What scale actually means
A scale ratio tells you how many times smaller your model or drawing is compared to the real thing. The format is always drawing size : real size.
1:20 means 1 unit in the drawing equals 20 units in reality.
So 1mm in the drawing = 20mm in reality.
Or: 1cm in the drawing = 20cm in reality.
Or: 1m in the drawing = 20m in reality — though you will never build a model that large.
So 1mm in the drawing = 20mm in reality.
Or: 1cm in the drawing = 20cm in reality.
Or: 1m in the drawing = 20m in reality — though you will never build a model that large.
The left number is always the drawing. The right number is always reality. A larger right number means a smaller drawing — 1:100 is smaller than 1:20.
Which scale to use for what
| Scale | Typical use | A 6m wall becomes |
|---|---|---|
| 1:5 | Construction details — door handles, window profiles | 1200mm — impractical for a full model |
| 1:10 | Detailed room sections, bathroom layouts | 600mm — large but workable |
| 1:20 | Individual house or room models — most common for students | 300mm — fits on a table, easy to work with |
| 1:50 | Apartment building, small commercial project | 120mm — compact, fine details are difficult |
| 1:100 | Full site, large building | 60mm — too small to show interior detail |
| 1:200 | Urban site plan, masterplan context | 30mm — symbolic only |
Calculating dimensions — the formula
Model dimension = Real dimension ÷ Scale factor
The scale factor is the right-hand number in your ratio.
The scale factor is the right-hand number in your ratio.
At 1:20:
- 6000mm wall → 6000 ÷ 20 = 300mm
- 3000mm wall → 3000 ÷ 20 = 150mm
- 2700mm ceiling → 2700 ÷ 20 = 135mm
- 900mm door → 900 ÷ 20 = 45mm
- 111mm wall thickness → 111 ÷ 20 = 5.55mm (round to 6mm for laser cutting)
At 1:50:
- 6000mm wall → 6000 ÷ 50 = 120mm
- 3000mm wall → 3000 ÷ 50 = 60mm
- 2700mm ceiling → 2700 ÷ 50 = 54mm
Common scale mistakes
Scaling twice. If you apply 1:20 scale in SketchUp during DXF export, then scale again in Illustrator, your pieces will be 400 times too small. Apply scale once — in SketchUp — and import at 1:1 in Illustrator.
Mixing units. SketchUp models are built in millimetres internally, even when the interface shows metres. When you draw a 3m wall, type
3000 in the dimension box — not 3. If you type 3, you have drawn a 3mm wall.
Not verifying before cutting. Always measure one known dimension in Illustrator with the ruler tool after importing your DXF. Confirm it matches your expected scaled dimension. This takes 30 seconds and prevents expensive material waste.
Scale on drawings vs scale on physical models
The word "scale" is used in two related but distinct ways:
- On a 2D drawing — a floor plan printed at 1:50 means the building layout is reduced 50× on paper. You can read dimensions with a scale ruler.
- On a physical model — a model built at 1:20 means every wall, every door, every window is physically one twentieth of real size. A 3mm sheet of plywood represents a 60mm structural element in reality.
They use the same ratios and the same maths — but they exist in different forms. A physical model at 1:20 looks like a physical model. A drawing at 1:20 looks like a drawing. Both are valid representations of the same design.