Proportions
Calculator
Enter any three values to instantly solve the missing variable. Beautiful, fast, and perfectly accurate.
Start CalculatingCalculate Your Proportions
Enter any three known values and leave one field as x (or empty) to find the unknown.
How Proportions Calculator Works
Proportions use cross-multiplication to find the unknown value.
Enter Values
Input any three known numbers into the A, B, C, D fields. Leave the unknown field empty.
Cross Multiply
The formula A × D = B × C is applied automatically to solve for the missing value.
Get Results
Instantly see the answer with step-by-step solution breakdown and verification.
Quick Examples
Click any example to auto-fill the calculator.
FAQ
What is a proportion? +
A proportion is an equation that states two ratios are equal. For example, 2/3 = 4/6 is a proportion because both ratios simplify to the same value. Proportions are fundamental in mathematics, science, cooking, and everyday problem-solving.
How does cross-multiplication work? +
In the proportion A/B = C/D, cross-multiplication means A × D = B × C. If one of the four values is unknown, you can solve for it by rearranging this equation. For example, if D is unknown: D = (B × C) / A.
Can I use decimals or negative numbers? +
Yes! The calculator supports integers, decimals, and negative numbers. Just type them normally — for example, -3.5 or 0.25.
What if I get a division by zero error? +
Division by zero occurs when the denominator in the calculation equals zero. This is mathematically undefined. Make sure the values you enter don't result in dividing by zero.
What is the proportion formula? +
The proportion formula is A/B = C/D, which can also be written as A:B = C:D. Using cross-multiplication, this becomes A × D = B × C, which you can rearrange to solve for any unknown variable.
Where are proportions used in real life? +
Proportions are used in cooking (scaling recipes), construction (scale drawings), finance (interest rates), science (unit conversions), photography (aspect ratios), and many more everyday situations.
Is this calculator free to use? +
Yes, ProportionCalc is completely free. All calculations are performed locally in your browser — no sign-up or payment required.