What is a denominator?
Year 3 compares fractions with the same denominator. Year 5 adds fractions whose denominators are multiples of the same number, and Year 6 adds fractions with different denominators. Below: what it means, a worked example, the mistake to watch for and an 11+ question.
The answer
The denominator is the bottom number of a fraction, and it tells you how many equal parts the whole is split into.
What it means
In ⅗ the denominator is 5: the whole is split into 5 equal parts.
To add or subtract fractions, their denominators must match first. A denominator both fractions can share is called a common denominator.
A worked example
Work out ¼ + ¼
- The denominators already match: both are quarters.
- Add the numerators only: 1 + 1 = 2, so the answer is 2/4, which simplifies to ½.
The common mistake
Adding the denominators as well: ¼ + ¼ is not 2/8. Two quarters are still quarters, so the answer is 2/4, or ½.
How it comes up in the 11+
Work out ⅓ + ¼.
Answer: 7/12
- The denominators are different, so find a common denominator. 12 is in the 3 and the 4 times tables.
- ⅓ = 4/12 and ¼ = 3/12.
- 4/12 + 3/12 = 7/12.
Try it yourself
The wall opens on 2/4, the answer to ¼ + ¼. Look straight down to see it lines up with ½, and how the pieces shrink as the denominator grows.
A fraction wall shows one whole split into halves, thirds, quarters and more, so equal fractions line up.
Related words and practice
More maths words
Questions parents ask
Can a denominator be 0?
No. You cannot split a whole into 0 parts, so a fraction can never have 0 as its denominator.
What is a common denominator?
A denominator that two fractions can both be written with. For ½ and ⅓ it is 6: ½ = 3/6 and ⅓ = 2/6.
Knowing the words is half of an 11+ maths question
11+ papers use maths vocabulary without explaining it. The free diagnostic checks maths, English and both kinds of reasoning, and shows what to practise next. No card is required.