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Proportion Solver
Solve a proportion for the missing value and see the cross products and simplified ratio.
Solve a proportion
Result: —
Cross products: —
Ratio a to b: —
How to use the Proportion Solver
- Fill in three of the four boxes with the values you know.
- Leave exactly one box empty for the value you want to find.
- Read the solved value, the cross products and the simplified ratio.
Features
- Solves any one missing value
- Shows the cross products check
- Reduces the ratio to simplest form
- Runs locally, no uploads
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Frequently Asked Questions
Common questions about the free online proportion solver.
Enter the three values you know, leave the unknown box empty and the tool solves it by cross
multiplication.
In a proportion a over b equals c over d, cross multiplication means a times d equals b times
c.
Yes. Leave whichever box is unknown empty and the other three filled, and the tool solves for
it.
Multiply across both diagonals. If the two cross products are equal, the proportion holds.
The tool asks for exactly one empty box, because one proportion cannot solve two unknowns.
Yes. Negative numbers and decimals work in every box, and results are rounded to six
decimals.
It is the ratio of a to b divided by their greatest common divisor, so 6 to 8 becomes 3 to 4.
No. All solving happens locally in your browser and nothing is ever transmitted anywhere.
Yes. The solver is fully responsive and works on phone, tablet and desktop.
Yes. It is completely free and unlimited, with no account or sign-up required.
Last updated: October 10, 2026