Adding Fractions Calculator
Add two to four fractions or mixed numbers. Shows the least common denominator, each converted fraction, the simplified sum, mixed number and decimal.
Adding fractions only works once they share a denominator. This calculator finds the least common denominator (LCD), rewrites every fraction over it, adds the numerators and reduces the answer to lowest terms. It accepts proper and improper fractions, mixed numbers such as 2 1/3, negative fractions and decimals, and adds up to four at once.
How to use this tool
- Type the first two fractions, for example 3/4 and 5/6. Write mixed numbers with a space: 1 1/2.
- Add a third and fourth fraction if you need them, or leave those boxes blank.
- Read the simplified sum, the mixed number and the decimal.
- Use the table to follow how each fraction was rewritten over the LCD.
Formula
a/b + c/d = (a × (L ÷ b) + c × (L ÷ d)) / L, where L = lcm(b, d)
L, the least common denominator, is the least common multiple of the denominators. Each numerator is multiplied by the same factor as its denominator, the numerators are added, and the result is divided top and bottom by their greatest common divisor to simplify.
Example
Add 3/4 + 5/6. The least common multiple of 4 and 6 is 12. Multiply 3/4 by 3/3 to get 9/12, and 5/6 by 2/2 to get 10/12. Then 9/12 + 10/12 = 19/12, which is already in lowest terms. As a mixed number that is 1 7/12, and as a decimal about 1.58333333.
Frequently asked questions
Do I have to use the least common denominator?
No. Any common denominator works, such as multiplying the denominators together (4 × 6 = 24). The LCD just keeps the numbers small and means less simplifying at the end.
How do I add mixed numbers?
Type them with a space, like 1 1/2. The calculator turns each into an improper fraction first (1 1/2 = 3/2), adds them, and converts the answer back to a mixed number.
Can I add a negative fraction?
Yes. Type -2/3 or -1 1/4. Adding a negative fraction is the same as subtracting it.
Why is my answer an improper fraction?
When the numerator is larger than the denominator the sum is more than 1. The mixed-number result shows the same value as a whole number plus a proper fraction.
Last updated: 2026-10-30