Prime number checker
Check a number or list, find primes in a range, and explore factors and nearby primes.
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Prime number checker — workspace
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How to use: Prime number checker
Choose One number, A list of numbers, or Primes in a range. Enter your values, or select an example for an immediate result.
Run the check. One-number results explain the answer, list divisors and prime factors, and show the previous and next prime. Lists flag invalid entries separately.
Explore a result, filter a list, then copy or download the complete output. Save the tool to find it in your browser next time; your numbers are not saved between visits.
What you can do
- Check a number from a math exercise and understand the answer.
- Check a list of up to 100 numbers, or find every prime in an inclusive range of up to 10,000 numbers.
What you get
- A prime, composite or neither result, with an explanation you can copy or download.
- Complete prime factorization and positive divisors for a single number, plus the previous and next prime within the supported limit. Copy or download list and range results as CSV.
What you need
- One number
- Whole numbers from 0 to 1,000,000,000,000. Arabic numerals work too.
- A list of numbers
- Up to 100 numbers. Separate with spaces, commas or new lines. Leave out digit-grouping separators.
- Primes in a range
- Up to 10,000 consecutive numbers, between 0 and 1,000,000,000,000.
Try an example
- 97
Prime. Its only positive factors are 1 and 97.
- 91
Composite: 7 × 13 = 91.
- 2
Prime. 2 is the only even prime number.
- 0, 1
Neither prime nor composite.
Good to know
- Accepts whole numbers from 0 to 1,000,000,000,000, including Arabic and Persian digits. In lists, commas separate numbers; do not use digit-grouping marks. Decimals, negative numbers and larger values are not supported.
- A prime number has exactly two positive factors: 1 and itself. A composite number is greater than 1 and has additional factors.
Your work stays on your device
Your files and entered values are processed in your browser and are not uploaded for processing. The website and any required processing engines still download from the site.
Questions & answers
How does the prime number checker work?
For a number greater than 1, it checks prime divisors up to the square root. Repeated division gives the full prime factorization. Range mode uses a segmented sieve to find all primes in the interval. Calculations run on your device within the stated limits.
Is 1 a prime number?
No. 1 has only one positive factor, so it is neither prime nor composite. Prime numbers need exactly two positive factors. 0 is also neither prime nor composite.
Is 2 prime, and can other even numbers be prime?
2 is prime because its only positive factors are 1 and 2. Every even number greater than 2 is divisible by 2 and is composite.
Why is 91 composite?
91 = 7 × 13. Its positive factors are 1, 7, 13 and 91, so it has more than the two factors required for a prime number.