Ratio solver
Fill in any three boxes and I will solve the fourth.
Enter two numbers and I will reduce them to simplest form.
Enter A, B, and a multiplier.
Solve Ratio
Solve proportions, simplify ratios, and scale them up or down, instantly and free, right in your browser.
Worked example: 3 : 4 = x : 20
Cross multiplication turns a missing-value proportion into one small division problem. Here is the full work for 3 : 4 = x : 20.
- Write the cross products: 3 × 20 = 60, and 4 × x = 4x.
- The cross products of equal ratios are equal, so 4x = 60.
- Divide both sides by 4: x = 60 / 4.
- So x = 15. Check: 15 : 20 simplifies to 3 : 4, so the proportion holds.
How it works
The solver tab treats a proportion as A : B = C : D. When the two ratios are equal, A times D always equals B times C. Fill in any three of the four boxes and the fourth is computed on every keystroke, then highlighted so you can see which value was solved for you.
The simplify tab reduces a ratio to its smallest whole numbers. Decimals are handled honestly: both sides are multiplied by a power of ten to clear the decimal places, and then the greatest common divisor does the rest. So 1.5 : 2 becomes 15 : 20 and finishes as 3 : 4.
The scale tab multiplies both sides of a ratio by the same factor, which keeps the proportion identical. Results are rounded to at most 6 significant digits with trailing zeros trimmed. Everything runs on your device with plain JavaScript: nothing you type leaves your browser, there is no account, and the page works even on a slow connection.
Frequently asked questions
What is a ratio?
A ratio compares two quantities and shows how much of one there is relative to the other. A ratio of 3 : 4 means that for every 3 units of the first quantity there are 4 units of the second.
How does cross multiplication work?
When two ratios are equal, their cross products are equal too. For A : B = C : D that means A times D equals B times C, so you can find any one missing value by multiplying the two known diagonal values and dividing by the remaining known value.
Can I use decimals?
Yes. The solver, simplifier, and scaler all accept decimals like 2.5 or 0.75. The simplifier converts decimals to whole numbers before reducing, so 2.5 : 1 simplifies to 5 : 2.
What if one of the values is zero?
A zero can appear in a ratio, but it cannot be the value the tool divides by during cross multiplication. When that happens the tool says so in plain words instead of showing NaN or Infinity. For simplifying, both numbers need to be nonzero.
How do you simplify a ratio that has decimals?
Multiply both sides by a power of ten large enough to clear every decimal place, then divide both whole numbers by their greatest common divisor. For example, 1.5 : 2 becomes 15 : 20, and then 3 : 4.
What is the difference between a ratio and a fraction?
A fraction compares a part to a whole, while a ratio can compare part to part. The ratio 1 : 2 means the first part is 1/3 of the whole, not 1/2, because the parts add up to 3.
Is my data stored anywhere?
No. Everything on this page runs locally in your browser with plain JavaScript. Nothing you type is sent to a server or saved.
Is Solve Ratio free?
Yes, completely free. There is no account, no subscription, and no limit on how many ratios you can solve.