Fractions can only be added directly when they are measured in the same-sized parts. That is why the calculator first finds the least common denominator. Each fraction is rewritten without changing its value, so all three fractions can be counted in one shared unit.

After the denominators match, only the numerators are added. The result may then simplify if the numerator and denominator share a common factor. This page keeps that flow visible with a formula board and a strip model that shows the equal parts being combined.

Example: 1/2 + 1/3 + 1/4 becomes 6/12 + 4/12 + 3/12. Adding the numerators gives 13/12, which is 1 1/12.