Proportion Calculator
Solve for any missing term in a ratio or proportion (A : B = C : D) with instant cross-multiplication steps and scale factors.
Inputs
- Find Missing Value
- Value A (Numerator 1)
- Value B (Denominator 1)
- Value C (Numerator 2)
- Value D (Denominator 2)
Worked examples
Recipe Scaling (4 servings to 10)
A recipe requires 3 cups of flour for 4 servings. How much flour is needed for 10 servings? Result: 7.5 cups of flour.
Formula: 3 / 4 = x / 10. Cross multiply: 4x = 30 → x = 7.5.
Architectural Blueprint Scale (1:50)
A wall is 6 cm on a 1:50 drawing. What is the real-world dimension in meters? Result: 300 cm (3.0 meters).
Formula: 1 / 50 = 6 / D. Cross multiply: D = 6 × 50 = 300 cm.
Frequently asked questions
How do you solve a proportion equation?
Use cross-multiplication: multiply the numerator of the first fraction by the denominator of the second fraction, set it equal to the other diagonal product (A × D = B × C), then divide by the known term to isolate the unknown.
Can denominators in a proportion be zero?
No. Division by zero is mathematically undefined. Denominators B and D must be non-zero numbers.
How is this used in real life?
Proportions are used constantly for recipe scaling (e.g., doubling ingredients for 8 people instead of 4), photo and screen aspect ratio resizing, scale model construction, currency conversions, and road map distances.