Drawing scale calculator — real size from a scaled drawing

Measure a length on a printed sheet, enter the scale ratio, and get the real dimension — or go the other way to find how long something will draw on the sheet. Imperial architectural scales are ratios too: 1/4" = 1'-0" is 1:48.

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drawing scale calculator

Length on drawing (mm)Scale ratio (1:N) (N)

Real length: 2.5m
Real length: 2,500mm
Real length: 8.202ft
1 m real draws as: 10mm on paper

Formula

real length = drawing length × N (scale 1:N)

A scale is a ratio, so the same factor works in any unit. Imperial architectural scales convert to a ratio by dividing 12 by the inch fraction: 1/4" = 1'-0" → 12 ÷ 0.25 = 48, so 1:48.

Worked examples

Input	Result
25 mm at 1:100	2.5 m real
40 mm at 1:50	2.0 m real
3" at 1/4" = 1'-0" (1:48)	12'-0" real

Imperial architectural scales as ratios

Scale	Ratio
3" = 1'-0"	1:4
1-1/2" = 1'-0"	1:8
1" = 1'-0"	1:12
3/4" = 1'-0"	1:16
1/2" = 1'-0"	1:24
3/8" = 1'-0"	1:32
1/4" = 1'-0"	1:48
3/16" = 1'-0"	1:64
1/8" = 1'-0"	1:96
3/32" = 1'-0"	1:128
1/16" = 1'-0"	1:192
1" = 10' (engineering)	1:120
1" = 20' (engineering)	1:240
1" = 50' (engineering)	1:600

What 1 metre draws as at common metric scales

Scale	1 m real on paper
1:1	1,000 mm
1:5	200 mm
1:10	100 mm
1:20	50 mm
1:25	40 mm
1:50	20 mm
1:100	10 mm
1:200	5 mm
1:250	4 mm
1:500	2 mm
1:1000	1 mm
1:1250	0.8 mm

Questions

What is 1/4 inch scale as a ratio?

1:48. A quarter inch on the drawing represents 12 inches, and 12 ÷ 0.25 = 48. 1/8" = 1'-0" is 1:96 and 1/2" = 1'-0" is 1:24.

Is 1:50 the same as 1/4 inch scale?

Close, but no. 1/4" scale is 1:48, so a dimension scaled with a 1:50 rule reads about 4% long. Use the matching rule or dimension from the model.

Why does my PDF measurement not match the scale?

The sheet was probably printed with "fit to page". Check against a known dimension or the graphic scale bar before measuring anything else.

How do I set scale in AutoCAD?

Draw at 1:1 in model space, then set the viewport scale in the layout (for example 1:100) and use annotative text and dimensions so they read correctly on the sheet.

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