Proportion Calculator
Enter three values of A : B = C : D to find the fourth, with the cross-multiplication worked out, or check whether two ratios are equal. Below are a ratio calculator that simplifies any ratio, a recipe and plan scaler, and a golden ratio tool.
Proportion calculator: solve A : B = C : D
Fill in three numbers and leave the unknown box empty.
Ratio calculator: simplify a ratio
Scale a recipe or plan
| Item | Original amount | Scaled |
|---|---|---|
Golden ratio
φ = (1 + √5) ÷ 2 ≈ 1.618034.
How to solve a proportion
A proportion says two ratios are equal: A : B = C : D, or A/B = C/D. Cross-multiplying gives A × D = B × C, so any one value can be found from the other three. For 3 : 4 = 15 : x, x = 4 × 15 ÷ 3 = 20.
Simplifying a ratio: divide both numbers by their greatest common divisor. 1920 : 1080 → both divide by 120 → 16 : 9.
Scaling: multiply every amount by new size ÷ original size. A recipe for 4 made for 10 uses a factor of 2.5.
Common questions
How do I solve a proportion?
Cross-multiply and divide. For 3/4 = 15/x: 3x = 60, so x = 20.
How do I simplify a ratio?
Divide both parts by their greatest common divisor. 12 : 18 → 2 : 3.
What's the difference between a ratio and a proportion?
A ratio compares two numbers (3 : 4). A proportion states that two ratios are equal (3 : 4 = 15 : 20).
How do I scale a recipe?
Multiply each ingredient by the number of servings you want ÷ the servings the recipe makes.
What is the golden ratio?
About 1.618. Two lengths are in the golden ratio when the longer ÷ shorter equals (longer + shorter) ÷ longer.
Sources
Related
General information, not financial, tax or legal advice. Check important numbers with the official source before acting on them.