Find the scale of a drawing
The scale note is missing. Work it out from the dimension and the measured length. Get 10 right to finish.
This wall is dimensioned 39'-0" and measures 4 7/8 in on the sheet. What is the scale?
How to work it out
Sometimes the scale note is missing, cut off a copy, or you just want to check it. One line with a written dimension is enough.
Feet and inches
Divide the real length in feet by the paper length in inches. That gives feet per inch. 4 feet per inch is 1/4" = 1'-0", 2 feet per inch is 1/2" = 1'-0", 8 feet per inch is 1/8" = 1'-0".
Metric
Divide the written dimension in mm by the measured length in mm. The answer is the scale number.
Common mistakes
- Dividing the wrong way round. The real length is always the bigger number.
- Forgetting to turn feet and inches into feet first: 14'-6" is 14.5 ft.
- Trusting the result on a copy of unknown size. Check a second dimension.
Questions
- How do I find the scale of a drawing with no scale note?
- Find a line with a written dimension, measure it on the sheet, and divide the real length by the paper length. In metric, 3600 mm over 72 mm gives 50, so the scale is 1:50.
- What if the answer is not a standard scale?
- Then the copy has probably been resized, or the line is not drawn to scale. Check against a second dimensioned line and look for a graphic scale bar before you trust any measurement.
Explained in our own words and checked against trade school textbooks. Metric scales follow the ISO 5455 series.