Eratosthenes

Sieve

Watch primes survive. Every composite is crossed out by its smallest prime factor. Tap a number to see why.

  1. 1The next unmarked number is prime.
  2. 2Cross out every multiple of it.
  3. 3Repeat with the next unmarked number.
  4. 4Whatever is still standing is prime.

Any window from 1 to 200.

The idea

Step 1 of 102

Every whole number from 1 to 100 starts unmarked. A prime has exactly two positive divisors: 1 and itself. The sieve crosses out the rest.

0 primes · 0 crossed out · 100 unmarked

  • Unmarked
  • Prime
  • Just crossed out
  • Composite

Five across on a narrow screen. Widen the window for the ten-column chart.