Ratio Calculator
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Solve a proportion in the form A:B = C:D by leaving one box blank, and see the missing term plus each ratio reduced to its simplest whole-number form.
Solving A:B = C:D
A proportion is a statement that two ratios are equal, written A:B = C:D. If you know three of the four numbers you can always find the fourth, because equal ratios obey the cross-multiplication rule: A × D = B × C. Rearranging that equation isolates whichever term is missing.
This calculator does the rearranging for you. Fill in any three boxes and leave one empty, and it returns the exact value that makes the two ratios equivalent. For example, 2:3 = 10:? gives 15, because 2 × 15 equals 3 × 10. If you fill in all four numbers instead, the tool checks whether the proportion is actually true and tells you so.
Simplifying and scaling ratios
Beyond solving, the tool reduces ratios to their simplest form using the greatest common divisor, so 8:12 is shown as 2:3 and 1920:1080 as 16:9. A simplified ratio is easier to read, compare and remember, and it reveals equivalent ratios at a glance.
Scaling is the same idea in reverse: multiply both sides of a ratio by the same number and the proportion is unchanged. That is exactly how you use the solver to scale a recipe, a mixture or a drawing. Set the known ratio on the left, put your new quantity for one ingredient on the right, and the missing quantity comes out already scaled to match.
- Cross-multiplication: A × D = B × C
- Missing term example: 3:4 = 9:? gives 12
- Simplify example: 15:25 reduces to 3:5
Everyday uses
Ratios appear far beyond the maths classroom. Cooks scale recipes up or down, builders mix concrete and mortar to fixed proportions, photographers and designers match aspect ratios, and finance uses ratios to compare figures like debt to equity. Whenever you need to keep two quantities in the same relationship while changing their size, a proportion is the tool.
Because the result updates as you type, it is quick to explore options. You can test how much of a second ingredient a larger batch needs, confirm that two ratios really are equivalent, or reduce an awkward pair of numbers to something memorable. The simplified outputs make it easy to communicate the result to someone else without the original figures.
Frequently asked questions
How does it find the missing number?
It uses cross-multiplication. For A:B = C:D the tool applies A × D = B × C and solves for whichever term you left blank.
What if I enter all four numbers?
It stops solving and instead verifies the proportion, showing True when A × D equals B × C and False when the two ratios are not equal.
Why can't it solve some inputs?
If solving would require dividing by zero — for example a needed denominator is 0 — the proportion has no valid answer and the tool reports that it cannot solve it.
Does it handle decimals?
Yes. You can enter decimal values and the missing term is calculated precisely. Simplified ratios scale decimals to whole numbers before reducing.
Can I scale a recipe with it?
Yes. Put the original ratio on the left, enter your new amount for one ingredient on the right, and the matching amount is calculated to keep the proportion.
Is anything stored or sent anywhere?
No. The calculations run entirely in your browser, so your numbers never leave your device.
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