Guide · Maths
Odds explained.
Three formulas every bettor should know: probability, margin and expected value.
Decimal odds
Odds show how many times your stake is multiplied if you win. A 10 USDT bet at 2.50 returns 25 USDT: your 10 stake plus 15 profit.
Odds → probability
To see the probability the bookmaker is implying, divide one by the odds:
1 / 2.50 = 0.40 → 40%
How to calculate the margin
Add up the implied probabilities of all outcomes and you get more than 100%. The difference is the bookmaker’s margin — their built-in edge.
1/2.10 + 1/3.40 + 1/3.60 = 0.476 + 0.294 + 0.278 = 1.048
Margin = 1.048 − 1 = 4.8%
1/1.85 + 1/1.95 = 0.541 + 0.513 = 1.053
Margin = 1.053 − 1 = 5.3%
The lower the margin, the closer the prices are to “fair” and the less you lose on average over time. That’s why we compare margins in every review.
Value: when a bet is worth it
A bet has positive expected value if your estimate of the probability is higher than the one built into the price.
You rate the chance at 50%, the odds are 2.20:
0.50 × 2.20 − 1 = +0.10 → +10% over time
The formula is simple, but estimating probability better than the bookmaker is very hard. Most bettors overrate their own predictions. Keep a record of your bets so you can check yourself honestly.
Other odds formats
| Format | Example | Decimal | How to convert |
|---|---|---|---|
| Fractional (UK) | 5/2 | 3.50 | 5 ÷ 2 + 1 |
| American, plus | +150 | 2.50 | 150 ÷ 100 + 1 |
| American, minus | −200 | 1.50 | 100 ÷ 200 + 1 |
In short
- Probability = 1 / odds.
- Margin = the sum of all outcomes’ probabilities − 1.
- There is value if your probability × odds is greater than 1.
- Compare prices across several operators — the simplest way to cut the margin you pay.