An implied probability calculator turns a price into a chance. 5/2 means 28.6%: the price only pays off if it wins at least 2 times in 7. That’s the break-even line, and the free tool above takes it two steps further. It strips out the bookmaker’s margin, and it tells you whether your own estimate makes the bet worth backing.
Enter fractional, decimal or American odds. 5/2, 3.50 and +250 all give the same 28.6%.
Implied probability calculator: the margin
Implied probability includes the bookmaker’s cut. Take a tennis match at 5/2 and 2/7. The implied chances are 28.6% and 77.8%, which adds up to 106.4%. The extra 6.35% is the margin. Remove it in proportion and the real chances come out at 26.9% and 73.1%, so the fair price of the outsider is 3.72, not 3.50.
Tick “Remove the margin” and enter the other price to do this for any two-way market. For three-way football markets or a whole race, the bookmaker margin calculator takes every outcome at once.
Finding value
A value bet is one where you think the chance is higher than the price says. Say you rate that tennis outsider at 35%. At 5/2 the price implies 28.6%, so your edge is 6.4 percentage points and a £10 bet is worth +£2.25 on average. At 2/1 (33.3%), your edge would shrink to 1.7 points and +£0.50. At 7/4 it’s gone.
The hard part is the estimate, not the maths. If you’re honest about where your 35% comes from, this is how professional bettors decide what to back, and the Kelly criterion calculator then tells you how much to stake.
The formulas
Implied probability = 1 ÷ decimal odds Margin (two-way) = 1 ÷ odds A + 1 ÷ odds B − 1 Fair probability = (1 ÷ odds A) ÷ (1 ÷ odds A + 1 ÷ odds B) Expected profit = stake × (your chance × decimal odds − 1)
Converting between formats rather than chances? The odds converter does decimal, fractional and American side by side.
A value bet can still lose, and often does. Stake for the long run, not for this one. BeGambleAware.org is free and confidential if it stops being fun.