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Probability Calculator

Enter the chance of two independent events and get the probability that both, either, or neither happen, plus a probability to odds converter.

Two independent events

Enter each chance as a percentage from 0 to 100. Both events are treated as independent. Everything is calculated in your browser. Nothing is uploaded.

Combined probability
10%
P(A and B)
both occur
60%
P(A or B)
at least one occurs
50%
P(exactly one)
one but not both
50%
P(not A)
A does not occur
80%
P(not B)
B does not occur
40%
P(neither)
neither occurs
Probability to odds
1 : 3
Odds in favor
for : against
3 : 1
Odds against
against : for
4.00
Decimal odds
return per 1 staked
+300
American odds
moneyline

How to calculate probability for two events

  1. Enter the two probabilities

    Type the chance of event A and event B as percentages, or drag the sliders between 0 and 100%.

  2. Read the combined results

    The probability of both, either, exactly one, and neither event, plus the complement of each, update instantly as stat cards.

  3. Convert a probability to odds

    Enter a single probability in the odds section to see odds in favor, odds against, decimal odds, and moneyline odds.

  4. Copy the summary

    Copy the full breakdown to your clipboard with one click to paste it wherever you need it.

Why use this tool

Combined event probabilities

From the chance of two independent events it works out the probability that both, either, exactly one, or neither of them happen.

Complement of each event

See P(not A) and P(not B) at a glance, the chance that each event does not happen, right alongside the combined results.

Probability to odds converter

Turn a single probability into odds in favor, odds against, decimal odds, and American moneyline odds, reduced to a tidy whole number ratio.

Percentages and decimals

Every result appears as a percentage and as a plain decimal, so 0.15 and 15% sit side by side ready to copy.

Sensible input handling

Inputs are capped to the 0 to 100% range and decimals are welcome, so partial percentages like 12.5% work without fuss.

Runs entirely in your browser

Everything happens on your device. Nothing you type is uploaded, stored, or logged.

About this tool

This probability calculator works with two independent events, A and B, entered as percentages from 0 to 100. Independent means the outcome of one has no effect on the other, so the chance of both happening is simply the product of the two. From your two inputs it computes the probability that both events happen, that at least one happens, that exactly one happens, that neither happens, and the complement of each event on its own. Every result is shown as a percentage and as a plain decimal so you can drop it straight into a report or a spreadsheet.

Use it to sanity check combined risks, quiz answers, dice and card questions, or any everyday what are the odds moment. Values are capped to the 0 to 100% range and decimals are accepted, so 12.5% or 0.5% both work. If you only care about a single event, the odds converter turns one probability into odds in favor, odds against, decimal odds, and American moneyline odds, reduced to a clean whole number ratio wherever possible. For plain percentage math see the percentage calculator, and for a hands on feel for chance try the dice roller.

Everything is calculated in your browser as you type. Nothing you enter is uploaded, stored, or logged, and there is no sign up. The formulas are the standard rules of probability for independent events: P(A and B) is the product, P(A or B) is P(A) plus P(B) minus P(A and B), and P(not A) is one minus P(A).

Frequently asked questions

What does independent mean here?
Two events are independent when the outcome of one does not change the chance of the other. For independent events the probability of both happening is the product of the two, which is the rule this calculator uses. If your events affect each other, these combined figures will not apply.
How is P(A or B) calculated?
P(A or B) is P(A) plus P(B) minus P(A and B). Subtracting the overlap avoids counting the case where both happen twice, so the result is the chance that at least one of the two events occurs.
What is the difference between odds and probability?
Probability is the chance of an event as a share of all outcomes, from 0 to 100%. Odds compare the chance of it happening to the chance of it not happening, so a 25% probability is odds of 1 to 3 in favor, or 3 to 1 against.
What input values are allowed?
Enter each probability as a percentage between 0 and 100. Decimals such as 12.5 or 0.5 are fine. Values outside the range are capped to 0 or 100%, and non-numeric text is ignored.
How are decimal and American odds worked out?
Decimal odds are one divided by the probability, so a 25% chance is 4.00. American odds show the moneyline, with negative values for events more likely than not and positive values for underdogs.
Is my data uploaded anywhere?
No. Everything runs in your browser; nothing is sent to a server. The numbers you enter are never uploaded, stored, or logged.

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