How drawing scales work
Measure on the sheet, then work out the real length. Get 10 right to finish. Pick feet and inches or metric in the box below.
The wall measures 2 3/4 in on the sheet. The scale is 3/8" = 1'-0". How long is the real wall?
How to read it
The scale is written in the title block or under each view. It tells you how much smaller the drawing is than the real thing. To get a real length, measure the line on paper and multiply.
Feet and inches (US)
US plans write the scale as paper inches per real foot: 1/4" = 1'-0" means a quarter inch on paper is one foot. So 1 inch on paper is 4 feet. A wall that measures 3 1/2 in is 3.5 x 4 = 14 feet long.
Another way: every architect's scale is also a ratio. Multiply the paper length in inches by the ratio to get real inches.
| Scale | Ratio | 1 in on paper is |
|---|---|---|
| 1/8" = 1'-0" | 1:96 | 8 ft |
| 1/4" = 1'-0" | 1:48 | 4 ft |
| 1/2" = 1'-0" | 1:24 | 2 ft |
| 1" = 1'-0" | 1:12 | 1 ft |
| 3" = 1'-0" | 1:4 | 3 in |
Metric (Europe)
Metric drawings write the scale as a ratio, such as 1:50. Multiply the length on paper by the second number. 30 mm on a 1:100 plan is 30 x 100 = 3000 mm, which is 3 metres. Building drawings give dimensions in millimetres, so keep your answer in mm unless it is a site plan.
Common mistakes
- Using the scale from another view. One sheet can carry several scales, so read the note under each view.
- Writing 14.5 ft as 14'-5". Half a foot is 6 inches, so 14.5 ft is 14'-6".
- Mixing up 1:50 and 1:48. Close, but not the same.
Questions
- What does 1/4" = 1'-0" mean?
- Every quarter inch on the drawing stands for one foot of the real thing. So 1 inch on paper is 4 feet. As a ratio it is 1:48.
- What does 1:50 mean on a drawing?
- The real thing is 50 times bigger than the drawing. 1 mm on paper is 50 mm real, and 20 mm on paper is 1 metre.
- Is 1/4" = 1'-0" the same as 1:50?
- No. It is 1:48. The two are close, about 4 percent apart, so a metric scale rule on an imperial drawing gives lengths that are slightly off.
- Which is bigger, 1:50 or 1:100?
- A 1:50 drawing is twice as big as a 1:100 drawing of the same thing. The bigger the second number, the smaller the drawing.
Explained in our own words and checked against trade school textbooks. Metric scales follow the ISO 5455 series.