How Betting Odds Work — Decimal, Fractional and Implied Probability Explained

Before you place a single bet, you need to understand what odds actually mean. Not just how to read them — what they tell you about probability, value and expected return.

This is the most important thing to learn. Everything else builds on it.

Decimal odds

Decimal odds are the standard format in the UK and Europe and the easiest to work with. The number represents your total return per unit staked — including your stake back.

If the odds are 2.00 and you bet £10, you get £20 back — £10 profit plus your £10 stake returned. If the odds are 1.50 and you bet £10, you get £15 back — £5 profit plus your stake. If the odds are 3.50 and you bet £10, you get £35 back — £25 profit plus your stake.

To calculate profit from decimal odds: (odds - 1) × stake = profit

So 1.85 odds on a £10 bet: (1.85 - 1) × £10 = £8.50 profit.

Fractional odds

Fractional odds are the traditional UK format — you'll still see them at horse racing events and some older bookmakers. They show profit relative to stake.

5/1 means you win £5 for every £1 staked. Your £10 bet returns £60 — £50 profit plus stake. 2/1 means you win £2 for every £1. Your £10 bet returns £30. 1/2 means you win £1 for every £2 staked. Your £10 bet returns £15.

Fractional odds are less intuitive than decimal — stick to decimal wherever possible.

American odds

You'll see American odds on NBA markets from US-facing sportsbooks. They work differently to decimal and fractional.

Negative numbers show how much you need to bet to win £100. -150 means bet £150 to win £100 profit. Positive numbers show how much you win from a £100 bet. +200 means bet £100 to win £200 profit.

To convert American odds to decimal: for negative odds, divide 100 by the absolute value and add 1. -150 becomes (100/150) + 1 = 1.67. For positive odds, divide by 100 and add 1. +200 becomes (200/100) + 1 = 3.00.

Implied probability

Every set of odds implies a probability. To convert decimal odds to implied probability, divide 1 by the odds.

Odds of 2.00 imply 50% probability (1 ÷ 2.00 = 0.50). Odds of 1.50 imply 67% probability (1 ÷ 1.50 = 0.67). Odds of 3.00 imply 33% probability (1 ÷ 3.00 = 0.33).

This is the single most useful calculation in sports betting. When a model gives a team a 70% chance of winning and the bookmaker's odds imply only 60%, that's a 10% edge — the model thinks the outcome is more likely than the bookmaker does.

The overround

Add up the implied probabilities across all outcomes in a market and they'll sum to more than 100%. That excess is the bookmaker's margin — built into every bet you place.

In a two-team market: if Team A is 1.85 and Team B is 1.95, the implied probabilities are 54.1% and 51.3% — a total of 105.4%. The extra 5.4% is the bookmaker's edge.

This is why you need to find value — bets where your assessed probability is higher than the bookmaker's implied probability — just to break even over time, let alone profit.

What this means for your betting

Every bet you place should start with one question: what do I think the true probability is, and is that higher than what the odds imply?

If yes — there may be value. If no — there isn't. It's that simple in principle. The hard part is accurately assessing probability, which is where models, data and structured analysis come in.

The HMJOROTips model calculates win probability for every tennis match and NBA game, then compares it against bookmaker odds to find the gap. That gap is the edge — the reason to bet.