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Proportion Calculator

Solve A : B = C : D for any missing value by cross-multiplying.

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Leave exactly one box empty and it will be solved for. All four boxes filled? Clear the one you want.

Quick Tool Overview

The Proportion Calculator solves A : B = C : D for whichever value you leave blank, by cross-multiplication, with the rearranged formula printed underneath. It is the most reusable piece of practical arithmetic there is, and there is an inverse mode for the cases where one value rises as the other falls.

What it does
Solves a proportion for its missing fourth value
You give it
Any three of the four values
You get back
The missing value and the rearranged formula
Best for
Scaling recipes, map distances, dosing, unit pricing and resizing
Calculators Free, unlimited use No sign-up Runs in your browser

About the Proportion Calculator

Enter any three values of a proportion in the form A/B = C/D and the missing fourth is solved by cross-multiplication, with the rearranged formula printed underneath so you can see exactly which numbers were multiplied and which was divided.

This is the most reusable piece of arithmetic in practical maths. Scaling a recipe, converting a map distance, working out a dose by body weight, resizing an image without distorting it, currency conversion at a fixed rate and unit pricing in a supermarket are all the same equation with different labels. There is also an inverse proportion mode for the cases where one value rises as the other falls, such as workers against days to finish a job.

Key features of the Proportion Calculator

Solve for any position

Leave whichever value you do not know blank — the calculator rearranges for it rather than only solving for D.

The rearranged formula shown

You can see exactly which numbers were multiplied and which was divided, so the answer is checkable.

Inverse proportion mode

For the cases where one value rises as the other falls, such as workers against days to finish a job.

One equation, many uses

Recipe scaling, map distance, dose by body weight, image resizing and unit pricing are all this same calculation.

Decimal-safe

Fractional inputs are handled exactly rather than rounded before the cross-multiplication.

How to use the Proportion Calculator

  1. 1

    Leave the unknown blank

    Fill in three of the four boxes and leave the value you want empty. The calculator solves for whichever one is blank.

  2. 2

    Read the answer

    The missing value appears with the cross-multiplication that produced it.

  3. 3

    Switch to inverse if needed

    Use inverse proportion when one quantity goes down as the other goes up, such as speed against journey time.

Frequently asked questions

In A/B = C/D the diagonal products are equal, so A x D = B x C. Rearranging that gives you whichever value is missing, for example A = (B x C) / D.

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