How Bookmaker Margins Work
A bookmaker margin is the built-in profit percentage baked into the odds. Add up the implied probabilities on every outcome of a market and you get more than 100% — that surplus is the margin (also called the overround, or vig). Across 238 sampled markets, Betway prices at an average margin of 6.49%; across 27 sampled markets, 10bet averaged 4.78%; and across 39 markets, Sportingbet averaged 8.63%. On the same match, that spread is the difference between a fair-ish price and a poor one. This guide shows exactly how the number is calculated, how to read it before you place a bet, and where measured South African prices currently sit.
Turning odds into implied probability
Every decimal price maps to a probability. The formula is simple: implied probability = 1 ÷ decimal odds.
- Odds of 2.00 → 1 ÷ 2.00 = 0.50 = 50%
- Odds of 1.50 → 1 ÷ 1.50 = 0.6667 = 66.67%
- Odds of 3.40 → 1 ÷ 3.40 = 0.2941 = 29.41%
In a perfectly fair two-way market, the two implied probabilities would sum to exactly 100%. They never do at a bookmaker. The sum always exceeds 100%, and the excess is the margin. That excess is why the house holds an edge regardless of which side wins.
Calculating the margin (overround)
For a market, add the implied probability of every outcome, then subtract 1 (100%).
Margin = (Σ 1/odds) − 1
Two-way example (a tennis match, no draw): - Player A at 1.90 → 1/1.90 = 0.5263 - Player B at 1.90 → 1/1.90 = 0.5263 - Sum = 1.0526 → margin = 5.26%
If both players were priced at a genuine 50/50, fair odds would be 2.00 each. The bookmaker shortens both to 1.90, and the 5.26% gap is the overround. A three-way market (home/draw/away in football) adds a third term, which is why football 1X2 margins are typically higher than two-way tennis or MMA prices — there are more outcomes to load.
Implied Probability Calculator
A worked PSL example
Take a Premiership (PSL) fixture priced as a three-way 1X2 market:
- Home 2.10 → 1/2.10 = 0.4762 (47.62%)
- Draw 3.30 → 1/3.30 = 0.3030 (30.30%)
- Away 3.60 → 1/3.60 = 0.2778 (27.78%)
- Sum = 1.0570 → margin = 5.70%
Now compare a higher-margin book pricing the identical fixture:
- Home 2.02, Draw 3.20, Away 3.45
- 0.4950 + 0.3125 + 0.2899 = 1.0974 → margin = 9.74%
Same match, same three outcomes, but the second book has loaded an extra ~4 percentage points. If you back the home side, you collect R210 on a R100 stake at the first book versus R202 at the second. Over a season of stakes, that gap compounds directly into your bottom line. The margin is the single cleanest number for judging whether a price is worth taking.
Removing the margin to find the 'fair' price
To estimate the true probability the bookmaker is implying, divide each outcome's raw implied probability by the market total.
Using the first PSL example (total 1.0570): - Home fair probability = 0.4762 / 1.0570 = 45.05% → fair odds ≈ 2.22 - Draw fair probability = 0.3030 / 1.0570 = 28.67% → fair odds ≈ 3.49 - Away fair probability = 0.2778 / 1.0570 = 26.28% → fair odds ≈ 3.80
Compare the fair odds (2.22) against the price you are actually offered (2.10). The difference is the margin you pay on that specific outcome. When your own probability estimate beats the fair figure, that is where value sits — and a lower-margin book leaves more of that value in the price.
Measured margins across South African books
BETAXIO samples live prices to compute two figures per operator: average margin (lower is better) and best-price share (the percentage of sampled markets where that book posted the top price). The sample sizes differ, so read them as indicative rather than final.
- 10bet: avg margin 4.78%, best-price share 85.2% (27 markets sampled)
- Betway: avg margin 6.49%, best-price share 90.8% (238 markets sampled)
- Sportingbet: avg margin 8.63%, best-price share 28.2% (39 markets sampled)
Interpretation: on the sampled data, 10bet posts the tightest average margin and Betway tops the most markets on best price — but Betway's sample is far larger (238 vs 27), which makes its 90.8% best-price share the more robust figure. Sportingbet's 8.63% average margin is the widest of the three sampled, and it led on price in fewer than one market in three. Hollywoodbets, Supabets, Sunbet, World Sports Betting, BetXchange, LulaBet and Easybets were not part of the priced sample here, so no margin figure is published for them yet.
Editorial read: sample size matters. A 4.78% average across 27 markets is promising for 10bet but sits on thin evidence; Betway's numbers rest on nearly nine times the data. Treat the smaller samples as directional signals, not verdicts.
Why margins vary by market and moment
Margin is not a fixed operator setting — it moves with the market.
- Number of outcomes: three-way football 1X2 carries more overround than two-way tennis or UFC head-to-heads.
- Liquidity and profile: a Champions League or Premier League match-winner market is typically tighter than a lower-profile PSL side-market, because competition on marquee fixtures forces books to trim.
- Bet type: outright and futures markets, plus multi-outcome props, usually carry heavier margins than the main match line.
- Live vs pre-match: in-play prices are often loaded more heavily to cover the risk of fast-moving events.
- Line movement: opening prices can differ from current prices as money and information arrive. Tracking that drift is what our market movers tool is built for.
Because of this, the smart approach is to check the margin on the exact market and moment you intend to bet, not a book's general reputation.
How to use margin data before you bet
Three practical steps:
1. Convert the offered odds on your chosen market to implied probability with 1/odds. 2. Sum every outcome and subtract 1 to get the market's margin. Anything under ~6% on a three-way football market is competitive; 9%+ is expensive. 3. Line-shop the same market across books. On the sampled data, that is what separates a 2.10 from a 2.02 on the same result.
A lower margin does not guarantee the single best price on every outcome — best-price share and average margin measure different things. A book can run a low average margin yet still be beaten on a specific selection. That is why comparing the actual price on your specific bet, every time, beats loyalty to one operator.
FAQ
What is a bookmaker margin?
It is the built-in profit percentage in a set of odds. Add the implied probabilities of every outcome in a market and the total exceeds 100%; that surplus is the margin, also called the overround or vig. On sampled South African markets it ranged from about 4.78% (10bet, 27 markets) to 8.63% (Sportingbet, 39 markets).
How do I calculate the margin on a bet?
Convert each outcome's decimal odds to probability using 1/odds, add them together, then subtract 1. For a three-way football market at 2.10 / 3.30 / 3.60, the implied probabilities sum to 1.0570, giving a margin of 5.70%.
Is a lower margin always the better price?
A lower average margin generally means better value across a book's markets, but it does not guarantee the top price on your specific selection. Average margin and best-price share are separate metrics — Betway led on best price in 90.8% of 238 sampled markets, while 10bet posted the lowest average margin (4.78%) on 27 markets. Always compare the actual price on your exact bet.
Why is the football 1X2 margin higher than a tennis margin?
A three-way market (home, draw, away) has an extra outcome for the bookmaker to load, so its overround is typically higher than a two-way market such as a tennis or MMA head-to-head, where there is no draw.
How do I find the 'fair' odds behind the margin?
Divide each outcome's implied probability by the market total. If a home side shows 47.62% implied and the market sums to 1.0570, the fair probability is 45.05%, equal to fair odds of about 2.22 — versus the 2.10 actually offered.
Do margins change during a match?
Yes. In-play prices are often loaded more heavily than pre-match to cover fast-moving risk, and lines drift as money and information arrive. Checking the margin on the exact market at the moment you bet is more reliable than relying on a general reputation.