Free Probability Calculator (+ Guide)

What is a probability calculator?

A probability calculator turns a question about chances into a number between 0% and 100%.

This one covers the 3 questions people ask most: 2 events, repeated tries, and how many ways to pick.

The 3 kinds of question

Pick the kind, type the numbers, and read the answer with its formula.

2 events

If 2 events are independent, like 2 coin flips, multiply their chances for both. If they can't both happen, like rolling a 1 and a 6 on 1 die, add them for either.

Repeated tries

The chance of at least 1 success is easier to find backwards: 1 minus the chance of failing every time.

For an exact count, the calculator uses the binomial formula as the NIST handbook gives it: C(n, k) × pᵏ × (1 − p)ⁿ⁻ᵏ.

Combinations

The number of ways to pick 6 numbers from 49 is C(49, 6) = 13,983,816, so 1 ticket has a 1 in 13,983,816 chance.

A worked example

Here's the example the calculator loads: a 16.67% chance (rolling a 6), 4 tries.

The chance of at least one 6 is 1 − 0.8333⁴ = 51.8%. The chance of exactly one 6 is 38.6%.

Mistakes to avoid

These are the classic probability traps.

• Adding chances of independent events: 2 coin flips aren't a 100% chance of heads
• Thinking a streak changes the next try: a fair die has no memory
• Using combinations when order matters, like a race's top 3
• Treating events that affect each other as independent

Who is the probability calculator for?

It's for students, gamers and anyone curious about odds, and these tools help around it.

• Statistics calculator – for summarizing data
• Percentage calculator – for any percent question
• Scientific calculator – for powers and factorials by hand

The odds are just math, so check them before you bet on them.