ToolsPopper
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Proportion Calculator

Rule of three: if A is to B, then C is to X.

What is the Proportion Calculator?

ToolsPopper's Proportion Calculator solves the classic rule-of-three problem: given that A is to B as C is to X, it finds the unknown X. Enter three known values and the tool cross-multiplies to return the missing fourth term — perfect for ratio homework, recipe scaling, and map distance problems.

All cross-multiplication runs locally in your browser. Your homework ratios and business calculations stay completely private.

When to use a proportion calculator

  • Math homework — solve ratio and proportion word problems quickly
  • Recipe scaling — adjust ingredient amounts when serving size changes
  • Map and model scales — convert distances using a known scale factor
  • Finance and statistics — find an unknown value in a proportional relationship

How the Proportion Calculator works

Enter A, B, and C in the input fields representing the proportion A : B = C : X. The tool computes X = (B × C) ÷ A using cross-multiplication. If A is zero, the result is undefined and the tool displays a dash instead of a number.

How to solve a proportion

  1. Identify your three known values from the proportion A/B = C/X.
  2. Enter A, B, and C in the corresponding fields.
  3. Click Calculate to find the unknown X.
  4. Verify the result by checking that A/B equals C/X.

ToolsPopper vs Calculator.net

  • Rule-of-three focus — direct A : B = C : X layout without extra modes
  • Unlimited free use — no throttled sessions or account walls
  • 100% private — ratio values processed on your device only
  • No signup — solve proportions immediately
  • Mobile-friendly — four fields and one button for quick homework checks

Frequently Asked Questions

Common questions about Proportion Calculator

What is the rule of three?

The rule of three states that if A is to B as C is to X, then X = (B × C) ÷ A. It is a standard method for finding an unknown in a proportional relationship.

What happens if A is zero?

Division by zero is undefined. If A equals zero, the calculator cannot compute X and will show a dash instead of a result.

Is this proportion calculator free?

Yes. Solve unlimited proportions with no account, subscription, or daily limit.

Can I use this for homework?

Yes. It is ideal for checking ratio and proportion problems in middle school through algebra. Show your cross-multiplication steps if your teacher requires written work.

Are my values sent to a server?

No. All proportion math runs entirely in your browser. ToolsPopper never receives or stores the numbers you enter.

How does this compare to Calculator.net's ratio calculator?

Calculator.net offers ratio tools on a server-rendered page. ToolsPopper provides unlimited free use, no signup, a focused rule-of-three layout, and fully private browser processing.

Does it work on mobile phones?

Yes. Open the page on any smartphone, enter your three known values, tap Calculate, and read X instantly.

How accurate are the results?

Results display to four decimal places using JavaScript floating-point math, accurate for typical homework and everyday scaling problems.