Maths calculators ยท free, runs on your device
Is it prime? Check any number.
Type a number to see if it is prime, how it breaks into prime factors, every factor it has and the primes either side of it. Or list all the primes up to a limit.
What is a prime number?
A prime number is a whole number greater than 1 whose only factors are 1 and itself. 2, 3, 5, 7 and 11 are prime. A number with other factors is called composite, and every composite number is a product of primes in exactly one way, which is why 360 = 2ยณ ร 3ยฒ ร 5.
- 1 is not prime. It has only one factor, so it is neither prime nor composite.
- 2 is the only even prime. Every other even number divides by 2.
- There are 25 primes below 100: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89 and 97.
- To test a number by hand, try dividing by each prime up to its square root. If none divides it, the number is prime.
Why does it matter? Primes are the building blocks of whole numbers, and they protect online banking and UPI through encryption. Next, use the LCM and HCF calculator to see prime factors at work.
FAQ
Common questions
Is 1 a prime number?
No. A prime must have exactly two different factors, 1 and itself. The number 1 has only one factor, so it is not prime.
Is 91 prime?
No. 91 = 7 ร 13. It looks prime because it is not divisible by 2, 3 or 5, which is why it is a favourite trick question.
How many prime numbers are there?
Infinitely many. The ancient Greek mathematician Euclid proved this more than 2,000 years ago.
More free tools
Keep going
Percentage calculator
Find X% of a number, what percent one number is of another, percentage change, add or take off a percentage, and reverse percentages.
Use the tool โFraction calculator
Add, subtract, multiply and divide fractions and mixed numbers, with the answer simplified and the working shown step by step.
Use the tool โLCM & HCF calculator
Find the lowest common multiple and highest common factor (GCD) of up to eight numbers, with a prime factor table that shows why.
Use the tool โQuadratic equation solver
Solve axยฒ + bx + c = 0 with the discriminant, both roots (including complex ones), the vertex and the factorised form, step by step.
Use the tool โMean, median & mode calculator
Mean, median, mode, range, quartiles and standard deviation for any list of numbers, such as marks, prices, runs or temperatures.
Use the tool โRatio calculator
Simplify ratios, share an amount in a ratio, or solve a : b = c : x. Handy for recipes, profit sharing, maps and scaling.
Use the tool โStart a conversation
Questions about classes?
A parent or guardian can share the learner's age, level and goals, and we'll suggest a starting point.
- Tell us the learnerโs age or grade and goals
- Weโll suggest a suitable class or learning path
- Weโll confirm format, timing and fees with a parent or guardian
Play it, don't just read it
Try it now: Factors and multiples
Eight quick questions with instant feedback. Your score is saved on this device and counts in the games and learning paths too.
Factors and multiples (Grade 4): Counting factors and listing multiples of a number. Open the practice page or print a worksheet.
Or play a game
- Prime HunterTap every prime, square, multiple or factor hidden in a grid of numbers.Play
- Maths PasswordCreate a password that follows ten maths rules, from digit sums andโฆPlay
- Mental Maths Tricks TrainerLearn a quick trick, then practise it: ร11, squaring numbers ending inโฆPlay