Ratio Calculator
Leave any one field blank to solve for it. Or fill A and B to see the simplified ratio.
Inputs
A : B = C : D
Result
Missing value
8
Simplified A:B = 2 : 3
Next useful calculators
Visual breakdown
A
2
B
3
C
8
D
12
Formula
From a/b = c/d, the unknown solves by cross-multiplication: a·d = b·c.
Example
2 : 3 = ? : 12 → ? = (2 × 12) ÷ 3 = 8.
Common examples
3 : 4 = 9 : ?
Cross-multiply → 12.
Simplify 12 : 18
GCD 6 → 2 : 3.
Recipe scale 2 : 5 → 8 : ?
Multiply by 4 → 20.
Frequently asked
How do I simplify a ratio?
Fill A and B and leave C and D blank — we show A:B reduced by the greatest common divisor.
Can I use decimals?
Yes, decimals are accepted. Simplification works best with whole numbers.
Why is one box blank?
Leave exactly one of A, B, C or D blank — that's the value we solve for.