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ToolWrenSolve a proportion of the form A:B = C:D by leaving the unknown value blank.
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Ratio is a free, private online tool that lets you solve a proportion A:B = C:D for the missing value. It runs entirely in your browser, so nothing you enter is uploaded to a server.
Privacy: this tool runs entirely in your browser. Your input is never sent to, received by, or stored on any server — there are no uploads and no tracking of what you enter.
A proportion states two ratios are equal, such as 2:3 = 10:15. If you know three of the four values, you can find the fourth.
Enter the three known values and leave C or D blank; the tool cross-multiplies to find the missing one.
Scaling recipes, mixing ratios, map scales, and resizing while keeping proportions.
A ratio compares two quantities, like 2:3, and a proportion states that two ratios are equal, like 2:3 = 10:15. This calculator solves a proportion for a missing value: if you know three of the four numbers in A:B = C:D, it finds the fourth. It is the maths behind scaling anything up or down while keeping it in correct proportion.
The tool solves proportions by cross-multiplication. If A:B = C:D, then A x D = B x C. Rearranging lets you find whichever value is unknown - for example, if D is missing, D = (B x C) / A. So for 2:3 = 10:?, the answer is (3 x 10) / 2 = 15. Enter the three known values and leave the unknown blank.
Proportional reasoning is everywhere: scaling recipes (a recipe for 4 served to 6), mixing ratios (paint, fuel-oil mixes, cleaning solutions), map scales and model-making, resizing images, currency and unit conversions, and adjusting quantities in chemistry or manufacturing. Anytime more of one means proportionally more of another, a proportion applies.
This calculator handles direct proportion, where the two quantities rise and fall together. Not every relationship is proportional - some are inverse (more workers, less time) or non-linear - so apply it only where doubling one really should double the other. For genuinely proportional problems it is fast and reliable.
Scale a recipe to a different number of servings, work out mixing quantities, resize while keeping proportions, convert between units, and solve school proportion problems. Leave one field blank and the tool fills it in, all in your browser.