Prime Number Checker
Check if a number is prime or composite and see its factors instantly with this free prime number checker that runs right in your browser.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter a number.
- Press Calculate.
How it works
Tests whether a number is prime — divisible only by 1 and itself — by checking for factors up to its square root.
Worked example
7. is prime; 8 is not.
Common mistakes
- Thinking 1 is prime — by definition a prime has exactly two factors, so 1 is not prime.
- Forgetting 2 is the only even prime.
Frequently asked questions
What is a prime number?
A whole number greater than 1 whose only factors are 1 and itself, such as 2, 3, 5, 7 and 11.
How do you check if a number is prime?
Test for divisibility by each integer up to its square root; if none divide it evenly, it's prime.
Is 1 a prime number?
No — a prime needs exactly two distinct divisors, and 1 has only one. The smallest prime is 2, which is also the only even prime.
Related tools
- GCD & LCM Calculator
- Factorial Calculator
- Prime Factorization Calculator
- Modulo Calculator
- Nth Root Calculator
- Square Root Calculator
Explore more in Maths, Education & Science.
Cite this tool
Zerdly. (2026). Prime Number Checker. Retrieved from https://www.zerdly.com/tools/prime-number-checker/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.