BUILDING & PLANNING · FIELD TOOL
A prime number has exactly two factors: 1 and itself. To check a number, try dividing it by every whole number up to its square root. 97 is prime because none of 2, 3, 5 or 7 divides it (√97 = 9.85), but 91 is not, because 91 = 7 × 13. There are 25 primes below 100.
Results update as you type.
Check the working before you order.
| Prime factors | 97 |
| Previous prime | 89 |
| Next prime | 101 |
Your prices are remembered on this device only.
Results are estimates for planning and material ordering only. They are not a substitute for structural design or approval by a qualified engineer. Read the full disclaimer.
n is prime if n ≥ 2 and no whole number from 2 to √n divides it exactly
Listing primes: cross out multiples of each prime (sieve of Eratosthenes)Is 91 prime?
| Up to | Number of primes |
|---|---|
| 10 | 4 |
| 100 | 25 |
| 1,000 | 168 |
| 10,000 | 1,229 |
| 100,000 | 9,592 |
| 1,000,000 | 78,498 |
Divide it by each prime up to its square root. If none divides exactly, it is prime. For 97 you only need to try 2, 3, 5 and 7, because √97 is under 10.
No. A prime has exactly two different factors, and 1 has only one. The first prime is 2.
Every other even number can be divided by 2, so it has at least three factors: 1, 2 and itself.
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: 25 primes.
A whole number above 1 that is not prime, so it has more than two factors. 91 is composite: 1, 7, 13 and 91.