Two tools in one: simplify any ratio to its lowest terms using the greatest common divisor, or solve a proportion to find the missing value.
Results update as you type. Whole numbers give an exact GCD reduction.
Find the greatest common divisor (GCD) of both numbers, then divide each part of the ratio by it. For example, 12:8 has a GCD of 4, so 12 ÷ 4 = 3 and 8 ÷ 4 = 2, giving the simplified ratio 3:2.
For a proportion a:b = c:x, cross-multiply and solve: x = b × c ÷ a. For example, 2:3 = 4:x gives x = 3 × 4 ÷ 2 = 6. The first term cannot be zero.
The greatest common divisor is the largest whole number that divides two numbers exactly with no remainder. It is what you divide a ratio by to reduce it to its simplest form.