---
title: "Prime Checker"
description: "Check whether a number is prime, and see the prime factorisation when it is not. no signup required, no upload required."
url: https://findutils.com/calculate/prime-checker/
category: calculate
---

# Prime Checker

Check whether a number is prime, and see the prime factorisation when it is not. no signup required, no upload required.

**Use this tool:** [Prime Checker](https://findutils.com/calculate/prime-checker/)

## Programmatic access

- REST id `prime-check`: POST https://api.findutils.com/api/tools/prime-check/execute (reference: https://findutils.com/api/prime-check/)
- MCP tool `prime_check` on https://mcp.findutils.com (reference: https://findutils.com/mcp/prime-check/)

## Why use this prime checker?

A prime number has exactly two divisors: one and itself. Everything else is composite, and every composite number breaks down into primes in exactly one way. This tool answers both questions at once: whether your number is prime, and if not, which primes multiply to make it. It runs entirely in your browser.

## Frequently Asked Questions

### What makes a number prime?

It has exactly two whole-number divisors, one and itself. 7 is prime because nothing between 2 and 6 divides it. 1 is not prime, because it has only one divisor, and that is why the tool starts at 2.

### What is prime factorisation?

Writing a number as a product of primes. 360 is 2 x 2 x 2 x 3 x 3 x 5, shown here as 2 cubed times 3 squared times 5. The fundamental theorem of arithmetic says every whole number above 1 has exactly one such factorisation.

### How large a number can it handle?

Up to 2^53-1, the largest integer JavaScript represents exactly. It stays fast to around 10^12; much beyond that, trial division starts to take noticeable time.

### How does it find the factors?

Trial division: it divides by 2, then 3, then each larger candidate, stopping at the square root of what remains. Anything left over at the end is itself prime.

### Is there an API for this?

Yes. The same check is available over REST and MCP as the tool id prime_check, taking a single integer n.

### Do I need an account?

No. This tool requires no signup, no usage limits, and no watermarks.

## Related Tools

- [GCD & LCM Calculator](https://findutils.com/calculate/gcd-lcm-calculator/)
- [Scientific Calculator](https://findutils.com/calculate/scientific-calculator/)
- [Fraction Calculator](https://findutils.com/calculate/fraction-calculator/)
- [Statistics Calculator](https://findutils.com/calculate/statistics-calculator/)
- [Percentage Calculator](https://findutils.com/calculate/percentage-calculator/)
- [Number Base Converter](https://findutils.com/convert/number-base-converter/)