Ratio Calculator
Solve a:b = c:d for the missing value, and simplify any ratio.
About this calculator
Two jobs, both instant. Fill three boxes of a:b = c:d and the fourth is solved by cross-multiplication, with the working shown. Or feed any two numbers to the simplifier and get the ratio in lowest terms โ 18:24 becomes 3:4, divided through by their greatest common divisor.
Where proportions run your life
The same one move โ cross-multiply, divide โ answers a startling share of everyday maths. Scaling a recipe (3 eggs for 4 people, so how many for 12?), unit prices (which bottle is actually cheaper per litre?), map scales, paint mixing, currency at a known rate, screen and photo shapes, medicine doses by weight. The key discipline is keeping matching quantities on matching sides: eggs over people on both ends, never eggs over people on one side and people over eggs on the other โ that flip is the single error behind most wrong proportion answers, and the calculator's cross-check line (the two products must match) is how you catch it. Percentages are the special case where the second pair is something:100.
Frequently asked questions
How do you solve a proportion like 3:4 = 9:x?
Cross-multiply: 3x = 4 ร 9, so x = 12. The check is that the two cross-products match โ 3 ร 12 and 4 ร 9 both equal 36.
How do I simplify a ratio?
Divide both sides by their greatest common divisor: 18:24 รท 6 = 3:4. The calculator finds the GCD for you and shows what it divided by.
Can ratios have decimals?
The proportion solver takes any numbers, decimals included. Simplifying to lowest terms is a whole-number idea โ for decimal ratios, multiply both sides up until they're whole (2.5:4 โ 5:8).
What's the difference between a ratio and a fraction?
A fraction is part-of-a-whole; a ratio compares two quantities. 3:4 as a mixture means 3/7 of the total is the first ingredient โ a distinction that trips up recipe scaling more than anything else.