Ratio Calculator
Solve proportions of the form A : B = C : D, simplify ratios and scale recipes, maps and images.
Solving a proportion
A proportion says two ratios are equal: A : B = C : D. If you know three of the values, cross-multiply to find the fourth.
D = B × C ÷ A
An image with a 4 : 3 aspect ratio that is 1,920 pixels wide must be 3 × 1,920 ÷ 4 = 1,440 pixels tall to avoid stretching.
Simplifying ratios
Divide both sides by their greatest common factor. 1920 : 1080 has a GCF of 120, so it simplifies to 16 : 9, the standard widescreen ratio.
Everyday ratio problems
- Recipes: a recipe for 4 uses 300 g of flour. For 6 people: 4 : 300 = 6 : D, so D = 450 g.
- Maps and models: on a 1 : 25,000 map, 4 cm represents 100,000 cm, or 1 km.
- Mixing: a 1 : 4 concentrate-to-water ratio needs 200 ml of concentrate for 800 ml of water.
- Exchange rates and unit prices are ratios too.
Ratios and fractions
A ratio of 3 : 2 means the first part is 3/5 of the whole and the second is 2/5. To split $500 in the ratio 3 : 2, give $300 and $200.
Frequently asked questions
How do I solve a ratio with a missing number?
Write it as A : B = C : D and cross-multiply. The missing value D equals B × C ÷ A.
How do I simplify a ratio?
Divide both numbers by their greatest common factor. For 18 : 24, the GCF is 6, so the simplified ratio is 3 : 4.
How do I split an amount by a ratio?
Add the parts of the ratio, divide the amount by that total, then multiply by each part.
Last reviewed Oct 20, 2026. Results are estimates; check important decisions with a qualified professional.