Free tool
No-vig calculator: fair odds and true probability
Enter the odds a bookmaker or a market offers on both players, in any format, and the calculator removes the vig. You get the margin the bookmaker built in, each player's fair chance of winning, and the fair odds in American, decimal and prediction-market cents. Free, and it runs in your browser.
What the vig is
Convert a bookmaker's odds on both players to probabilities and add them up. You'll get more than 100%. That extra is the vig, also called the juice, the overround or the margin. It's how the bookmaker gets paid: the prices are set so that whichever player wins, the money paid out is a bit less than the money taken in.
Say Player A is -150 and Player B is +130. Those imply 60.0% and 43.5%, which add up to 103.5%. The 3.5% over the top is the margin. If you want to know what the bookmaker really thinks each player's chance is, you have to take the margin back out. That's a no-vig price, or fair odds.
How the calculator removes it
- Proportional (the standard way). Divide each implied chance by the total. 60.0% ÷ 103.5% = 58.0% for Player A and 43.5% ÷ 103.5% = 42.0% for Player B. This is what most people mean by a no-vig calculator, and it's what I show by default.
- Additive. Take the same amount off each side: half the margin each. Here that's 1.75 points off each, giving 58.3% and 41.8%. Some traders prefer it when one side is a long shot, because the proportional method takes more off the favourite in absolute terms.
The two methods agree when the match is close to a coin flip and drift apart as the favourite gets shorter. Neither is "correct". Both are estimates of where the bookmaker's own probabilities sit, and they differ by less than a point for most tennis matches.
The calculator also shows two measures of the same margin, because people quote both:
- Overround is how far over 100% the two chances add up. At 103.5%, the overround is 3.5%.
- Vig as a share of money staked is how much of the total money the bookmaker keeps if the money is spread in proportion to the prices: 1 minus 1 ÷ 1.035, which is 3.4%. It's always a touch smaller than the overround.
Worked example in dollars and cents
A bookmaker has Player A at -150 and Player B at +130. You back A with $60 and B with $43.48, which is $103.48 in total. If A wins you get back $60 × 250 ÷ 150 = $100. If B wins you get back $43.48 × 2.30 = $100. Either way the bookmaker hands back $100 of the $103.48 and keeps $3.48, or 3.4% of the money staked.
In prediction-market cents, -150 is a 60¢ share and +130 is a 43.5¢ share, so the pair costs 103.5¢ for a guaranteed payout of $1. The fair prices are 58¢ and 42¢, which add up to exactly $1. The bookmaker is charging 2¢ over fair on A and 1.5¢ over fair on B.
What different margins look like
Each row shows a pair of American odds, the margin, and Player A's fair chance by the proportional method.
| Player A | Player B | Chances add up to | Margin | Fair chance, A | Fair odds, A |
|---|---|---|---|---|---|
| -110 | -110 | 104.8% | 4.8% | 50.0% | +100 |
| -150 | +130 | 103.5% | 3.5% | 58.0% | -138 |
| -200 | +165 | 104.4% | 4.4% | 63.9% | -177 |
| -300 | +240 | 104.4% | 4.4% | 71.8% | -255 |
| -500 | +375 | 104.4% | 4.4% | 79.8% | -396 |
| -1000 | +650 | 104.2% | 4.2% | 87.2% | -682 |
Tennis match odds at the big bookmakers usually carry a margin of 3% to 6%. Smaller tournaments, doubles and in-play prices tend to carry more.
Prediction markets: the vig is in the spread
On a prediction market such as Polymarket, the two players' prices are set by traders, not by a bookmaker, so the "yes" prices for both players often add up to about 100¢, and sometimes a little less. Put the two cents prices into the calculator with the "prediction market price" format and you'll see a margin near zero, or even negative.
That doesn't make trading free. The cost sits in two other places: the gap between the buy and sell price, which you pay every time you get in and out, and any trading fee. Polymarket's tennis fees explains the fee, and the profit calculator shows what the two together do to your break-even price. For the background, read tennis trading vs tennis betting.
What to do with fair odds
Fair odds tell you what the price-setter believes. They don't tell you who will win. Use them in three ways:
- Compare bookmakers and markets. Strip the vig from each and you can see whether they disagree about the match or just charge different margins.
- Test your own view. If you think a player's chance is 65% and the fair chance is 58%, you've found a 7-point disagreement. Now ask why. The win-chance calculator gives a model-based estimate from the score and serve-hold rates to compare against.
- Know the toll. The "you pay over fair" line is the cost of a bookmaker's price per $1 share. A 2¢ toll on a 58¢ chance means you need to be right more than 60% of the time just to break even.
To move between odds formats without removing the vig, use the odds converter. To understand why a price and a probability are the same thing on a prediction market, start with how tennis prices work.
FAQ
What does no-vig mean?
Odds with the bookmaker's margin removed, so the two players' implied chances add up to exactly 100%. They show what the price-setter believes each player's chance is, before adding their own cut.
How do I calculate no-vig odds by hand?
Convert both odds to implied probabilities, add them together, then divide each one by that total. For -150 and +130: 60.0% and 43.5% add to 103.5%, so the fair chances are 60.0 divided by 103.5, which is 58.0%, and 42.0%.
What is a normal vig on tennis?
At the big bookmakers, about 3% to 6% on match odds. Smaller events, doubles and in-play prices usually carry more. On a prediction market the two prices often add up to about 100 cents, and the cost sits in the spread and any fee instead.
Which method should I use, proportional or additive?
Proportional is the standard and the default here. Additive takes the same amount off each side, which some traders prefer when one player is a heavy favourite. For most tennis matches the two differ by less than one percentage point.
Does this work for three-way or multi-runner markets?
No. It's built for two-player markets like a tennis match. The same idea extends to more runners, but the inputs here are two prices.
Demo only. 77 Tennis uses demo money with no cash value. Nothing here is financial or betting advice. Prices come from Polymarket's public tennis markets, and I'm not affiliated with Polymarket. Prediction markets aren't legal everywhere, so check the rules where you live. If trading stops being fun, stop.
Keep reading
- Free tool
Odds converter
Turn any odds or price into a probability and every common odds format.
Free, no sign-up - Free tool
Tennis trading profit calculator
See your shares, profit or loss and break-even price before you trade.
Free, no sign-up - Learn
How tennis prices work: why 62¢ means a 62% chance
Price equals probability. Here's why, and how it moves with every point.
7 min read - Learn
Tennis trading vs tennis betting: what's actually different
Exits, prices vs odds, costs and risk, compared side by side.
6 min read - Learn
Polymarket fees explained: tennis and every sport
The formula, the rate for each market category, who pays, and what it does to your break-even price.
8 min read - Free tool
Tennis win-chance calculator
Type in the score and get each player's estimated chance of winning the match.
Free, no sign-up