How to use this calculator
Write the number of favorable outcomes in the first box and unfavorable outcomes in the second. A ratio of 1:35 in favor is often described as 35:1 against, but is a 1-in-36 probability. It is not 1 in 35. These are outcome odds, not payout or bookmaker odds. At least one side must be positive. The results also show the complementary probability of failure.
The formula
p = favorable / (favorable + unfavorable)
Here p is a probability from 0 to 1, n is a whole number of attempts, C(a,b) counts combinations, and ∏ means multiply all the factors. The input labels define the other symbols.
A worked example
Odds of 1 to 35 in favor mean 1 favorable outcome out of 36 total: 2.78%.
Does this predict what I will get?
No. It describes a probability under the rules you enter. A single run can differ from the average. A displayed rounded 100% may still be slightly below certainty unless the model includes a guarantee.
Where do the rates come from?
You enter them. Check the rules and rate disclosure for your exact outcome. We do not supply odds for real products, collect account details or fetch rates from a game or seller.