Ratio calculator

The cross-multiplication rule

Two ratios are equal when their cross products are equal: A × D = B × C. Rearrange the equation and you can isolate any one of the four terms. Pick which side you want as the unknown, fill in the other three, and we'll do the algebra.

Where ratios show up

• Recipes: scale a 4-person dish to 7 people without splitting eggs.
• Maps and plans:"1 cm represents 50 m" is a ratio; convert any measurement on the page to a real-world distance.
• Mixing solutions: if 50 ml of concentrate dilutes to 1 L of working solution, how much concentrate for 4.5 L?
• Currency and pricing: per-unit cost comparisons across different pack sizes.

Ratios vs fractions vs percentages

All three are ways of expressing the same relationship. 3:4 is the same proportion as 3/7 (of the whole) and ~43%. Ratios are best when you're comparing two parts; fractions are best for arithmetic; percentages are best for headlines. We have separate tools for fractions and percentages if you need them.