How to Subtract Fractions: A Step-by-Step Guide

Pie Chart Fractions Maths 2020

Last updated: 7 June 2026

Subtracting fractions is one of the trickier topics in primary school maths, but it follows a clear set of rules. Once your child understands the method, they can apply it to any subtraction — whether the denominators are the same, different, or the fractions are mixed numbers. This guide breaks it down step by step.

The Key Rule: Denominators Must Be the Same

Before you can subtract fractions, the denominators (bottom numbers) must match. This is the single most important rule to remember:

  • If the denominators are already the same → subtract the numerators (top numbers) directly.
  • If the denominators are different → find a common denominator first, then subtract.

Step 1: Subtracting Fractions with the Same Denominator

When both fractions have the same denominator, subtraction is straightforward — simply subtract the numerators and keep the denominator the same.

Example 1

5/8 − 3/8 = ?

  • Denominators are the same (8), so subtract the numerators: 5 − 3 = 2.
  • Answer: 2/8, which simplifies to 1/4.

Example 2

7/10 − 2/10 = ?

  • Subtract the numerators: 7 − 2 = 5.
  • Answer: 5/10, which simplifies to 1/2.

Step 2: Subtracting Fractions with Different Denominators

When the denominators are different, you need to find a common denominator — a number that both denominators divide into evenly. The easiest method is to find the lowest common multiple (LCM) of the two denominators.

Example 3

3/4 − 1/3 = ?

  1. Find the LCM of 4 and 3. The LCM is 12.
  2. Convert both fractions:
    • 3/4 → multiply top and bottom by 3 → 9/12
    • 1/3 → multiply top and bottom by 4 → 4/12
  3. Subtract: 9/12 − 4/12 = 5/12.

Example 4

5/6 − 1/4 = ?

  1. LCM of 6 and 4 is 12.
  2. Convert:
    • 5/6 → multiply by 2 → 10/12
    • 1/4 → multiply by 3 → 3/12
  3. Subtract: 10/12 − 3/12 = 7/12.

Step 3: Subtracting Mixed Numbers

A mixed number has a whole number part and a fraction part (like 2 1/3). To subtract mixed numbers:

Method A: Convert to Improper Fractions

  1. Convert each mixed number to an improper fraction.
  2. Find a common denominator if needed.
  3. Subtract.
  4. Convert back to a mixed number.

Example 5

3 1/2 − 1 3/4 = ?

  1. Convert: 3 1/2 = 7/2 and 1 3/4 = 7/4.
  2. Common denominator (4): 7/2 = 14/4.
  3. Subtract: 14/4 − 7/4 = 7/4.
  4. Convert back: 7/4 = 1 3/4.

Method B: Subtract Whole Numbers and Fractions Separately

Sometimes it is simpler to subtract the whole numbers first, then the fractions:

Example 6

4 2/5 − 2 1/5 = ?

  • Whole numbers: 4 − 2 = 2.
  • Fractions: 2/5 − 1/5 = 1/5.
  • Answer: 2 1/5.

This method works well when the first fraction is larger than the second. If not (e.g., 3 1/4 − 1 3/4), you need to borrow from the whole number, which is where Method A is simpler.

Subtracting a Fraction from a Whole Number

To subtract a fraction from a whole number, convert the whole number into a fraction first:

Example 7

3 − 2/5 = ?

  1. Convert 3 to fifths: 3 = 15/5.
  2. Subtract: 15/5 − 2/5 = 13/5 = 2 3/5.

Always Simplify Your Answer

After subtracting, check whether your answer can be simplified by dividing the numerator and denominator by their highest common factor:

  • 6/8 → divide both by 2 → 3/4
  • 4/10 → divide both by 2 → 2/5
  • 9/12 → divide both by 3 → 3/4

Common Mistakes to Avoid

  • Subtracting the denominators. Only the numerators are subtracted — the denominator stays the same (or becomes the common denominator).
  • Forgetting to convert. If the denominators are different, you must find a common denominator before subtracting.
  • Not simplifying. Always check if your answer can be reduced to its simplest form.
  • Converting mixed numbers incorrectly. To convert 2 3/4 to an improper fraction: (2 × 4) + 3 = 11, so it is 11/4.

Practice Problems

Try these at home with your child:

Same Denominator

  • 7/9 − 4/9 = ? (Answer: 3/9 = 1/3)
  • 11/12 − 5/12 = ? (Answer: 6/12 = 1/2)

Different Denominators

  • 3/4 − 1/6 = ? (Answer: 9/12 − 2/12 = 7/12)
  • 2/3 − 1/5 = ? (Answer: 10/15 − 3/15 = 7/15)

Mixed Numbers

  • 4 1/3 − 2 1/6 = ? (Answer: 2 1/6)
  • 5 − 2 3/8 = ? (Answer: 2 5/8)

Related Reading

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