How to use this calculator
- Enter or choose first term (A).
- Enter or choose second term (B).
- Enter or choose scale A to (optional).
- The simplest form updates as you type. Use Copy result or Share to keep it.
Formula
A : B = C : D, so D = B × C ÷ A
- A, B
- the original ratio
- C
- the new first term
- D
- the missing second term
Worked example
- The greatest common divisor of 4 and 6 is 2, so 4 : 6 = 2 : 3.
- To scale to 10 : D, D = 6 × 10 ÷ 4 = 15.
4 : 6 simplifies to 2 : 3, and is the same proportion as 10 : 15.
What the result means
Decimal ratios are converted to whole numbers first, so 1.5 : 2.5 simplifies to 3 : 5. Use scaling for recipes, map scales and screen aspect ratios.
Assumptions and limits
- Both terms are positive.
- Three-part ratios (A : B : C) are not supported in this version.
Questions
How do I simplify a ratio?
Divide both numbers by their greatest common divisor. 18 : 24 → divide by 6 → 3 : 4.
How do I split an amount in a ratio?
Add the parts (2 : 3 has 5 parts), divide the amount by the total, then multiply by each part. 100 in 2 : 3 is 40 and 60.