Proportion Calculator
Solve a proportion a/b = c/d by leaving one value blank. The calculator finds the missing number.
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Leave exactly one field blank to solve for it.
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This calculator solves proportions of the form a/b = c/d. Fill in any three of the four values, leave one blank, and the missing value is calculated instantly.
- Enter three known values. Place your numbers in the a, b, c, and d fields. The layout mirrors the proportion: a/b on the left equals c/d on the right.
- Leave one field blank. The blank field is the unknown the calculator will solve for.
- Read the result. The missing value appears along with the formula used for the calculation. Use Copy or Share to save or send the result.
The calculator uses cross multiplication to solve: a x d = b x c. From this, any missing value can be isolated. For example, if a is missing, a = (b x d) / c.
About Proportions
A proportion states that two ratios are equal. Proportions are fundamental in scaling recipes, converting units, resizing images, calculating distances from maps, and solving many real-world problems. If you know that two quantities maintain a constant ratio, you can use a proportion to find an unknown value when you have three of the four numbers.
Cross multiplication is the standard solving technique: multiply the numerator of each fraction by the denominator of the other, set them equal, and solve for the unknown. This calculator does that step automatically.
Frequently Asked Questions
How do you solve a proportion?
Use cross multiplication. For a/b = c/d, multiply a x d and b x c. Set them equal and solve for the unknown. For example, in 3/4 = x/8, cross multiply to get 3 x 8 = 4 x x, so 24 = 4x, and x = 6.
Can I leave more than one field blank?
No. A proportion with two unknowns has infinite solutions. You must provide exactly three values for the calculator to find the unique fourth value.
What is the difference between a ratio and a proportion?
A ratio compares two numbers (like 3:4). A proportion is an equation stating that two ratios are equal (like 3/4 = 6/8). A ratio is a single comparison; a proportion links two comparisons.