Enter A, B and C to solve the proportion A:B = C:D for the missing value D, and see A:B in its simplified form. Everything runs in your browser — nothing you enter here is uploaded, stored, or logged.
Calculated locally in your browser — nothing is sent to our server.
D (A:B = C:D)
A:B Simplified
How This Calculation Works
A proportion states that two ratios are equal: A:B = C:D. Cross-multiplying and solving gives D = (B × C) ÷ A. The calculator also simplifies A:B to its lowest terms (for whole-number inputs) using the greatest common factor, the same method used in the GCF Calculator.
FAQ
What if A is zero?
A cannot be zero, since the proportion would require dividing by zero.
Is my data uploaded anywhere?
No. The calculation happens entirely in your browser using JavaScript.