Finding the Lowest Common Denominator (LCD) for Adding Fractions

The lowest common denominator (LCD) is simply the LCM of the denominators. Once you have it, adding or comparing fractions becomes straightforward.

This guide shows how to find the LCD and rewrite fractions so they share it.

What the LCD is

The LCD is the smallest number that every denominator divides into — exactly the definition of the LCM. So an LCM tool is also an LCD tool: use the LCM Calculator on your denominators.

Worked example: 1/4 + 1/6

LCM(4, 6) = 12, so the LCD is 12. Rewrite: 1/4 = 3/12 and 1/6 = 2/12. Now add: 3/12 + 2/12 = 5/12.

Step-by-step method

1. Find the LCM of all denominators — that's your LCD.
2. Scale each fraction so its denominator becomes the LCD.
3. Add or compare the numerators.
4. Simplify the result with the Fraction Simplifier.

Why not just multiply the denominators?

Multiplying works but often gives larger numbers than necessary. Using the LCM keeps the arithmetic small and the final answer easier to simplify.