Solve A:B = C:D for the missing value using cross multiplication.
If A/B = C/D then D = (B × C) ÷ A
2:3 = 8:? Cross-multiplying gives 2D = 24, so D = 12.
Two ratios are proportional when their cross products match: A × D equals B × C. Knowing three of the four values pins down the fourth — that is the entire trick, and it powers unit pricing, recipe scaling, map scales and currency conversion.
Leave blank whichever value you want to find — A, B, C or D. Fill in the other three.
No. This solves direct proportions. Inverse relationships (more workers, less time) need the product form A×B = C×D instead.
Full expression calculator with parentheses, powers, roots, trigonometry, logarithms and constants π and e.
Calculate X% of Y, find what percent one number is of another, and solve percentage increase problems in one place.
Find the percentage increase between an original value and a new value, including the absolute change.
Raise any base to any power, including negative and fractional exponents.
Work out how much a value dropped in percent terms, with the absolute reduction shown alongside.
Add, subtract, multiply and divide fractions and mixed numbers, with results automatically simplified.