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Prime numbers for kids: rectangles, tests and a first hundred chart

For: Ages 8-11. Updated 2026-10-15.

Prime numbers look mysterious until you build them. If the only rectangle you can make with n blocks is a single line, n is prime. If you can make a wider rectangle, n is composite.

Build first, define second

  • 7 blocks: only 1 × 7. Prime.
  • 9 blocks: 1 × 9 and 3 × 3. Composite.
  • 1 block: only one factor, so neither prime nor composite.

A simple test children can remember

  1. Is it 0 or 1? Neither.
  2. Is it 2 or 3? Prime.
  3. Even and bigger than 2? Composite.
  4. Digit sum a multiple of 3? Composite (except 3 itself).
  5. Ends in 0 or 5 and bigger than 5? Composite.
  6. Otherwise try dividing by primes up to the square root.

Numbers that trick people

51, 57, 87 and 91 look prime but are not. Ask children to try dividing by 3 or 7 before guessing.

Open 91 in the tool and read the rectangles and prime-factor line. Open the tool with 91

Prime hunt

Mark each as prime, composite or neither. Give a factor pair when composite.

  1. 2
  2. 1
  3. 15
  4. 29
  5. 51
  6. 49
Show answers
  1. Prime.
  2. Neither.
  3. Composite: 3 × 5.
  4. Prime.
  5. Composite: 3 × 17.
  6. Composite: 7 × 7.

Keep learning

Skip counting practice: 2s, 5s, 10s and linking to multiplesSkip-counting drills tied to place value and the Number Blocks Generator, with forward and backward patterns and answers.Hundreds, tens and ones practice with expanded formFocused practice on three-digit place value: reading digits by place, expanded form, and comparing numbers that look similar.Place value explained with blocks: hundreds, tens and onesWhy the same digit can be worth 3, 30 or 300, how to show it with blocks, and the mistakes to watch for when children first meet three-digit numbers.