What makes a number prime
A prime has exactly two divisors: 1 and itself. The checker tests divisibility only up to √n because any factor larger than √n must pair with one smaller than it.
Prime Number Checker
Test whether a number is prime and see its smallest factors when it isn't.
Enter your details and press Calculate to see results.
Formula
Trial division up to √n
Example
97 has no divisors between 2 and 9, so it is prime.
Advertisement
Frequently asked questions
Why is 1 not prime?
By definition primes need two distinct divisors. 1 has only one, which keeps factorizations like 6 = 2×3 unique.
What is the largest number I can test?
Values up to about 10¹⁵ respond instantly.
Related calculators
Random Number Generator
Generate random integers or decimals in any range, with optional unique values for draws and raffles.
Percentage Calculator
Calculate X% of Y, find what percent one number is of another, and solve percentage increase problems in one place.
Percentage Increase Calculator
Find the percentage increase between an original value and a new value, including the absolute change.
Fraction Calculator
Add, subtract, multiply and divide fractions and mixed numbers, with results automatically simplified.
Square Root Calculator
Find √ of any non-negative number, plus perfect-square detection and nearby roots.
Scientific Calculator
Full expression calculator with parentheses, powers, roots, trigonometry, logarithms and constants π and e.