Ratio Calculator

Simplify a ratio to its lowest terms, or solve a proportion to find a missing value. Works with whole numbers and decimals.

Simplify a ratio
Simplest form
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Solve a proportion: A : B = C : x
x =
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What is a ratio?

A ratio compares two amounts, written as A:B and read "A to B". If a class has 12 girls and 18 boys, the ratio of girls to boys is 12:18, which simplifies to 2:3. Ratios stay the same when you multiply or divide both sides by the same number.

How to simplify a ratio

  1. Find the greatest common factor (GCF) of both numbers.
  2. Divide both sides by it.

1920:1080 has a GCF of 120, so it simplifies to 16:9, the standard widescreen shape. For decimals like 1.5:2.5, first multiply by 10 to get 15:25, then simplify to 3:5.

How to solve a proportion

A proportion says two ratios are equal: A:B = C:x. Cross-multiplying gives A × x = B × C, so:

x = B × C ÷ A

For 3:4 = 12:x, x = 4 × 12 ÷ 3 = 16.

Everyday uses

  • Recipes: a 2:1 ratio of rice to water means 2 cups of water for every cup of rice.
  • Maps and models: on a 1:24 scale model, 1 inch stands for 24 inches.
  • Mixing: 50:1 two-stroke fuel means 50 parts gas to 1 part oil.
  • Screens and photos: 16:9, 4:3 and 3:2 describe width to height.

Last reviewed October 2026.

Frequently asked questions

How do I simplify a ratio?

Divide both sides by their greatest common factor. 12:18 simplifies to 2:3 because both numbers divide by 6. If the ratio has decimals, multiply both sides by 10 or 100 first to make them whole numbers.

How do I solve a proportion?

Cross-multiply. For 3:4 = 12:x, multiply 4 × 12 = 48 and divide by 3 to get x = 16. The calculator does this for you.

What is the difference between a ratio and a fraction?

A ratio compares two quantities, like 2 cups of flour to 1 cup of sugar (2:1). A fraction describes part of a whole. The ratio 2:1 means flour makes up 2/3 of the mixture.

How do I scale a recipe with ratios?

Keep the ratio the same and change the amounts. If 2 cups of flour make 3 batches of cookies and you want 5 batches, solve 3:2 = 5:x to get x = 3 1/3 cups.

What does a 16:9 ratio mean?

For every 16 units of width there are 9 units of height. Screens with resolutions like 1920 × 1080 and 3840 × 2160 are all 16:9.