Ratio Calculator

Solve and simplify ratios

The Ratio Calculator does two jobs: it solves a proportion A : B = C : D for whichever value you leave blank, and it simplifies a ratio to its lowest terms. Enter the numbers you know and get the missing one instantly — ideal for scaling recipes, mixing solutions, resizing images, and any "if this, then that" math.

What is a proportion?

A proportion states that two ratios are equal. If A is to B as C is to D, then A/B = C/D. Knowing any three of the four values lets you find the fourth using cross-multiplication. This is the everyday math behind scaling things up or down while keeping them in the same relationship.

How to use it

1. Enter three of the four values in A : B = C : D.
2. Leave the unknown field blank.
3. Read the solved value (and the simplified ratio when all four are filled).

Real-world examples

Task	Ratio setup
Scale a recipe for more people	servings : ingredient = new servings : ?
Mix a 1:8 solution	1 : 8 = concentrate : water
Resize keeping aspect ratio	width : height = new width : ?
Convert map scale	map cm : real cm = measured : ?

Simplifying ratios

A ratio like 4 : 6 can be reduced to 2 : 3 by dividing both sides by their greatest common divisor. Simplified ratios are easier to understand and compare. When you fill in all four values, the tool shows the simplified form of A : B so you can see the relationship in its clearest terms.

Tips

• Keep units consistent on both sides of the proportion.
• Cross-multiply to check: A × D should equal B × C.
• For scaling, put the known pair on the left and the unknown on the right.

Private and free

All calculations run in your browser — nothing is uploaded. The tool is completely free with no sign-up.