Prime Number Checker

Instantly check whether any number is prime or composite — up to 200 digits. Get the full factor list, prime factorization, nearest primes, and a divisibility chart. Check multiple numbers at once in Batch mode, or find every prime in a range. All results are calculated privately in your browser.

🔒 Calculated in your browser — your data never leaves your device

Enter a whole number (up to 200 digits)

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Result
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Digit Count

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Even or Odd

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Number of Factors

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Previous Prime

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Next Prime

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Divisibility Check

Results are for informational purposes only. All calculations are performed locally in your browser.

Enter multiple numbers (separated by commas, spaces, or new lines)

Enter a range (up to 1,000,000)

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How to Use the Prime Number Checker

Enter Your Number

Type any whole number into the input field. The tool accepts numbers up to 200 digits long — so you can check anything from small classroom examples to very large numbers. You can also paste a number directly from your clipboard.

Click Check

Click the Check button or simply press Enter. Your result appears instantly — showing whether the number is prime or composite, along with all of its factors, its prime factorization, its nearest primes, and a visual divisibility chart.

Use Batch Mode or Range Finder

Switch to the Batch Check tab to test a whole list of numbers at once — just paste them in separated by commas or new lines. Use the Find in Range tab to discover every prime number between any two values up to one million.

Print or Copy Your Results

Use the Print button to produce a clean printout of your results — including the divisibility chart. Copy individual results or full tables to your clipboard with one click.

Frequently Asked Questions

What is a prime number?

A prime number is a whole number greater than 1 that can only be divided evenly by 1 and itself. For example, 7 is prime because nothing other than 1 and 7 divides into it without a remainder. The first few primes are 2, 3, 5, 7, 11, and 13. Numbers with more than two factors — such as 6, which divides by 1, 2, 3, and 6 — are called composite numbers.

Is 1 a prime number?

No. By definition, a prime number must have exactly two distinct factors: 1 and itself. The number 1 has only one factor, so it does not qualify. It is classified as neither prime nor composite — a special category all its own.

What is a composite number?

A composite number is a whole number greater than 1 that has at least one factor other than 1 and itself. For example, 15 is composite because it is divisible by 3 and 5 in addition to 1 and 15. Every composite number can be expressed as a unique product of prime factors — this is known as its prime factorization.

What are twin primes?

Twin primes are pairs of prime numbers that differ by exactly 2. Common examples include 3 and 5, 11 and 13, 17 and 19, and 29 and 31. They are special because primes generally become more spread out as numbers get larger — yet twin prime pairs keep appearing. When you check a prime number, this tool will tell you whether it belongs to a twin prime pair.

Why are prime numbers important?

Prime numbers are sometimes called the building blocks of all whole numbers, because every number can be broken down into a unique product of primes. Beyond mathematics education, they play a central role in securing the internet. Modern encryption relies on the fact that it is easy to multiply two large primes together, but extraordinarily difficult to reverse the process and find the original primes from the result.

Is my data safe when using this tool?

Yes. All calculations happen entirely within your browser. The numbers you enter are never sent to any server and never leave your device.