in any format — decimal from European sportsbooks, fractional from UK slips, American from US sportsbooks — and stores them consistently so your analytics aren't skewed by format. Set your display preference once; every bet shows in your chosen format. Zort Pro adds odds-range analytics so you can see exactly which price range is profitable for you.
Zort scans any odds format, stores them in decimal internally, and displays in your preference. Free forever.
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