CAD Scale Calculator
Calculate the exact CAD scale ratio from a known real-world distance and the same distance measured on paper or in a PDF.
Enter your values, then select Calculate.
Runs in your browser. No calculation data is sent to an external service.
What this calculator does
Use this when you know what a distance represents in the model and can measure the same line on the plotted sheet. The calculator converts both values to one unit, divides real length by paper length, and returns the exact 1:N ratio.
Example
If 10 metres in the drawing measures 100 mm on the sheet, the ratio is 10,000 ÷ 100 = 100, so the plotted scale is 1:100.
How to use the result
- Measure a long known dimension when possible; a longer test distance reduces measurement error.
- Measure the PDF or paper at 100% viewing/printing scale.
- If the result is close to a standard scale such as 1:50 or 1:100, verify the viewport and Page Setup before accepting a slightly odd ratio.
Frequently asked questions
Can this calculate scale from a PDF?
Yes. Measure a known line in the PDF, enter its real-world length and measured paper/PDF length, and compare the result with the intended scale.
Why do I get 1:96 instead of 1:100?
1:96 is the exact ratio for 1/8 inch = 1 foot. Architectural and metric scale families do not always have identical equivalents.
Should I round every result to a standard scale?
No. First determine why the exact ratio differs. A viewport, PDF viewer or printer may have introduced unintended scaling.
Explore more in CAD Plotting Tools & Calculators.