Ratio Calculator
Simplify a ratio, solve a proportion, or scale one up and down — instantly and completely free.
Simplify a ratio
Reduce a ratio to its lowest whole-number terms.
Solve a proportion
Fill in any three values and we'll find the missing one in A : B = C : D. Leave exactly one box empty.
Scale a ratio
Multiply or divide a ratio by a factor to scale it up or down.
How the ratio calculator works
Three tools in one — every result is worked out instantly on your device.
Simplify
Enter two numbers and we divide both by their greatest common divisor to reach lowest terms, like 16:9.
Solve A:B = C:?
Give any three of the four values and we cross-multiply to find the missing one for an equivalent ratio.
Scale
Multiply or divide both sides by the same factor to size a ratio up or down while keeping its proportions.
A quick refresher on ratios
A ratio compares two quantities — written A : B — and stays the same when you multiply or divide both sides by the same number. Simplifying means dividing both terms by their greatest common divisor so the ratio is expressed with the smallest whole numbers, for example 8 : 12 becomes 2 : 3. Decimal terms are supported too and are scaled to whole numbers where possible.
A proportion says two ratios are equal, A : B = C : D. Because the cross products are equal (A × D = B × C), knowing any three values lets you solve for the fourth — handy for recipes, maps, mixing, and unit conversions. Scaling keeps a ratio's shape while changing its magnitude: 3 : 4 multiplied by 2 gives 6 : 8, the same proportion at twice the size.
Private by design. Your numbers never leave your device. Every calculation happens right here in your browser — nothing is uploaded or stored.