Ratio and Proportion Calculator

Ratio and Proportion Formula

Worked Example

The proportion 3 : 4 = 15 : x:

1. a : b = c : x means a/b = c/x, so x = b·c ÷ a
2. x = 4 × 15 ÷ 3 = 20

Test-day tip

SAT ratio questions often hide unit conversions. Write units in the proportion (miles/hour = miles/hour) so they cancel correctly before cross-multiplying.

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Last reviewed: 2026-10-15. Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.