Why do the implied probabilities of a betting market add up to more than 100%?

The amount above 100% is the bookmaker's margin — also called the vig, juice or overround. If the implied probabilities added up to exactly 100%, the bookmaker would have no built-in profit. The surplus is how they earn money regardless of the result.

How do I calculate the bookmaker's margin?

Convert each outcome's odds to implied probability (1 divided by the decimal odds), add them up, and subtract 100%. For example, two outcomes both priced at 1.91 give 52.36% each, totalling 104.72% — a margin of about 4.7%.

What is a typical bookmaker margin?

It varies. Sharp bookmakers often run around 102–106%, mainstream books around 110–118%, and smaller or newer books can exceed 120%. Lower overround means better value for the bettor, but the margin always favours the bookmaker on average.

Why betting odds never add up to 100%

Last updated: 2026-07-12 · Gamblerfy editorial team

Take any betting market, convert every outcome to an implied probability, and add them up. You'll never get 100% — you'll get more. That extra slice above 100% is the bookmaker's margin, and it's the single most important number that most bettors never check.

Implied probability, in one line

The implied probability of a price is just 1 ÷ decimal odds, written as a percentage. Odds of 2.00 imply 50%. Odds of 4.00 imply 25%. (Our Odds Converter does this for you in any format — decimal, fractional or American.)

The coin-flip that isn't fair

A coin flip is genuinely 50/50, so the fair odds are 2.00 on each side — and the two implied probabilities add up to exactly 100%. But no bookmaker offers 2.00 on both sides. You'll typically see 1.91 on each instead.

1 ÷ 1.91 = 52.36% implied for heads
1 ÷ 1.91 = 52.36% implied for tails
Total: 104.72%

That extra 4.72% is the margin. It's why, if the bookmaker takes balanced action on both sides, it pays out less than it takes in — no matter which way the coin lands.

How to measure the margin on any market

Convert each outcome's odds to implied probability (1 ÷ decimal odds).
Add them all together.
Subtract 100%. What's left is the margin.

A three-way football market (home / draw / away) works the same way — you just add three implied probabilities instead of two. The total still comes out above 100%, and the excess is the overround.

What a "good" margin looks like

Lower margin means more of the true value stays with you. As a rough guide to what's out there:

Sharp bookmakers: often around 102–106% total book (2–6% margin).
Mainstream bookmakers: around 110–118%.
Smaller or newer books: sometimes 120% or more.

Same bet, different book, very different long-run cost. Comparing the margin is one of the few things fully in a bettor's control.

Why this matters (and the honest part)

The margin is the betting equivalent of a casino's house edge / RTP: a built-in mathematical advantage that means, on average and over time, the odds are not in your favour. A lower margin softens that edge; it never removes it. This is exactly why we treat gambling as entertainment with a cost, never as a way to make money — and why understanding the numbers is part of staying in control.

Put it into practice

Our Margin & Fair Odds Calculator does all of this for you: enter the odds for every outcome and it shows the total book, the margin, and the fair no-vig odds. To convert a single price between formats, use the Odds Converter. For bonuses rather than single bets, the Bonus Value Calculator does the equivalent job — the real value once the terms are decoded.

Gambling should be entertainment, not a source of income — the margin means the numbers favour the bookmaker on average. Read our responsible gambling guide.