What is it?
The Prime Number Checker tells you whether a number is prime. It also shows the number’s prime factors, all its divisors and the next prime number.
Who is it for?
Students learning factors and primes, teachers, and puzzle lovers.
How to use it
- Type a whole number.
- Read whether it’s prime, its factors and divisors.
- See the next prime.
The answer appears as you type; there is no Calculate button. Tap Copy result to copy it, or Share on WhatsApp to send it.
What is a prime?
A whole number above 1 that divides evenly only by 1 and itself: 2, 3, 5, 7, 11, 13… 1 is not prime, and 2 is the only even prime.
Prime factorisation
Every whole number above 1 is a product of primes in exactly one way: 360 = 2³ × 3² × 5. This is used for HCF and LCM, simplifying fractions and square roots.
Number of divisors
From the factorisation: add 1 to each power and multiply. 360 = 2³ × 3² × 5¹ has (3+1)(2+1)(1+1) = 24 divisors.
Quick tests
- Divisible by 3 if the digit sum is (123 → 6). By 9 if the digit sum is divisible by 9. By 11 if the alternating digit sum is.
- To test by hand, you only need to try primes up to the square root of the number.
Very large numbers take longer to check; numbers beyond about 15 digits may be rounded by normal arithmetic.
Errors and how to fix them
- “Enter a whole number from 2 up”: 0, 1, decimals and negatives aren’t checked.
Open the tool: Prime Number Checker
Still stuck?
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Questions, or an error not listed here?
Tell us in the comments: what you were doing, the exact message you saw, and your device and browser. We read every comment and add new fixes to this article.