⚖️ Proportion Calculator
A proportion states that two ratios are equal: a/b = c/d. If you know any three of the four values — scaling a recipe, converting map distances, working out exchange rates — cross-multiplication finds the fourth.
Enter the three values you know, tell the calculator which one is missing, and it solves the proportion instantly, showing the completed ratio for checking.
How to use this calculator
Enter three of the four values in the proportion a/b = c/d, and tell the calculator which one is missing. It solves for the unknown and shows the completed proportion.
- Type the three values you know into Value A, Value B, Value C and Value D.
- Use the Unknown value dropdown to select which of the four is missing.
- Press Calculate to see the missing value highlighted at the top.
- Check the completed proportion shown below the answer to confirm the ratios balance.
Frequently asked questions
What does this proportion calculator solve?
It solves the proportion a/b = c/d: you supply any three of the four values, mark the unknown in the dropdown, and it returns the missing value plus the full completed ratio for checking.
What method does it use?
It uses cross-multiplication - a times d equals b times c - rearranged to isolate the unknown. For example, with D unknown, the answer is B times C divided by A.
Can you show a worked example?
Enter A = 1, B = 2, C = 3, and mark D as unknown. The calculator computes 2 * 3 / 1 = 6, so the completed proportion is 1 : 2 = 3 : 6.
What should I watch out for?
Any value the formula must divide by cannot be zero, or you will get an error. Keep the two ratios in the same order - if A pairs with C then B must pair with D - which is how recipes and scale conversions stay consistent.