What the Bookmaker Margin Costs You on a Tennis Bet

The margin is a price you pay on every bet

Every set of betting odds contains a fee. It is not charged separately and it is not on the slip, which is exactly why it is easy to ignore. It sits inside the prices themselves.

Take a tennis match between two evenly matched players. Fair odds would be 2.00 on each side, because each has a 50% chance. A bookmaker will not offer that. It offers something like 1.93 on each side, and the gap between 2.00 and 1.93 is the margin. Over many bets it is the one number that works against you with complete reliability, whether you pick winners or not.

How to measure it

Convert both prices to implied probabilities (1 divided by the decimal odds), add them up and subtract 100%. What is left is the margin, also called the vig or overround.

With odds of 1.91 on both players: 1 ÷ 1.91 = 52.4% on each side, together 104.7%. The margin is 4.7%. With 1.934 on both sides the total is 103.4%, a margin of 3.4%.

You can skip the arithmetic with the no-vig calculator: enter both prices, and it shows the margin together with the fair probabilities and fair odds with the margin removed.

What it costs when you have no edge

If your estimate of a player is exactly what the market implies, you have no edge, and every bet loses the margin on average. The expected loss per unit staked is 1 − 1 ÷ (1 + margin).

Margin	Expected loss per unit	Per 100 bets of 1u
2.4%	2.34%	-2.3u
3.4%	3.29%	-3.3u
5.0%	4.76%	-4.8u
8.0%	7.41%	-7.4u

At a margin of 3.4%, a hundred one-unit bets with no edge lose 3.3 units on average. That is not variance. It is the expected result, and it only shows up reliably once the sample is large. In a short run luck dominates. That is the reason a few good weeks say little about whether a method has an edge.

The hit rate you need to break even

The margin raises the bar. A bet at decimal odds q needs a hit rate of 1 ÷ q just to break even:

Decimal odds	Break-even hit rate
1.80	55.6%
1.90	52.6%
2.00	50.0%
2.50	40.0%

Here the margin becomes concrete. A player whose true chance is 60% would be priced at 1.667 with no margin. At a margin of 3.4% the price becomes 1.612, and you need to win 62.0% of such bets to break even, 2.0 percentage points more than the player’s true chance. Your estimate has to beat the market by more than the margin to have an edge.

What we measured in tennis moneylines

We logged the margin of the two-way market behind each pick when it was recorded. Across the 230 tennis moneyline markets where it could be measured, it ranged from 2.4% to 5.0%, with a median of 3.4%. Our model skips any market above 8%, and none of the logged markets came close to that limit.

Two caveats belong next to this. These are the markets we looked at when we found something to pick, not a survey of every tennis price on offer. And a margin is only one property of a price: a bookmaker with a slightly higher margin can still be the better place to bet if its price on your side is longer. Compare the price you can actually take, after removing the margin, not the headline margin alone.

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