Proportion Calculator

Proportion calculator for ratios and scaling

A proportion is a statement that two ratios are equal. In plain terms, it lets you compare two relationships like A to B and C to D, written as A/B = C/D. This is useful whenever you need to scale something up or down, keep a mixture consistent, or solve for an unknown value without guessing. People most often use proportions for recipes, mixing solutions, resizing measurements, map scales, speed and distance comparisons, and school or exam problems.

This calculator is built for the most common real-world case: you already know three of the four values and you want the missing one. You can enter numbers in A, B, C, and D, then leave exactly one field blank. The calculator solves the missing value using cross multiplication, which is the standard method for proportions. It also gives you a practical scale factor so you can see how much bigger or smaller one side is compared with the other.

To use it, decide how your proportion is structured and put the numbers into the matching positions. If you are working with a recipe, A and B might represent an original ingredient and original yield, while C and D represent the new ingredient amount and new yield. If you are working with measurement scaling, A and B might represent the original dimension and original reference, and C and D represent the new dimension and new reference. The key is to keep the relationship consistent. If you swap terms around, you will still get a number, but it will not describe what you intended.

Assumptions and how to use this calculator

• Leave exactly one input blank. If none are blank, the calculator will treat it as a consistency check instead of solving for a missing value.
• All entered values must be greater than 0. Proportions that include zero can be valid in some contexts, but they often cause divide-by-zero issues and are rarely what normal users mean.
• The calculator assumes A/B and C/D describe the same relationship. If you entered values from different contexts, the result is not meaningful.
• Results are rounded to two decimals for readability. For precise work, use the unrounded value idea and keep more digits in your own calculations.
• Scale factor is shown as a helpful interpretation, not a law of nature. If your situation is not linear, scaling may not apply.

Common questions

What does A/B = C/D actually mean?

It means the ratio of A to B is the same as the ratio of C to D. If A is half of B, then C should also be half of D. When the ratios are equal, the cross products match: A × D equals B × C.

Which value should I leave blank?

Leave blank the value you are trying to find. If you are scaling a recipe and want to know the new ingredient amount, leave that ingredient field blank. If you want to know the new total, leave the total field blank. The calculator will solve for whichever one is empty.

What is the scale factor and why should I care?

The scale factor is how much the right side is scaled relative to the left side. If C/A is 3, then the right side is three times the left side, and you should expect D to be three times B as well. This helps you sanity-check your result and avoid a common mistake where values are flipped.

What if I fill in all four values?

The calculator will treat it as a check. If A/B equals C/D, the cross products A × D and B × C should match (within small rounding differences). If they do not match, your four numbers do not describe a true proportion.

Why does it complain about missing or invalid values?

Because the method requires three known values and one unknown. If you leave two blanks, there are infinitely many answers. If you leave none blank, there is nothing to solve. If you enter zero or negative values, the ratio may not represent a usable real-world relationship and can create division problems.

How do I improve accuracy for real-world scaling?

Keep your inputs in consistent units and avoid rounding early. For example, convert measurements first (grams to grams, centimeters to centimeters), then solve the proportion, and only round at the end. If your scenario is not linear (like some chemical reactions or non-uniform resizing), a proportion may not apply.