Math & Scientific
Proportion Calculator
Proportion — it solves the proportion a/b = c/d for the unknown by cross-multiplication. This page runs that same calculation on your numbers, in your browser: no signup, no uploads, nothing stored.
Proportion Calculator turns your A, B and C into the unknown value using cross-multiplication of a∶b = c∶d. The maths runs locally, so nothing you enter leaves the page.
About this calculator
Get proportion right and the rest of the task gets easier. It solves the proportion a/b = c/d for the unknown by cross-multiplication. It matters when you need the exact relationship checked quickly rather than worked through by hand. Solve 3/4 = x/20: x = 3 × 20 ÷ 4 = 15 . The tool above does the same computation in your browser — free, private, nothing stored.
How to use
- Enter the three known values A, B, and C.
- Press Calculate to run the proportion calculator.
- Read the result: the missing value X that completes the proportion.
- Press Reset to clear the proportion calculator and start again.
Why use Proportion Calculator
Fast
Calculations run in your browser. No round trip to a server.
Private
Your inputs never leave your device. Nothing is uploaded.
Free
No signup, no paywall, no ads-in-results, no watermarks.
Accurate
Based on standard, well-established formulas.
Mobile-ready
Optimised for phones, tablets, and desktops alike.
Transparent
Honest disclaimers wherever browser limits or regional rules apply.
Common uses
Use it when revising for exams
Use it when preparing worksheets
Use it for science-fair and project numbers
Proportion in research and report calculations
Use it for verifying answers from a textbook
Proportion in competitive-exam practice
Technical notes
Proportion Calculator solves the proportion a/b = c/d for the unknown by cross-multiplication.
Inputs are taken exactly as entered, so check for typos before trusting an answer.
Because it runs locally, results appear instantly with no server round-trip.
FAQ
What does this calculator solve?
It solves the proportion A : B = C : X for the unknown X.
How is X found?
From A/B = C/X, the calculator computes X = B × C ÷ A.
What do I enter in the Proportion Calculator?
The three known values A, B, and C.
Does the Proportion Calculator send my data to a server?
No — the Proportion Calculator runs entirely in your browser, so the numbers you enter never leave your device.
Worked example
Solve 3/4 = x/20: x = 3 × 20 ÷ 4 = 15.