How the proportion solver works
A proportion states that two ratios are equal: A : B = C : D. Because the cross-products are equal (A × D = B × C), you can rearrange to find any missing term:
D = B × C ÷ A | C = A × D ÷ B | B = A × D ÷ C | A = B × C ÷ D
Pick which letter to solve for, fill in the other three, and the calculator cross-multiplies to find it. In 2 : 3 = 10 : D, the answer is 3 × 10 ÷ 2 = 15.
Simplifying ratios
To reduce a ratio to its simplest form, divide both sides by their greatest common divisor. The calculator simplifies the A : B ratio automatically — so 18 : 24 becomes 3 : 4, and 50 : 100 becomes 1 : 2.
Ratios, fractions, decimals, and percentages
Every ratio is also a fraction. The ratio 1 : 4 is the fraction 1/4, which is 0.25 as a decimal and 25% as a percentage. The results panel shows all of these so you can use whichever form fits your task — scaling a recipe, mixing a solution, or setting an aspect ratio.
Frequently asked questions
How do you solve a proportion?
A proportion sets two ratios equal: A:B = C:D. To find a missing term, cross-multiply. If you know A, B, and C, then D = B × C ÷ A. For example, in 2:3 = 10:?, the missing value is 3 × 10 ÷ 2 = 15.
How do you simplify a ratio?
Divide both numbers by their greatest common divisor. The ratio 18:24 has a greatest common divisor of 6, so it simplifies to 3:4. A simplified ratio expresses the same relationship in the smallest whole numbers. This calculator simplifies the left-hand ratio for you.
How do you scale a recipe or mixture with a ratio?
Treat it as a proportion. If a recipe uses a 2:3 ratio of sugar to flour and you want to use 10 cups of sugar, set up 2:3 = 10:?, and solve for 15 cups of flour. Enter your known three values and the calculator finds the fourth.
What is the difference between a ratio and a fraction?
They are closely related. A ratio A:B compares two quantities, and it can be written as the fraction A/B or as a decimal. The ratio 1:4 is the same as the fraction 1/4, the decimal 0.25, and 25% — this calculator shows all of those forms.
Tip: The field you're solving for is filled in for you — just enter the other three values.