Quick answer: to add or subtract, give both fractions the same denominator first: 2/3 + 3/4 = 8/12 + 9/12 = 17/12 = 1 5/12. To multiply, multiply tops and bottoms: 2/3 × 3/4 = 6/12 = 1/2. To divide, flip the second fraction and multiply: 2/3 ÷ 3/4 = 2/3 × 4/3 = 8/9.
The four operations
| Operation | Rule | Example |
|---|---|---|
| Add | Common denominator, add numerators | 1/4 + 1/6 = 3/12 + 2/12 = 5/12 |
| Subtract | Common denominator, subtract numerators | 3/4 − 1/3 = 9/12 − 4/12 = 5/12 |
| Multiply | Top × top, bottom × bottom | 2/5 × 3/4 = 6/20 = 3/10 |
| Divide | Flip the second, then multiply | 1/2 ÷ 3/4 = 1/2 × 4/3 = 2/3 |
Mixed numbers
Turn a mixed number into an improper fraction before calculating: 2 ⅓ = (2 × 3 + 1)/3 = 7/3. After calculating, divide the numerator by the denominator to turn it back: 17/12 = 1 remainder 5, so 1 5/12. The calculator does both conversions and shows the working.
Simplifying
Divide the top and bottom by their greatest common divisor. For 18/24 the GCD is 6, giving 3/4. Using the least common multiple as the shared denominator (12 rather than 24 for quarters and sixths) keeps numbers small and the answer easier to simplify.
Fractions in everyday life
Scaling a recipe (¾ cup × 1½), woodwork in inches (5⅝ − 2¾), sharing a bill three ways, or homework – the decimal and percentage forms in the result make it easy to switch to a tape measure or a spreadsheet.
To turn a fraction into a percentage of something real – a discount, a share of a bill – use the percentage calculator. For powers, roots and trigonometry, switch to the scientific calculator.
How to calculate with fractions
- Enter the first fraction. Numerator on top, denominator below. Add a whole number for a mixed number.
- Choose the operation. Add, subtract, multiply or divide.
- Enter the second fraction. Negative fractions work too – put the minus sign on the whole number or numerator.
- Read the answer. Simplified fraction, mixed number, decimal, percentage and the steps.
Worked example: subtracting mixed numbers
2⅓ − ¾
- Make improper fractions: 2⅓ = 7/3.
- Common denominator of 3 and 4 is 12: 7/3 = 28/12 and 3/4 = 9/12.
- Subtract: 28/12 − 9/12 = 19/12.
- As a mixed number: 19 ÷ 12 = 1 remainder 7, so 1 7/12 (≈ 1.583).
Common fraction mistakes
- Adding the denominators: 1/2 + 1/3 is not 2/5 – it is 5/6.
- Forgetting to flip the second fraction when dividing.
- Not simplifying: 6/8 should be written as 3/4.
- Mixing up negative mixed numbers: −1½ is −3/2, not −1/2.
Frequently asked questions
How do I add fractions with different denominators?
Find a common denominator (the least common multiple), convert both fractions, then add the numerators. 1/4 + 1/6 = 3/12 + 2/12 = 5/12.
How do I divide fractions?
Keep the first fraction, flip the second and multiply: 2/3 ÷ 3/4 = 2/3 × 4/3 = 8/9.
How do I enter a mixed number?
Type the whole number in the small “whole” box and the fraction in the boxes beside it, e.g. 1 and 1/2.
How do I simplify a fraction?
Divide top and bottom by their greatest common divisor. The calculator always simplifies the result.
Can I enter decimals?
Fractions need whole numbers. Convert decimals first (0.5 = 1/2) or use the percentage or scientific calculator.
What is an improper fraction?
One where the numerator is bigger than the denominator, like 17/12. As a mixed number that is 1 5/12.
How do I turn a fraction into a decimal?
Divide the numerator by the denominator: 3/8 = 3 ÷ 8 = 0.375. The calculator shows the decimal under every result.
What is a common denominator?
A number both denominators divide into. The smallest one is the least common multiple – 12 for thirds and quarters.