Probabilities and fair odds
- Maths reviewed by Josh Miller
- Tested by Diana Westfield
This over under calculator turns two numbers, each side’s expected goals, into the chance of every goal line from 0.5 to 6.5. Feed in 1.6 for the home side and 1.1 for the away side and over 2.5 comes out at about 51%, which is fair odds of 1.96. If your bookmaker is offering 2.10, that’s a bet the numbers like; at 1.80 it isn’t. Team totals sit underneath for when you only care about one side.
Over under calculator: where the line comes from
Bookmakers set the main line where the over and under are close to a coin flip, which is why 2.5 is the standard in most football. The CasinoCalcs calculator shows why: with 2.7 expected goals in a match, exactly two goals is about 25% likely and exactly three about 22%, so the line naturally falls between them. Push the total up to 3.2 expected goals and over 2.5 rises to roughly 62%; drop it to 2.1 and it’s down near 35%.
Expected goals can come from a stats site, from each team’s average goals for and against, or from the prices themselves. The Poisson calculator goes into building those numbers from league averages if you want to start from scratch.
Reading a price
Put the bookmaker’s over and under odds in and you get three things: their margin, what their prices imply once that margin is stripped out, and whether either side is value against your model. Over/under markets tend to carry a smaller margin than correct score or first scorer, often 5% to 7% on big leagues, so the gap between a fair price and a good one is narrow.
A word on the model. Treating each team’s goals as independent works well for totals, but it slightly underrates 0-0 and 1-1, so unders at 1.5 and 2.5 come out a touch too long. If the calculator says an under is just barely value, it may not be. The total goals calculator works from a single match total if you don’t have a figure for each team, and the BTTS calculator handles both teams to score with the same model.
Goals markets move fast in play. Decide your stake before kick-off. BeGambleAware.org has free support if that gets harder.