What is a proportion in math?

A proportion states that two ratios are equal: a/b = c/d. It means 'a is to b as c is to d.' Proportions are used in scaling, unit conversion, recipe adjustments, and many real-world problems.

What is cross multiplication?

Cross multiplication is the method for solving proportions. Given a/b = c/d, multiply the numerator of each fraction by the denominator of the other: a × d = b × c. Then solve for the unknown variable.

Proportion Calculator – Solve Proportions & Ratios

Enter three values and leave the unknown as blank. The calculator solves for the missing value.

a / b = c / d | Leave one field blank

Scale a Recipe or Measurement

Original amount → new amount → find scaled quantity.

Original quantity

Original scale

New scale

Proportion Calculator FAQ

Common questions about solving proportions and ratios.

How do you solve a proportion step by step?

Step 1: Set up the proportion: a/b = c/d.
Step 2: Cross-multiply: a × d = b × c.
Step 3: Solve for the unknown.
Example: 3/4 = x/12 → 3 × 12 = 4 × x → 36 = 4x → x = 9.

How do I solve 5/8 = x/40?

Cross-multiply: 5 × 40 = 8 × x → 200 = 8x → x = 25. Check: 5/8 = 25/40 = 0.625 ✓

What is the difference between a ratio and a proportion?

A ratio compares two quantities (e.g., 3:4 or 3/4). A proportion is an equation saying two ratios are equal (e.g., 3/4 = 9/12). Proportions let you scale one ratio to another — which is what this calculator solves.

How are proportions used in real life?

Proportions appear everywhere: scaling recipes (if 4 servings need 3 cups, how much for 10?), map reading (1 inch = 50 miles), unit conversion, medicine dosages, architectural blueprints, and photography (aspect ratios). Anywhere you need to scale one relationship to match another.

What if the proportion has no solution?

A proportion always has exactly one solution as long as the denominator isn't zero. If you get division by zero (b = 0 or d = 0), the proportion is undefined. The calculator will alert you in that case.