What is a proportion?

A proportion is a statement that two ratios are equal, written a/b = c/d. Proportions are used whenever two quantities scale together at a fixed rate.

How do you solve a proportion?

Cross-multiply: a × d = b × c. Then divide both sides by the coefficient of the unknown to isolate it. For example, to solve for a in a/b = c/d, a = (b × c) / d.

What happens if I leave more than one field blank?

The calculator needs exactly three known values to solve for the fourth. If you leave more than one field blank, or fill in all four, it will show an error asking you to leave exactly one blank.

Can I use decimals or negative numbers?

Yes, a, b, c, and d can be any real numbers, including decimals and negative values, as long as the denominators involved in the cross-multiplication aren't zero.

What if the denominator I need to divide by is zero?

Division by zero is undefined, so if solving for a value would require dividing by a zero term, the calculator shows an error instead of an incorrect result.

How is a proportion different from a ratio?

A ratio (like 3:4) compares two quantities. A proportion is an equation stating that two ratios are equal (like 3:4 = 6:8). You need a proportion, not just a ratio, to solve for an unknown scaled value.

Can I use this to scale a recipe?

Yes — a very common use. If a recipe for 4 servings needs 2 cups of flour, set up 4/2 = 6/x to find how much flour you need for 6 servings, leaving x blank.

Can I use this for map scales or similar triangles?

Yes. Map scales (distance on map / real distance) and similar triangle side ratios are both classic proportion problems this calculator solves directly.

Proportion Calculator — Solve for the Missing Value in a Proportion

Our Proportion Calculator solves any proportion of the form a/b = c/d. Enter the three values you know and leave the fourth blank — the calculator cross-multiplies and isolates the missing value automatically, showing every step of the algebra so you can follow exactly how the answer was reached. Useful for scaling recipes, converting map distances, solving similar-triangle problems, and any 'if this, then that' ratio question.

Quick Answer

A proportion states two ratios are equal: a/b = c/d. Enter any three values and leave the fourth blank — this calculator cross-multiplies (a × d = b × c) to solve for the missing term instantly, with the full algebra shown step by step.

a / b = c / d — leave one field blank to solve for it

Enter three values and leave one blank, then click Calculate.

How to Use the Proportion Calculator — Solve a/b = c/d Online

1
Enter the three known values of your proportion a/b = c/d.
2
Leave the field you want to solve for blank.
3
Click 'Calculate' to cross-multiply and solve for the missing value.
4
Review the step-by-step cross-multiplication to see exactly how the result was derived.

Why Use Proportion Calculator — Solve a/b = c/d Online?

A proportion states that two ratios are equal, and solving for an unknown term is one of the most common real-world math tasks — scaling a recipe up or down, converting a scale-model measurement to real size, or figuring out how much of an ingredient you need for a bigger batch. The classic technique, cross-multiplication, is quick but easy to mess up when a sign or a term gets swapped. This calculator performs the cross-multiplication exactly and shows the algebra, so you can verify your own work or get a fast, reliable answer.

Frequently Asked Questions

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