How architectural scale conversion works
A drawing scale is a ratio between the size on paper and the size in real life. To convert a measurement from a plan to its real dimension, multiply by the scale factor:
Real size = Drawing measurement × Scale factor
For metric scales the factor is the second number (1:100 → × 100). For imperial architectural scales, the factor is 12 ÷ the fraction: at 1/4″ = 1′ one foot is a quarter inch, so the factor is 48; at 1/8″ = 1′ it is 96.
Common architectural scales and factors
| Scale | Factor | Typical use | 1 unit on paper = |
|---|---|---|---|
| 1:20 | 20 | Details, joinery | 1 cm = 20 cm |
| 1:50 | 50 | Floor plans, room layouts | 1 cm = 50 cm |
| 1:100 | 100 | Floor plans, sections | 1 cm = 1 m |
| 1:200 | 200 | Site plans | 1 cm = 2 m |
| 1/4″ = 1′ | 48 | US residential floor plans | 1 in = 4 ft |
| 1/8″ = 1′ | 96 | Larger plans, site layouts | 1 in = 8 ft |
| 1/2″ = 1′ | 24 | Enlarged plans, details | 1 in = 2 ft |
Worked examples
- 1:100 plan: a wall measures 4.2 cm on paper → 4.2 × 100 = 420 cm = 4.2 m.
- 1:50 plan: a bedroom is 7 cm wide on paper → 7 × 50 = 350 cm = 3.5 m (11′ 6″).
- 1/4″ = 1′ plan: a room measures 3 inches → 3 × 48 = 144 in = 12 ft.
Converting between scales
To redraw a plan at a different scale, divide the old factor by the new one. Going from 1:100 to 1:50 doubles every measurement (100 ÷ 50 = 2); going from 1:100 to 1:200 halves it. Scaling 1:100 drawings is the most common conversion because many plans are issued at 1:100 but printed or modeled at 1:50.