Calculate cross multiplication and direct proportions online with our free Rule of Three Calculator. Solve proportions quickly and easily. The simple rule of three (also known as cross-multiplication) is the most practical mathematical method to find an unknown fourth value when you already know three values that have a direct proportional relationship. Our calculator makes it effortless: whether you're resizing an image, scaling a recipe, calculating percentages, or doing homework, just enter the three known values and we will automatically compute and reveal the missing fourth value instantly.
