Expected value (EV) in tennis betting, explained
Expected value is the single number that tells you whether a price is in your favor over the long run. It is also widely misused. Here's the math, and the honesty around it.
The one formula
Expected value (EV) per unit staked is:
- EV = probability × odds − 1
If your probability estimate times the decimal odds is above 1, EV is positive; below 1, it is negative. That is the whole engine.
A worked example
Say a model gives a player a 60% chance and the market offers @1.95. Then EV = 0.60 × 1.95 − 1 = +0.17, or +17% per unit staked over the long run. The price is longer than the fair price of @1.67, so the math is on your side.
Now a heavy favorite at 90% priced @1.04: EV = 0.90 × 1.04 − 1 = −0.064, or −6.4%. Very likely to win, but the price is too short — you would be paying the margin. Same formula, opposite verdict.
EV is a long-run average, not a promise
Positive EV does not mean you win this bet. It means that across many repeats of the same kind of spot, the math favors you. A +17% EV bet can and will lose plenty of individual matches. EV describes the process, not the outcome of one night.
Why positive EV is hard to find
The odds already encode almost everything a model knows, and they carry a margin on top. So genuine positive-EV prices are rare, and a model that disagrees with the market is as likely to be wrong as right on any single call. This is exactly why Baseline frames itself as calibrated insight and public transparency — not a betting edge.
Confidence is not value
The worked example makes the key point: the 90% favorite is the safer *outcome* but the worse *price*. "Most likely to win" and "priced with value" are different questions, and they often point at different matches. That split is exactly why Baseline separates a confidence pick from a value pick — and tracks them as separate series.
You can see today's value read, with fair odds and EV, on the daily pick page.
Frequently asked questions
How do you calculate expected value in betting?
EV = probability × decimal odds − 1. If a 60% chance is offered at @1.95, EV = 0.60 × 1.95 − 1 = +0.17, i.e. +17% per unit staked over the long run.
Does positive EV mean I will win the bet?
No. Positive EV means the odds favor you on average across many similar bets. Any single positive-EV bet can still lose; EV is a long-run property, not a guarantee.
Can a favorite have negative expected value?
Yes, and it is common. A 90% favorite priced at @1.04 has EV of about −6.4% — very likely to win, but at a price too short to be worth it.
Why is positive EV so hard to find?
Betting markets are efficient and carry a built-in margin. The price already reflects most available information, so genuine value is rare and apparent edges are often just estimation noise.