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Fraction Calculator: The Operations Most People Forgot After Middle School

By David Brown · January 2026 · 3 min read

Fraction arithmetic is one of those things most adults know they once knew. Here's the quick reference.

Addition and Subtraction: Common Denominator Required

To add or subtract fractions, they must have the same denominator.

Same denominator: just add/subtract the numerators.

3/8 + 1/8 = 4/8 = 1/2

Different denominators: find the least common denominator (LCM).

1/3 + 1/4: LCM of 3 and 4 is 12.

1/3 = 4/12; 1/4 = 3/12

4/12 + 3/12 = 7/12

Quick method when denominators share no common factors: multiply both denominators.

1/3 + 1/4: use 3×4=12 as common denominator. ✓

Multiply numerators together, multiply denominators together. No common denominator needed.

2/3 × 3/5 = (2×3)/(3×5) = 6/15 = 2/5

Simplify first when possible (cross-cancel) to keep numbers smaller:

2/3 × 3/5: the 3 in numerator and denominator cancel → 2/1 × 1/5 = 2/5. Same answer, less arithmetic.

Flip the second fraction, then multiply.

2/3 ÷ 4/5 = 2/3 × 5/4 = 10/12 = 5/6

Memory aid: "Keep, Change, Flip" — keep the first fraction, change division to multiplication, flip the second fraction.

Convert to improper fractions first.

2¾ = (2×4 + 3)/4 = 11/4

Then apply normal fraction arithmetic. Convert back at the end if needed.

11/4 + 3/2 = 11/4 + 6/4 = 17/4 = 4¼

[Calculate with fractions →](https://doesitaddup.com)

Try it yourself: All free calculators at doesitaddup.com →

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