4th grade prime & composite numbers worksheets

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Prime and composite worksheets ask fourth graders to classify numbers from 2 to 99 by counting their factors. Primes have exactly two factors; composites have more. Deciding takes a quick divisibility check with 2, 3, 5, and 7.

Free printable PDF worksheet, aligned to Common Core 4.OA.B.4.

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The kind of problems you'll get

Write prime, composite, or neither for each number.

  1. Classify 13 and 15.

    Answer: 13 is prime: factors 1 and 13. 15 is composite: factors 1, 3, 5 and 15.

Most problems on this sheet sit beside a picture, model or diagram, so this is the worked example the sheet itself prints. The answer key on page two prints the answer under every problem.

Every print pulls a fresh set of problems at this level, so a make-up test or a second sibling never gets the same sheet.

What's on a 4th grade sheet

Every version prints on US Letter or A4, with its answer key after the problems.

How to teach this

Give your child a short test sequence instead of guessing: is it even? Do the digits add to a multiple of 3? Does it end in 5 or 0? Then try 7. Any yes means composite; all no's on a number under 100 means prime. Keep the trap numbers coming back: 51, 57, 87, and 91 catch almost everyone once.

Watch for: Kids call every odd number prime, but 9, 15, 21, and 51 are all odd and all composite, so odd only means 'not divisible by 2'. Kids label 1 as prime. It has only one factor, so it is neither prime nor composite.

Common questions about 4th grade prime & composite numbers

Why isn't 1 a prime number?
A prime needs exactly two different factors, and 1 has only itself. Mathematicians count it as neither prime nor composite.
How can my child check a bigger number like 91 without a calculator?
Try the small primes in order: 2, 3, 5, then 7. For 91, the first three fail but 7 × 13 = 91, so it's composite, and for any number under 100, checking up to 7 is enough.

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Aligned to Common Core 4.OA.B.4. Reviewed by the One more sheet team. Content version 240, updated October 2026.

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