The Proportion Calculator solves the classic ratio equation a/b = c/d in one click. Enter any three of the four values and the missing number is computed instantly using cross-multiplication — or enter all four values to check whether two ratios are truly proportional. Perfect for scaling recipes, converting map distances, comparing prices, and countless everyday math problems. All calculations run locally in your browser, so your numbers never leave your device.
How to Use
- 1Enter three values: Fill any three of the four boxes in a/b = c/d. Leave the value you want to find blank.
- 2Get the answer instantly: The missing value appears in real time, with a step-by-step cross-multiplication breakdown.
- 3Verify a proportion: Fill all four values and the calculator tells you whether the two ratios are proportional.
- 4Copy or reset: Use the Copy Result button or click Reset to start over.
The Proportion Formula
A proportion states that two ratios are equal: a/b = c/d
To find a missing value, cross-multiply and divide:
To check a proportion: two ratios are proportional when a × d = b × c (the cross products are equal).
Common Uses of Proportions
If 2 cups of flour make 12 cookies, how much flour makes 36 cookies? That is a proportion.
A 1:1000 map scale means 1 cm on the map equals 1000 cm in real life — scale any distance with a proportion.
Compare prices per unit: 500 g for $3 versus 750 g for $4.50 — the cheaper option wins the proportion test.
Keep aspect ratio intact when resizing images: width/height stays constant.
Mix paint, chemicals, or juice concentrate in the correct ratio by solving for the needed amount.
If a car uses 5 L per 150 km, how much fuel is needed for 600 km? Solve it in seconds.
How It Works
The calculator reads the three values you provide and uses cross-multiplication to isolate the unknown. For example, to solve d in a/b = c/d, it multiplies b × c and divides by a. When all four values are present, it compares the cross products a × d and b × c — if they match (within a tiny floating-point tolerance), the ratios form a valid proportion. The simplified ratio uses the greatest common divisor to reduce the pair to lowest terms.
Frequently Asked Questions
A proportion is an equation stating that two ratios are equal, written as a/b = c/d. For example, 1/2 = 2/4 is a proportion because both ratios equal 0.5.
Use cross-multiplication: multiply across the equals sign and divide by the remaining known value. To solve d in a/b = c/d, compute d = (b × c) ÷ a. The calculator does this instantly for any missing position.
Cross-multiply the two ratios. If a × d equals b × c, the ratios form a proportion. The calculator checks this automatically when all four values are entered.
Scaling recipes, converting map distances, calculating prices per unit, resizing images, mixing solutions, and solving percentage and rate problems.
Yes, decimal values are fully supported. For example, 2.5/5 = 1/2 works perfectly. Results display up to 6 decimal places.
If the value you are dividing by is zero, the calculator shows a clear error message instead of an undefined result. Simply enter a valid set of values.
Yes, the Proportion Calculator is 100% free with no subscriptions, hidden fees, or usage limits.
Absolutely. All calculations happen locally in your browser. Your numbers never leave your device.
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