Common drawing scales
Three kinds of scale cover almost every drawing: the architect's scale and the engineer's scale in the US, and metric scales in Europe and most of the world.
Architect's scale vs engineer's scale
An architect's scale is written as inches on paper per real foot, such as 1/4" = 1'-0". It reads feet and inches and is used for buildings: plans, elevations, sections and details.
An engineer's scale (or civil scale) is written as real feet per paper inch, such as 1" = 20'. It reads feet and tenths of a foot and is used for site plans, roads and surveys. If the title block says 1" = 30', reach for the engineer's scale.
Architect's scales
| Scale | Ratio | 1 in on paper is |
|---|---|---|
| 1/8" = 1'-0" | 1:96 | 8'-0" |
| 1/4" = 1'-0" | 1:48 | 4'-0" |
| 3/8" = 1'-0" | 1:32 | 2'-8" |
| 1/2" = 1'-0" | 1:24 | 2'-0" |
| 3/4" = 1'-0" | 1:16 | 1'-4" |
| 1" = 1'-0" | 1:12 | 1'-0" |
| 1 1/2" = 1'-0" | 1:8 | 0'-8" |
| 3" = 1'-0" | 1:4 | 0'-4" |
Most architect's scales also carry 3/32" = 1'-0" (1:128) and 3/16" = 1'-0" (1:64).
Engineer's scales
| Scale | Ratio | Parts per inch on the rule |
|---|---|---|
| 1" = 10' | 1:120 | 10 |
| 1" = 20' | 1:240 | 20 |
| 1" = 30' | 1:360 | 30 |
| 1" = 40' | 1:480 | 40 |
| 1" = 50' | 1:600 | 50 |
| 1" = 60' | 1:720 | 60 |
The same edges also read 1" = 100', 1" = 200' and so on, by adding a zero.
Metric scales
| Scale | 1 cm on paper is | Used for |
|---|---|---|
| 1:5 | 0.05 m | Details |
| 1:10 | 0.1 m | Details |
| 1:20 | 0.2 m | Details, small parts of plans |
| 1:50 | 0.5 m | Floor plans of small buildings, sections |
| 1:100 | 1 m | Floor plans, elevations |
| 1:200 | 2 m | Site plans, large buildings |
| 1:500 | 5 m | Site and location plans |
These follow the ISO 5455 series. 1:1, 1:2 and 1:1250 are also used.
Imperial and metric are close, not equal
1/4" = 1'-0" is 1:48 and 1:50 is the nearest metric scale. 1/8" is 1:96 against 1:100. The gap is about 4 percent, which is enough to put a 10 m wall about 400 mm out. Measure imperial drawings with an architect's scale and metric drawings with a metric rule.