Fraction rules at a glance
| Operation | How | Example |
|---|---|---|
| Add | common denominator, add numerators | 1/2 + 1/3 = 3/6 + 2/6 = 5/6 |
| Subtract | common denominator, subtract numerators | 3/4 − 1/6 = 9/12 − 2/12 = 7/12 |
| Multiply | numerator × numerator, denominator × denominator | 2/3 × 3/5 = 6/15 = 2/5 |
| Divide | multiply by the reciprocal | 3/4 ÷ 1/2 = 3/4 × 2/1 = 3/2 |
| Simplify | divide both by the greatest common factor | 12/18 = 2/3 (GCF 6) |
The least common denominator is the least common multiple (LCM) of the denominators. For 4 and 6 it is 12, not 24, which keeps the numbers small.
Mixed numbers
A mixed number like 2 ¼ is a whole number plus a fraction. To calculate with it, turn it into an improper fraction: whole × denominator + numerator, so 2 × 4 + 1 = 9, giving 9/4. In the calculator, type the whole number in the small box on the left.
Common fractions as decimals and percentages
| Fraction | Decimal | Percent |
|---|---|---|
| 1/2 | 0.5 | 50 % |
| 1/3 | 0.333… | 33.3 % |
| 2/3 | 0.666… | 66.7 % |
| 1/4 | 0.25 | 25 % |
| 3/4 | 0.75 | 75 % |
| 1/8 | 0.125 | 12.5 % |
| 3/8 | 0.375 | 37.5 % |
| 1/16 | 0.0625 | 6.25 % |
Eighths and sixteenths come up all the time with tape measures and recipes. For percentages, use the percentage calculator; for ratios, the rule of three calculator.
Frequently asked questions
How do you add fractions with different denominators?
Rewrite both with the least common denominator, then add the numerators: 1/4 + 1/6 = 3/12 + 2/12 = 5/12.
How do you simplify a fraction?
Divide the numerator and denominator by their greatest common factor. 18/24: the GCF is 6, so 3/4.
How do you divide fractions?
Keep, change, flip: keep the first fraction, change ÷ to ×, flip the second. 2/3 ÷ 4/5 = 2/3 × 5/4 = 5/6.
How do you turn 0.375 into a fraction?
0.375 = 375/1000, which simplifies by 125 to 3/8. The “Decimal → fraction” mode does it for you.