Prime Number Checker — Is It Prime? + Prime Factorization
Check if any number is prime online free — get an instant yes or no, plus the complete prime factorization. Works for very large numbers. Shows why a number is or isn't prime.
Last updated: June 2026
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Maximum span: 10,000 numbers.
About this tool
Primes, factors and the building blocks of numbers
Prime numbers are the atoms of arithmetic — every whole number is either prime or a unique product of primes. This prime number checker gives you three related tools in one: a fast primality test, a complete prime factorization, and a generator that lists every prime within a range you choose.
The is-it-prime tab tells you immediately whether a number has exactly two factors. When a number isn't prime, it also shows its prime factors so you understand why. The prime factorization tab breaks any number into the primes that multiply to make it, written in compact exponent form like 360 = 2³ × 3² × 5 — the same decomposition that underpins finding lowest common multiples, highest common factors and simplifying fractions.
The primes in a range tab is handy for study and curiosity alike, listing all the primes between two numbers and counting them. The primality test checks only up to the square root of each number, which keeps it quick even into the billions. Everything is computed locally in your browser, so the tool is instant, free and private — no number you enter is ever sent anywhere.
How it works
Three quick steps — no account, nothing uploaded to a server.
Pick a mode
Check primality, factorize, or list primes in a range.
Enter your number
Type a whole number — results appear as you type.
Read the result
See whether it's prime, its factors, or every prime in range.
FAQ
Frequently asked questions
What is a prime number?
A prime number is a whole number greater than 1 that has exactly two factors: 1 and itself. So 2, 3, 5, 7, 11 and 13 are prime, while 4, 6, 8 and 9 are not because they can be divided evenly by other numbers. The number 1 is neither prime nor composite by definition.
How does the checker decide if a number is prime?
It tests divisibility efficiently — checking 2 and 3 first, then candidate factors up to the square root of the number (the highest factor you ever need to test). If nothing divides it evenly, the number is prime. This works quickly even for numbers in the billions.
What is prime factorization?
Prime factorization breaks a number down into the prime numbers that multiply together to make it. For example, 12 = 2 × 2 × 3, written compactly as 2² × 3. It's the foundation for finding the LCM, HCF and simplifying fractions, and the factorization tab shows it for any number.
How large a number can I check?
You can check whether a number up to 10 billion is prime and find its prime factors. For listing every prime in a range, keep the span within 10,000 numbers so the list stays fast and readable. These limits keep the tool instant in the browser.
Are 0 and 1 prime numbers?
No. By definition a prime must be greater than 1 and have exactly two distinct factors. 0 and 1 don't qualify — 1 has only a single factor (itself), and 0 has infinitely many — so both are classed as neither prime nor composite.
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