How to use the Ratio Calculator
- Enter both sides of your ratio.
- Read the simplified form and the decimal equivalent.
- Enter a target value for side A to find the matching side B.
- Round the scaled result to whatever precision your task allows.
How the calculation works
Simplifying divides both terms by their greatest common divisor, so 1920 : 1080 reduces to 16 : 9. The decimal form — 1.778 for 16:9 — is what you compare when checking whether two ratios match, since visually different pairs can be equivalent.
Scaling relies on the fact that equivalent ratios have equal cross-products: if a : b equals x : y, then a·y = b·x. Solving for the unknown gives y = (x·b) ÷ a. This one relationship covers map scales, model kits, aspect-ratio letterboxing, currency conversion at a fixed peg, and the classic recipe question of how much of ingredient two you need for a given amount of ingredient one.
Mixing ratios add a wrinkle: a 1:4 concentrate-to-water ratio means five parts total, so a litre of finished mixture contains 200 ml of concentrate, not 250 ml. Confusing part-to-part with part-to-whole is the most common ratio mistake in dilution work.
Simplify by gcd ; if a : b = x : y then y = (x × b) / aSource: Standard proportional reasoning; Euclidean algorithm for greatest common divisor.
Worked example
Fitting a 16:9 video into a frame that is 1440 px wide.
- Ratio 16 : 9 is already simplified; decimal 1.778.
- Scale side A to 1440.
- Side B = 1440 × 9 ÷ 16.
810 px tall — an exact 16:9 fit with no letterboxing.
Frequently asked questions
Is 1:4 the same as 20%?+
Only if you mean part-to-whole. As a part-to-part mixing ratio, 1:4 is 20% of the total, since there are five parts.
How do I compare two ratios quickly?+
Convert both to decimals. Equal decimals mean equivalent ratios regardless of how the numbers look.
Can ratios have decimals in them?+
Yes, though multiplying both sides to clear the decimals usually makes the ratio easier to read.
What is a three-term ratio?+
Same principle extended — divide all three terms by their common divisor, and scale all three by the same factor.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.