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Proportion calculator

Solve a proportion a/b = c/x and find the missing value x.

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Enter a, b, c and the Proportion calculator works out x straight away. For instance, with a = 2, b = 5 and c = 8 it returns x = 20.

How to use it

  1. Enter your values: a, b, c.
  2. Read the result instantly: x.

Frequently asked questions

What does the Proportion calculator actually compute?

It takes a, b and c and derives x from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

3 values: a, b and c. Nothing else is required — no account, no file upload.

Can you show a worked example?

With a = 2, b = 5 and c = 8, the calculator returns x = 20. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

What happens if I enter larger values?

It moves a lot. Using a = 4, b = 10 and c = 16 instead, x goes from 20 to 40 — which is why it is worth testing a few scenarios rather than trusting a single figure.

What does it give for smaller values?

Scaled down to a = 1, b = 3 and c = 4, x comes out at 12. The relationship is worth checking at both ends before you rely on a single result.

When would I actually use this?

The everyday cases: a discount at the till, a rise or fall between two figures, the price before tax, and working backwards from a total.

What is the most common mistake?

Undoing a percentage by applying it again. Take 20% off 100 and you get 80; add 20% to 80 and you get 96, not 100 — the two operations use different bases.

What is the difference between the Proportion calculator and the Confidence interval for a proportion?

This one returns x; the Confidence interval for a proportion returns Sample proportion and Wald lower. That is the whole difference — open the one whose figure you need.

Where do the figures come from, and how current are they?

Percentage arithmetic is exact and universal. Where a result is a rate rather than an amount, the tool states the base it was taken on, because that is where the ambiguity lives.

Further reading

All guides →
How-toHow to Convert a Percentage to a Fraction: Over 100, Then ReduceA percentage is just a fraction out of 100. Put the number over 100, reduce to lowest terms, and handle decimal percentages like 12.5% by clearing the decimal first. With examples.How-toHow to Calculate a Percentage: 4 Methods with ExamplesA clear guide to percentages: find X% of a number, work out what percent one number is of another, and add or subtract a percentage — with worked examples.How-toPercentage Change: Increase and DecreaseSubtract old from new, divide by the old value, times 100. Here's the formula, why increases and decreases aren't symmetric, and where it's used.ExplainerStacked Discounts Do Not Add UpTwenty percent off and then a further ten percent off is not thirty percent off — it is twenty-eight, because the second discount applies to the already-reduced price. The general rule, the cases where order matters, and how sales tax changes the answer.How-toHow to Simplify a RatioDivide both sides by their greatest common divisor. Here's what a ratio is, how to reduce it, why units must match first, and how to compare ratios.ExplainerPercentage Points Are Not PercentMoving from 4% to 5% is a rise of one percentage point and a rise of twenty-five percent. Both statements are true, which is exactly why the ambiguity is so easy to exploit — and why basis points exist.