Ratio calculator
Reduce a ratio to its simplest form, or fill in the missing number when two ratios are equal. Works with two or three parts and with decimals.
Leave the third box empty for a two-part ratio.
Leave exactly one box empty and it will be solved for you.
How to use it
Choose a mode
Pick "Simplify" to reduce a ratio, or "Solve a proportion" when two ratios are equal and one value is missing.
Enter your numbers
Type the parts of the ratio. For a proportion, leave the unknown box blank and fill in the other three.
Read the answer
Simplify shows the ratio in lowest terms and as 1 : n. Proportion returns the missing value with the working shown.
Why it is handy
Recipes and scaling
Doubling a mix or resizing a recipe is a proportion. Enter the known amounts and read off the quantity you need.
Cleaner ratios
A ratio like 30:45:75 is hard to read. Reduced to 2:3:5 it is instantly meaningful, and this does the reducing for you.
Private and instant
All the arithmetic happens in your browser as you type. Nothing is uploaded, and there is nothing to install.
Understanding ratios and proportions
A ratio compares quantities of the same kind — parts of a mixture, ingredients in a recipe, or the split of a bill. Writing it as 2:3 says "two of the first for every three of the second". The order matters, and the total number of parts here is five.
Simplifying a ratio
To reduce a ratio you divide every part by their greatest common divisor. For 8:12 the largest number that divides both is 4, giving 2:3. The relationship is identical; the numbers are just smaller and easier to compare.
What a proportion is
A proportion states that two ratios are equal, like 2:3 = 10:15. Because the cross-products match (2 × 15 = 3 × 10), the statement holds. If one value is unknown, cross-multiplication solves it: in 2:3 = 10:? the missing number is 3 × 10 ÷ 2 = 15.
The 1 : n form
Sometimes it helps to express a ratio against a single unit, so 8:12 becomes 1:1.5. This makes it easy to see how much of the second quantity accompanies one of the first — useful for scale factors, odds and mixing rates.
Common questions
How do I simplify a ratio?
Divide each part by the greatest common divisor of all the parts. For 8:12 that divisor is 4, so the simplified ratio is 2:3. The Simplify tab does this automatically.
How do I solve a proportion for a missing value?
Use cross-multiplication. In A:B = C:D, any missing term equals the product of its diagonal partners divided by the remaining known value. Leave one box blank and the tool applies this for you.
Can I simplify a three-part ratio?
Yes. Fill in all three boxes and it reduces the whole ratio using the greatest common divisor of the three numbers, for example 30:45:75 becomes 2:3:5.
Does it work with decimals?
Yes. Decimal inputs are scaled up to whole numbers first, so a ratio like 1.5:3 is correctly simplified to 1:2.
What is the 1 : n form for?
It rewrites the ratio so the first part is 1, which shows how much of the second quantity goes with a single unit of the first. It is convenient for scale factors and mixing rates.
Is my data private?
Yes. Every calculation runs locally in your browser. Nothing you type is sent anywhere, and the page works offline after loading.