Proportion Calculator
Solve a : b = c : d for whichever term is missing.
Work out Proportion. Solve a : b = c : d for whichever term is missing. Exact where exactness is possible.
Value of d
12
2 : 3 = 8 : 12
A proportion holds exactly when its cross products match, which is what cross-multiplication is testing. Both come to 24 here. The scale factor of 4 is the multiplier taking the first pair to the second — the same idea as a map scale or a recipe scaled up.
How the Proportion Calculator works
A proportion holds exactly when its cross products match, which is what cross-multiplication is testing. This solves for any of the four terms and shows both cross products so the balance is visible rather than asserted.
Also known as: solve for x proportion · cross multiplication calculator · ratio and proportion calculator · direct proportion calculator
Frequently asked questions
How do I solve a proportion?
Cross-multiply and divide. In 2 : 3 = 8 : x, multiply 3 by 8 and divide by 2 to get 12.
Why does cross-multiplication work?
Because a proportion is an equation between two fractions. Multiplying both sides by both denominators clears them, leaving the cross products equal.
What is direct proportion?
A relationship where doubling one quantity doubles the other, so their ratio stays fixed. Recipe scaling and map scales both work this way.
How is this different from inverse proportion?
In direct proportion the ratio is constant; in inverse proportion the product is. Twice the workers halving the time is inverse, not direct.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/proportion-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Proportion Calculator"></iframe>The only condition is that the credit line stays visible. It sits inside the frame, so you do not have to do anything to keep it.
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