Proportion calculator
Solve for the one missing number in two equal ratios, such as 3:5 = 9:x. Students use it to check cross multiplication, and cooks and builders use it to scale amounts up or down.
- Runs in your browser
- Free, no account needed
- Copy or download the result
Result
Example
3:5 = 9:x
x = 15 Completed proportion: 3:5 = 9:15
How to use the proportion calculator
- Type the proportion in the form a:b = c:d, such as 3:5 = 9:x.
- Put x in exactly one of the four spots. The other three can be whole numbers, decimals or fractions.
- Press Run to see x and the completed proportion.
- Press Copy result or Download to keep the answer.
How it works
The tool uses cross multiplication. In a:b = c:d, a x d = b x c, so it solves that equation for whichever value is x.
The answer is exact. If x is not a whole number it is shown as a simplified fraction, such as 4/3, not a rounded decimal.
x can be lowercase or uppercase. Use exactly one x, one equals sign and two colons.
The second number in each ratio cannot be zero. Setups with no single answer, such as 0:5 = 3:x, are rejected with an error.
For 3:5 = 9:x the output is x = 15, followed by the completed proportion 3:5 = 9:15.
Everything runs in your browser. Your input is not uploaded or stored.
Questions and answers
How do you solve a proportion?
Cross multiply and divide. In 3:5 = 9:x, 3 times x equals 5 times 9, so x = 45 / 3 = 15.
What is the difference between a ratio and a proportion?
A ratio compares two numbers, like 3:5. A proportion says two ratios are equal, like 3:5 = 9:15.
Can I use fractions or decimals in a proportion?
Yes. Any of the three known values can be a decimal like 2.5 or a fraction like 3/4, and the answer stays exact.
How do I scale a recipe with a proportion?
Write the original amount and servings as one ratio and the new servings with x as the other. For 2 cups for 4 servings scaled to 6 servings, enter 2:4 = x:6 to get x = 3.