Proportion Calculator
Free Proportion Calculator for fractions. Enter values to get step-by-step solutions with formulas and graphs. See charts, tables, and visual results.
Reviewed by Manoj Kumar, Mathematics Educator
Formula
A/B = C/D → D = B × C / A
A proportion states that two ratios are equal. Cross-multiplying A/B = C/D gives A × D = B × C; dividing both sides by A isolates the unknown term D = (B × C) / A.
Worked Examples
Example 1: Medication dosage scaling
Problem:A medication is dosed at 5 mg per 10 kg of body weight. How much should a 42 kg patient receive?
Solution:Set up 5:10 = D:42, i.e. 5/10 = D/42. Cross-multiply: 10 × D = 5 × 42 = 210, so D = 21.
Result:Dose = 21 mg
Example 2: Currency-style unit conversion
Problem:If 8 kilometers equal 5 miles, how many miles equal 60 kilometers?
Solution:Set up 8/5 = 60/D. Cross-multiply: 8 × D = 5 × 60 = 300, so D = 300/8 = 37.5.
Result:60 km ≈ 37.5 miles
Frequently Asked Questions
What is a proportion in mathematics?
A proportion is a statement that two ratios are equal, written A/B = C/D (or A:B = C:D). It asserts that the relationship between A and B is exactly the same as the relationship between C and D, even if the actual numbers are different sizes. Solving a proportion means finding the one unknown value that keeps that equality true.
What is the difference between a direct proportion and an inverse proportion?
In a direct proportion, both quantities increase or decrease together at the same rate (more workers, more output in the same time). In an inverse proportion, one quantity increases as the other decreases (more workers, less time to finish a fixed job) — inverse proportions are written A×B = C×D rather than A/B = C/D. Proportion Calculator solves the standard direct-proportion form.
References
Reviewed by Manoj Kumar, Mathematics Educator · Editorial policy