What is overround? A 5-minute reference
Overround is the bookmaker's built-in margin — the amount by which implied probabilities in a betting market exceed 100%. A quick reference with worked examples.
Overround is the percentage by which the sum of implied probabilities in a betting market exceeds 100%. It is the bookmaker's built-in margin — the structural reason a punter's expected return on a fair-looking bet is below the stake.
The 30-second example
A coin toss has a true probability of 50% each side. Fair odds would be 2.00 / 2.00. A bookmaker priced market on a coin toss might offer 1.91 / 1.91.
Each side at 1.91 implies a probability of 52.4% (1 ÷ 1.91 = 0.524). Sum the two: 52.4% + 52.4% = 104.8%. The amount the implied probabilities exceed 100% — in this case 4.8% — is the overround.
That 4.8% is what the bookmaker expects to keep, on average, across every £100 staked across both sides of the market.
Why it matters
- Every quoted odd has overround built into it. Without it, bookmakers would break even in the long run — they don't.
- Overround compounds across accumulator legs. A 5% per-leg margin becomes 18.5% on a four-fold and 33.7% on an eight-fold.
- Different markets carry different overrounds. Premier League match betting is around 4-7%, lower-league football and niche sports can be 8-12%, in-play markets often higher again.
- Lower overround usually means sharper prices. Exchanges (Betfair, Smarkets) replace overround with a commission charge; their effective margin is typically lower than fixed-odds books.
How to spot it on any market
Convert every outcome's decimal odds to implied probability (1 ÷ odds), then sum them. The amount over 100% is the overround.
For three-way markets like football match betting, the maths is the same — implied probabilities of home, draw and away should sum to 100% in a fair market. The odds converter does the conversion in one click for any odds format.
Overround vs margin vs vig
Three names, one concept measured from slightly different angles:
- Overround — the amount by which implied probabilities exceed 100% in a market.
- Margin — the bookmaker's expected hold expressed as a percentage of total stake.
- Vig (or vigorish, juice) — US terminology for the same idea, more common in sports betting contexts than in UK racing.
For two-way markets, overround and margin are numerically very close. For three-way markets like football match betting, overround is the cleaner term to use.
For the full mathematical treatment with worked examples on how overround compounds across accumulators and translates into expected-value loss per bet, see the cornerstone reference on bookmaker margin and overround. For sport-specific deep dives, the football accumulator margin breakdown shows how a 5% per-leg margin becomes 18.5% on a four-fold.