Ratio Calculator

Reduces a ratio of two whole numbers, or finds the missing term in a proportion.

1. Choose simplify or solve.
2. Enter the two parts of the ratio. Solving also needs the third term.
3. Read the reduced ratio or the missing term.

Greatest common divisor and a proportion

A simplified ratio divides both whole numbers by their greatest common divisor. The missing term in A : B = C : D is D = B × C / A.

Source. A ratio of two whole numbers is reduced by dividing both by their greatest common divisor. The missing term in a : b = c : d is d = b × c / a.

Updated November 1, 2026

8 : 12 simplifies to 2 : 3. In the proportion 2 : 3 = 10 : D, the missing term is 15.

• Simplify needs whole numbers other than zero.
• The proportion works with decimals. The first term cannot be zero.

Can I simplify 1.5 : 3?

Not on the simplify setting, which keeps the greatest-common-divisor step exact for whole numbers. Use the proportion setting when the terms are decimals.