Solve for any missing term in a proportion by cross multiplication, with the ratios and scale factor shown.
The method behind this calculator
A proportion states that two ratios are equal. Cross multiplying turns it into a single multiplication and division, which is why the same trick handles recipe scaling, map distances, currency conversion and similar triangles.
The scale factor line tells you how much the second ratio has been enlarged relative to the first, which is often the number you actually wanted.
Try it with these numbers
As an example, with a solve for of d, the calculator returns a missing value of 20. Underneath, Completed proportion comes out at 3 : 4 = 15 : 20 and Cross products at 60 = 60. Change any field and every figure updates as you type.
How to use it
- Choose which of the four terms is unknown.
- Enter the three you know.
- Read the missing value and the cross-product check.
How the number is calculated
a : b = c : d ā a Ć d = b Ć c
solve for the unknown by dividing through
Frequently asked questions
What is a proportion?
A statement that two ratios are equal. If 3 is to 4 as 15 is to 20, the two fractions 3/4 and 15/20 are the same number.
How does cross multiplication work?
Multiplying both sides by both denominators clears the fractions, leaving aĆd = bĆc.
What is a direct proportion?
One where doubling the first quantity doubles the second. Inverse proportion, where one rises as the other falls, needs a different setup.