The overround is the hidden tax on every bet you place. It is the margin a bookmaker builds into the odds to ensure they profit regardless of which outcome wins.
How the Overround Works
In a fair market, the implied probabilities of all outcomes sum to exactly 100%. A bookmaker inflates these probabilities so they total more than 100%. The excess is the overround.
Example: Arsenal v Chelsea, draw market
| Outcome | Fair Odds | Fair Probability | Bookmaker Odds | Implied Probability |
|---|---|---|---|---|
| Arsenal | 2.20 | 45.5% | 2.10 | 47.6% |
| Draw | 3.50 | 28.6% | 3.40 | 29.4% |
| Chelsea | 3.80 | 26.3% | 3.60 | 27.8% |
| Total | 100.0% | 104.8% |
The overround is 4.8%. The bookmaker has shortened the odds on every outcome, ensuring they collect slightly more in stakes than they pay out on average.
Calculating the Overround
The formula is straightforward:
Overround (%) = (1/Odds₁ + 1/Odds₂ + 1/Odds₃ ... ) x 100 - 100
For a two-way market (e.g., tennis match): (1/1.85 + 1/1.85) x 100 - 100 = 108.1 - 100 = 8.1%
For the same match at (1/1.95 + 1/1.95): 102.6 - 100 = 2.6%
The difference between those two bookmakers is enormous over hundreds of bets.
Why Overround Matters for Your Bankroll
Even a small margin compounds over time. With a 5% overround, you need to win more than 52.5% of your even-money bets just to break even. Over 1,000 bets, the difference between a 3% and an 8% overround market costs you roughly £50 per £100 in turnover.
Overround by Market Type
| Market | Typical Overround |
|---|---|
| Premier League match result | 3-5% |
| Championship match result | 5-8% |
| Correct score | 15-30% |
| First goalscorer | 20-40% |
| Betting exchange | 1-3% (plus commission) |
How to Minimise the Overround's Impact
- Line shop: Compare odds across multiple bookmakers and always take the best price.
- Use exchanges: Betting exchanges charge commission (typically 2-5%) but offer much tighter odds.
- Avoid exotic markets: Stick to main markets (match result, over/under, both teams to score) where competition between bookmakers drives margins down.
- Track your effective margin: Record which bookmakers consistently offer the best odds for your preferred markets.
Understanding the overround transforms you from a passive bettor into an informed one. Every percentage point of margin you avoid keeping more of your edge in your own pocket.