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Proportion Solver

Solve a/b = c/d for any missing value with full cross multiplication steps.

About this tool

Proportion Solver

Solve a/b = c/d for any missing value with full cross multiplication steps.

Proportion Solver finds the missing term in a / b = c / d. Pick which of the four values is unknown, type the other three, and the tool cross multiplies to isolate it while printing every algebra step. It also reports both ratios as decimals, the simplified whole-number ratio and the scale factor between the two sides — handy for students checking homework and for anyone scaling a recipe, a map distance or a unit price.

How to use Proportion Solver

  1. 1

    Open Proportion Solver on AltFTool — it loads instantly in your browser.

  2. 2

    Enter the values you already know.

  3. 3

    Fine-tune the options to match your scenario.

  4. 4

    Read the result and use it in your planning or reporting.

Why use Proportion Solver

  • Solves any position

    Leave a, b, c or d unknown and the tool rearranges the equation for that term.

  • Shows the working

    Every cross-multiplication step is listed, so you can copy it into homework or a worksheet.

  • Catches bad inputs

    Divide-by-zero cases are explained in plain language instead of printing NaN or Infinity.

Common use cases

  • Scale a recipe: 2 cups of flour serves 5 people, so how much flour serves 8?
  • Convert a map measurement at 1:250 scale into the real distance on the ground.
  • Work out unit pricing — if 3 items cost Rs 149, what should 7 items cost at the same rate?

Frequently asked questions

What is cross multiplication?

For a / b = c / d, multiplying both sides by b and by d gives a x d = b x c. You then divide by whichever known term sits beside the unknown to isolate it.

Why does it say it cannot solve for my term?

Isolating each term requires dividing by one specific known value. If that value is 0 the division is undefined, so the tool names the term that must be non-zero.

Can I use decimals and negative numbers?

Yes, any finite decimal or negative value works. The simplified whole-number ratio only appears when both left-hand terms are integers.

Is a proportion the same as a ratio?

A ratio compares two quantities (a to b). A proportion is the statement that two ratios are equal, which is exactly what this tool solves.

Embed this tool on your site

Add the Proportion Solver widget to your blog or website — free, responsive, no signup. Just keep the “Widget by AltFTool” credit link visible.

<iframe src="https://www.altftool.com/embed/widget/proportion-solver"
  title="Proportion Solver — free AltFTool widget"
  width="100%" height="640" style="border:0;border-radius:12px;overflow:hidden"
  loading="lazy" referrerpolicy="no-referrer-when-downgrade" allow="clipboard-write"></iframe>
<p style="font-size:12px;margin:4px 0 0">Widget by <a href="https://www.altftool.com/tools/all/proportion-solver?utm_source=embed&amp;utm_medium=widget" rel="nofollow">AltFTool — free online tools</a></p>

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