Proportion Calculator
Use the Proportion Calculator to solve A/B = C/X for X. Enter three nonzero values, review the formula, and get a four-decimal result.
Formula: A / B = C / X; X = (B × C) / A
Enter nonzero values for A, B, and C. Negative and decimal values are supported.
Use the Proportion Calculator when three nonzero values can be arranged as A / B = C / X and the unknown value is X. The calculator applies one deterministic equation, so the same A, B and C inputs produce the same X result. It does not choose which value is unknown or interpret a word problem for you.
The default example uses A = 2, B = 4 and C = 5. It solves 2 / 4 = 5 / X and returns X = 10.0000. Check that your known quantities occupy the matching positions in both ratios before relying on the result.
How to Use the Proportion Calculator
- Arrange the relationship in the form
A / B = C / X, with the unknown in the X position. - Enter the known values for A, B and C. Each one must be a finite number other than zero.
- Select Calculate.
- Review the displayed X value and confirm it in the original proportion.
Positive, negative and decimal inputs are supported. Use consistent units and matching meanings across the two ratios. For example, 2 cups / 12 cookies = 5 cups / X cookies gives X = 30 cookies.
Proportion Formula and Worked Examples
A / B = C / X
A × X = B × C
X = (B × C) / ACross-multiplication gives A × X = B × C. Dividing both sides by A gives the formula used by the calculator: X = (B × C) / A. OpenStax’s Prealgebra 2e section on solving proportions defines a proportion as an equation of equal ratios and states that a proportion’s cross products are equal, which is the algebra used here. With A = 3, B = 4 and C = 6, X is (4 × 6) / 3 = 8; substituting it back gives 3 / 4 = 6 / 8.
Negative values can also form a valid numeric proportion. With A = -2, B = 4 and C = 5, the result is X = -10 because -2 / 4 = 5 / -10. A negative result is not automatically an error; its meaning depends on the quantities being compared.
Input Rules, Precision and Related Calculators
A cannot be zero because the calculation divides by A. B cannot be zero because it is a denominator in the entered ratio. C is also required to be nonzero so the resulting X does not create the undefined ratio 0 / 0. Blank, nonnumeric and non-finite values are rejected before calculation.
The result is displayed to four decimal places. Like other browser calculators, the underlying arithmetic uses standard floating-point numbers, so long decimals and very large magnitudes can be rounded. Verify units and significant figures when precision matters.
Use the Aspect Ratio Calculator for width-to-height relationships. For percent-based questions, try the Percentage Calculator, Percentage Change Calculator or Percentage Difference Calculator. These tools answer different questions and should not be treated as interchangeable with this fixed-position proportion solver.
Frequently asked questions
What is a proportion?
A proportion is an equation stating that two ratios are equal. This calculator uses the specific arrangement A / B = C / X.
What equation does the Proportion Calculator solve?
It solves A / B = C / X for X by using X = (B × C) / A. Enter the three known values in their matching A, B and C positions.
Why must A, B and C be nonzero?
A is the divisor in the calculation, B is a denominator in the entered ratio, and C = 0 would make the resulting C / X ratio equal to the undefined form 0 / 0.
Can I use negative or decimal values?
Yes. Positive, negative and decimal values are accepted as long as A, B and C are finite and nonzero. Interpret negative results in the context of your quantities.
Is the Proportion Calculator free to use?
Yes. The Proportion Calculator is currently free to use in a modern browser and does not require account sign-up.
Why is the result shown to four decimal places?
Four decimal places make many ratio results easier to read. Standard browser floating-point arithmetic can still round long decimals or very large values.
Can this calculator solve any missing value in a proportion?
No. This widget solves only the X position in A / B = C / X. Rearrange your known values into that form before entering them, then verify the answer in the original equation.