Betting odds explained: decimal, fractional and American
Three ways of writing the same price, how to convert between them, and how to turn any odds into a probability.
Updated October 4, 2026
Betting odds tell you two things at once: how much you win, and how likely the bookmaker thinks an outcome is. In the UK you'll still see fractional odds like 5/2 in betting shops; online, and across Europe, decimal odds like 3.50 are standard; in the US, American odds like +250 or −150 rule. This guide explains how betting odds work in each format, how to convert them, and how to read the probability hidden inside every price.
Decimal odds
Decimal odds show the total return for each unit staked, stake included. At 3.50, a £10 bet returns £35: £25 profit plus your £10 back.
- Return = stake × decimal odds
- Profit = stake × (decimal odds − 1)
Decimal is the simplest format for comparing prices and for accumulators, because you just multiply. It's the format used at Nocturn Sport.
Fractional odds
Fractional odds show profit relative to stake. 5/2 ("five to two") means £5 profit for every £2 staked, so a £10 bet wins £25 profit and returns £35 — exactly the same as 3.50 decimal. Evens (1/1) doubles your money. Odds-on prices, like 1/2, mean you stake more than you win: £10 at 1/2 gives £5 profit.
To convert fractional to decimal, divide and add 1: 5/2 = 2.5 + 1 = 3.50. 1/2 = 0.5 + 1 = 1.50. 11/8 = 1.375 + 1 = 2.375.
American odds
American (moneyline) odds use a plus or minus sign around a base of 100:
- Positive (+250): the profit on a $100 stake. +250 → $250 profit → decimal 3.50.
- Negative (−150): how much you must stake to win $100. −150 → stake $150 to win $100 → decimal 1.67.
Conversions: for positive odds, decimal = 1 + (American ÷ 100). For negative odds, decimal = 1 + (100 ÷ |American|). Going the other way, decimal 2.50 becomes +150, and decimal 1.40 becomes −250.
Conversion table
| Decimal | Fractional | American | Implied probability |
|---|---|---|---|
| 1.25 | 1/4 | −400 | 80.0 % |
| 1.50 | 1/2 | −200 | 66.7 % |
| 1.91 | 10/11 | −110 | 52.4 % |
| 2.00 | 1/1 (evens) | +100 | 50.0 % |
| 2.50 | 6/4 | +150 | 40.0 % |
| 3.00 | 2/1 | +200 | 33.3 % |
| 5.00 | 4/1 | +400 | 20.0 % |
| 11.00 | 10/1 | +1000 | 9.1 % |
Implied probability: the number behind the odds
Every price implies a probability. In decimal, it's simply 1 ÷ odds. Odds of 2.40 imply 1 ÷ 2.40 = 41.7 %. Odds of 1.80 imply 55.6 %. If you believe a team wins more often than the implied probability, the bet has value in your eyes; if less, it doesn't.
Add up the implied probabilities of all outcomes in a market and you'll get more than 100 %. Take a match priced 2.10 / 3.40 / 3.60: 47.6 % + 29.4 % + 27.8 % = 104.8 %. The extra above 100 % is the bookmaker's margin, also called the overround. At Nocturn Sport this margin is spread proportionally across the outcomes, so each one is shaded by the same relative amount — our pricing guide shows the formula.
To get a rough "fair" probability, divide each implied probability by the total: 47.6 ÷ 1.048 ≈ 45.4 % for the home win.
Asian formats in brief
If you follow Asian handicap markets you may also meet Hong Kong odds (profit only, so decimal minus 1: 2.50 decimal = 1.50 HK) and Malay or Indonesian odds, which use positive and negative values in a similar way to American odds. They are less common for UK and US bettors, and every price can be converted back to decimal. More on those lines in our Asian handicap guide.
Which format should you use?
Whichever you think in most naturally. Decimal is the easiest for comparing and multiplying; fractional is traditional for racing; American is what US sports media quote. Once you can convert between them, read prices as probabilities — it's the habit that makes the biggest difference. Then move on to betting markets explained.
Frequently asked questions
What does 5/1 mean in betting?
You win £5 profit for every £1 staked, plus your stake back. In decimal that's 6.00, an implied probability of 16.7 %.
What does −110 mean in American odds?
You stake $110 to win $100 profit. In decimal it's about 1.91, an implied 52.4 %.
How do I calculate implied probability?
Divide 1 by the decimal odds. For 4.00, that's 25 %.