Free Online Prime Number Checker

Check if any number is prime. Find factors, prime factorization, and nearby prim...

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Checker Features

Primality Test - Instantly check if any number is prime or composite
Prime Factorization - Complete prime factorization for composite numbers
All Factors - View every divisor of the number sorted in order
Navigate Primes - Jump to previous or next prime number instantly
Quick Check Buttons - One-click testing for common primes and composites
Check History - Color-coded history of your recent primality checks

Why Use Free Online Prime Number Checker?

Instant Primality Test

Check if any number is prime or composite with instant results using trial division.

Prime Factorization

See the complete prime factorization of any composite number.

All Factors

View all divisors/factors of the number, not just prime factors.

Nearby Primes

Navigate to the next or previous prime number with one click.

Check History

See your recent checks color-coded as prime (green) or composite (red).

100% Private

All calculations happen in your browser. No data is sent to any server.

Common Uses for Prime Checker

Math Education

Learn about prime numbers, factorization, and number theory concepts.

Programming

Verify primality algorithms and test edge cases in your code.

Cryptography

Understand prime numbers used in RSA encryption and security protocols.

Number Theory

Explore mathematical properties and relationships between numbers.

How It Works

1

Enter a Number

Type any positive integer in the input field, or use the quick check buttons for common numbers.

2

Check the Result

Click 'Check Number' or press Enter to see if the number is prime or composite with full factorization.

3

Explore Nearby Primes

Use the Previous/Next Prime buttons to navigate through prime numbers near your input.

Tips & Information

Quick Navigation

Use the Previous/Next Prime buttons to quickly explore sequences of prime numbers.

Pattern Recognition

All primes greater than 3 are of the form 6k +/- 1. Check numbers near multiples of 6 to find primes efficiently.

Large Numbers

For very large numbers, the check may take a moment. The tool handles numbers up to JavaScript's safe integer limit.

Frequently Asked Questions

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. For example, 2, 3, 5, 7, 11, 13 are prime numbers. The number 1 is not considered prime.
The tool uses trial division to check if a number is prime. It tests divisibility by all numbers from 2 up to the square root of the input. If no divisor is found, the number is prime. This is efficient for numbers up to several billion.
There is no largest prime number - primes go on forever (proven by Euclid around 300 BC). The largest known prime has over 24 million digits. Our tool can check numbers up to JavaScript's safe integer limit (about 9 quadrillion).
Prime factorization breaks a number into its prime components (e.g., 12 = 2 x 2 x 3). Factors are all numbers that divide evenly into a number (e.g., factors of 12 are 1, 2, 3, 4, 6, 12). Our tool shows both for non-prime numbers.
By mathematical convention, 1 is not considered prime. This ensures the uniqueness of prime factorization (Fundamental Theorem of Arithmetic). If 1 were prime, every number would have infinitely many prime factorizations.
Yes! 2 is the only even prime number. Every other even number is divisible by 2, so no other even number can be prime.

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