Math & Algebra Tool
Proportion Calculator
Solve for the missing value in a proportion A/B = C/D.
Σ The Formula
A/B = C/D → Cross-multiply to solve for unknown
Real World Examples
Simple Proportion
1/2 = x/4 → x=2
Recipe Scaling
2 cups/4 servings = x cups/10 servings
Currency
1 USD / 0.9 EUR = 100 USD / x EUR
Map Scale
1 cm / 50 km = 3.5 cm / x km
# About This Calculator
A Proportion is an equation stating that two ratios or fractions are equal. It is often written as A/B = C/D or A:B :: C:D.
Proportions are the mathematical backbone of scaling. Whether you are adjusting ingredients for a larger party, calculating currency exchange rates, or working with map scales, you are using proportions.
Cross-Multiplication Principle: If A/B = C/D, then A × D = B × C. This allows you to easily find any single missing value if you know the other three.
How To Use
- Enter values for three of the fields (A, B, C, or D).
- Leave the field you want to find empty.
- Click Calculate.
- The missing value will be computed automatically.
Frequently Asked Questions
What is the detailed cross-multiplication formula?+
To solve for A: (B×C)/D. For B: (A×D)/C. For C: (A×D)/B. For D: (B×C)/A.
Can I use decimals?+
Yes, this calculator supports integers, decimals, and creates precise floating-point results.
What is a 'Direct' proportion?+
As one amount increases, another amount increases at the same rate. (e.g., buying more apples costs more money).
What is an 'Inverse' proportion?+
As one amount increases, the other decreases. (e.g., more workers usually means less time to complete a job). This tool solves Direct proportions.
Why do I get 'Infinity'?+
You likely divided by zero (e.g., having B=0 or C=0 in the denominator position). Proportions require non-zero denominators.
Is Proportion Calculator free to use?+
Yes, Proportion Calculator on Matheric is completely free to use. We believe in accessible education and utility for everyone.