How to use this ratio calculator
- Enter three values across the boxes A, B, C and D.
- Leave one box blank, that is the term you want to find.
- Read the missing value and the simplified ratio, they update as you type.
Nothing is submitted or stored, so the numbers never leave your device.
How to solve a ratio
Two equal ratios form a proportion, written A : B = C : D. Because the two sides are equal, the cross-products match: A × D = B × C. That single rule lets you find any missing term. If D is unknown, rearrange to D = B × C ÷ A. The same idea works for any position: C = A × D ÷ B, B = A × D ÷ C, or A = B × C ÷ D.
To simplify a ratio, divide both sides by their greatest common divisor (GCD), the largest number that divides both terms exactly. Reducing a ratio does not change its meaning, it just states it in the smallest whole numbers, which makes proportions far easier to read and compare.
A worked example
Suppose a recipe uses flour and sugar in the ratio 2 : 5, and you have 8 cups of flour. How much sugar keeps the same proportion? Set it up as 2 : 5 = 8 : D and cross-multiply: D = 5 × 8 ÷ 2 = 20 cups. To simplify a ratio such as 18 : 24, the GCD of 18 and 24 is 6, so dividing both by 6 gives 3 : 4.
Frequently asked questions
How do I solve a ratio with a missing value?
Write the two ratios as a proportion, A:B = C:D, then cross-multiply. If D is the unknown, D = B × C ÷ A. For example, 2:5 = 8:? gives ? = 5 × 8 ÷ 2 = 20.
How do I simplify a ratio?
Divide both numbers by their greatest common divisor. For 18:24 the GCD is 6, so the simplified ratio is 3:4. The calculator does this automatically for whole numbers.
What is a proportion?
A proportion is a statement that two ratios are equal, written A:B = C:D. If three of the four terms are known you can always solve for the fourth.
Can I use decimals?
Yes. Enter decimal values in any box and the missing value is solved the same way, shown rounded to four significant figures.
Is my data uploaded?
No, everything is calculated on your device and nothing is sent anywhere.