Everyday math & dates
Ratio Calculator
The first box scales a ratio up or down, such as a recipe or a screen size: fill in three numbers and leave one empty. The second reduces a ratio like 1920:1080 to its simplest form.
Solve A : B = C : D
Your result will appear here.
Simplify a ratio
Your result will appear here.
How it's calculated
In a proportion the cross products are equal: A × D = B × C. Leave any one box empty and the calculator rearranges that to find it. For 3 : 4 = 15 : D, D is 4 × 15 ÷ 3 = 20.
To simplify, both numbers are divided by their greatest common divisor. 1920 and 1080 share 120, so the ratio is 16 : 9. Decimals are scaled up to whole numbers first.
A × D = B × C
How to do it in Excel
=A2/GCD(A2,B2)&":"&B2/GCD(A2,B2)- Simplified ratio as text, from whole numbers in A2 and B2.
=B2*C2/A2- The missing D in A : B = C : D.