# Prime Number Checker & Factoriser

> Check whether a number is prime and see its full prime factorisation.

To check whether a number is prime, enter it above. The tool performs trial division up to the square root and returns the full prime factorisation in exponent notation, along with the total number of divisors.

**URL:** https://convertto.tech/t/prime-number-checker
**Category:** Calculators (https://convertto.tech/c/calculators)
**Privacy:** Runs entirely in the browser; no upload
**Cost:** Free, no sign-up
**Last updated:** 2026-08-01

## Key facts

- **Privacy:** Runs entirely in your browser — nothing is uploaded
- **Cost:** Free, unlimited, no sign-up

## How to use

1. Set the number.
2. The result appears immediately — copy or download it.

## FAQ

### Why is 1 not prime?

Because a prime has exactly two distinct divisors, one and itself, and 1 has only one. The definition is chosen deliberately: if 1 were prime, every number would have infinitely many prime factorisations and the fundamental theorem of arithmetic would fail.

### How far do you need to test for divisors?

Only to the square root. If a number has a factor above its square root, it must have a matching factor below it, which the search would already have found. That reduces testing a million from a million steps to a thousand.

### Why do primes matter outside mathematics?

RSA encryption rests on the fact that multiplying two large primes is easy while factoring the product back is not. The security of a great deal of internet traffic is exactly that asymmetry.

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