Prime Number Checker
Check whether a number is prime, show its prime factorisation and the next prime.
Enter values
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Step by step
The formula
trial division up to √n
A prime has exactly two divisors: 1 and itself. Testing only up to the square root is the key shortcut — any factor above √n must already be paired with one below it, so if nothing small divides the number, nothing will.
All calculations run locally in your browser and update instantly as you type.
Example calculations
Common questions this calculator answers — select one to load its values.
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Frequently asked questions
What is a prime number?+
A prime is a whole number greater than 1 whose only divisors are 1 and itself — for example 2, 3, 5, 7 and 11.
All calculations run locally in your browser. Nothing you type is sent to a server.