Prime Checker

Check whether a number is prime and list its prime factorization. It trial-divides up to the square root, so it's fast for everyday numbers.

Try it now

💡 Good to know: You only need to test factors up to a number's square root — any larger factor would already have a smaller partner you'd have found.

Worked example

Prime?: no · Factors: 2 × 2 × 3 × 7

How it’s solved

Worked on the example above, step by step — follow along and you can do it on paper next time, no tool required.

Trial-divide by 2, 3, 5… up to √n, peeling off each prime factor. A number is prime only if the single factor left is itself.
Break the number into prime factors
84 = 2 × 2 × 3 × 7
Judge: exactly one prime factor and n ≥ 2 means prime
4 prime factor(s) → not prime

Learn the method

Inputs

Number
84

Frequently asked

Is 84 prime?

No — it factors as 2 × 2 × 3 × 7.

What counts as prime here?

A whole number 2 or greater with exactly one prime factor (itself).