What does the Prime Number Checker work out?

Test a number, factor it, or list the primes in a range. It is one of the Math tools on Mondo Knows.

How is the Prime Number Checker calculated?

Tell Mondo a number and it will say whether it is prime, break it into prime factors, and show how far it had to divide to be sure. There is also a mode for listing primes across a range and one for finding the next prime after any number. The maths is short enough to check by hand: A number n is prime when nothing from 2 up to √n divides it exactly. Trial division stops at √n because any factor larger than that must pair with one smaller.

Do I need to sign up, and is my data stored?

No sign-up, and nothing you type is sent anywhere. The calculation runs in your own browser, so the figures stay on your device and the page works offline once loaded.

Mondo Knows Tools

Prime Number Checker

Test a number, factor it, or list the primes in a range. Use the free prime number checker below to get an instant answer, with the formula shown so you can check the maths and trust the result. Free, private, and no sign-up.

What are you after?

Number

Mondo says

Is 97 prime?

Yes

Answer97 is prime

Divisors checkedup to 9

Nearest primes below89, 83

97 has no divisors other than 1 and itself.

How it works

Tell Mondo a number and it will say whether it is prime, break it into prime factors, and show how far it had to divide to be sure. There is also a mode for listing primes across a range and one for finding the next prime after any number.

The formula

A number n is prime when nothing from 2 up to √n divides it exactly. Trial division stops at √n because any factor larger than that must pair with one smaller.

Prime Number Checker: common questions

What does the Prime Number Checker work out?: Test a number, factor it, or list the primes in a range. It is one of the Math tools on Mondo Knows.
How is the Prime Number Checker calculated?: Tell Mondo a number and it will say whether it is prime, break it into prime factors, and show how far it had to divide to be sure. There is also a mode for listing primes across a range and one for finding the next prime after any number. The maths is short enough to check by hand: A number n is prime when nothing from 2 up to √n divides it exactly. Trial division stops at √n because any factor larger than that must pair with one smaller.
Do I need to sign up, and is my data stored?: No sign-up, and nothing you type is sent anywhere. The calculation runs in your own browser, so the figures stay on your device and the page works offline once loaded.

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Estimates for general guidance only. This is not financial, medical or legal advice. Everything runs in your browser and nothing you type is stored.