---
title: "Composite Numbers, and the Odd One People Miss"
url: https://www.learningloftinstitute.com/glossary/composite-number
source: Learning Loft Institute
language: en
updated: 2026-08-21
section: "Glossary › Composite Numbers, and the Odd One People Miss"
---
# Composite Numbers, and the Odd One People Miss
> **Answer.** A composite number is a whole number above 1 that has more than two factors, so it can be made by multiplying two smaller whole numbers. 4, 6, 8, 9 and 10 are composite. The number 1 is neither prime nor composite, and the smallest odd composite number is 9.
## Key facts
| | |
| --- | --- |
| Definition | A composite number is a whole number greater than 1 with more than two factors, meaning it can be written as one smaller whole number multiplied by another. |
| Where students meet it | Grade 5 mathematics, alongside primes and factors, and relied on again wherever prime factorisation, HCF or LCM is needed at GCSE and IGCSE. |
## The test is how many factors, not how big the number is
Every whole number greater than 1 is either prime or composite, and the sorting is done by counting factors. Exactly two factors — itself and 1 — makes it prime. More than two makes it composite.
The number 1 has exactly one factor, so it fits neither description, and this is a definition rather than an oversight. If 1 counted as prime, then 12 could be written as 2 × 2 × 3 or 1 × 2 × 2 × 3 or 1 × 1 × 2 × 2 × 3, and the promise that every number has one prime factorisation would break.
## The odd composites
Asked for the smallest odd composite number, most people say 15 or 21. The answer is 9, because 9 = 3 × 3. Odd does not mean prime; it only means not divisible by 2.
Every even number above 2 is composite, since 2 divides it, which makes 2 the only even prime. That leaves the odd numbers as the interesting ones, and plenty of them are composite:
- 9 = 3 × 3
- 15 = 3 × 5
- 21 = 3 × 7
- 25 = 5 × 5
- 27 = 3 × 9
- 33 = 3 × 11
- 35 = 5 × 7
- 49 = 7 × 7
## How far you have to check
To decide whether a number is composite, divide by the primes in order — 2, 3, 5, 7, 11 — and stop once the prime you are testing, squared, passes the number. For 91 that means testing up to 9, and 7 × 13 = 91 turns up on the way, so 91 is composite.
That is the checkable answer to the numbers people misclassify. 51 is 3 × 17. 57 is 3 × 19. 91 is 7 × 13. All three look prime and none of them are, and the digit-sum test for 3 catches the first two in a second.
## Questions
### Is 1 a composite number?
No, and it is not prime either. A composite number needs more than two factors and 1 has only one factor, itself. Mathematicians treat 1 as a special case so that prime factorisation stays unique — every number breaks into primes in exactly one way, which would fail if 1 were allowed in.
### Is 2 a composite number?
No. 2 has exactly two factors, 1 and 2, so it is prime — the only even prime there is. Every other even number is divisible by 2 as well as by 1 and itself, giving it at least three factors and making it composite.
### What is the smallest odd composite number?
9. Its factors are 1, 3 and 9, so it has three factors rather than two. The odd numbers below it — 3, 5 and 7 — are all prime, and 1 is neither. This question is a favourite precisely because the instinct is to jump to 15.
### Is 0 a composite number?
No. The prime and composite labels apply only to whole numbers greater than 1. Zero sits outside that: every whole number divides into it, so counting its factors gives no useful answer, and it is left unclassified rather than called composite.
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