Drawing Scale Calculator
Convert between real dimensions and drawn dimensions at any scale — and find the scale that fits a building on a sheet, which is the question that actually comes up.
At 1:100, one millimetre on paper is 100 in the world; a 12-metre wall is 120 mm long.
How the drawing scale calculator works
At 1:100, one millimetre on paper is 100 in the world; a 12-metre wall is 120 mm long. The other way round, a line you measure at 35 mm on a 1:50 drawing is 1.75 metres.
The practical question is which scale fits: a 45-metre building on an A1 sheet 841 mm wide needs at least 1:60, so it goes on at 1:100 with room for dimensions.
Formula: drawn = real / scale; real = drawn × scale
Worked examples
| Inputs | Drawn dimension (mm) | Note |
|---|---|---|
| 12 m at 1:100 | 120 | 120 mm on paper |
| A detail at 1:5 | 90 | 90 mm |
| A site plan | 360 | fits an A1 at 1:250 |
FAQFrequently asked questions
What does 1:100 mean?
One unit on the drawing is 100 in reality. A metre becomes a centimetre; a 12-metre wall becomes 120 mm.
Which scale for which drawing?
Site plans at 1:500 or 1:200, floor plans at 1:100 or 1:50, details at 1:20 to 1:5. The scale is chosen so the sheet fits and the information reads.
Why does area scale by the square?
Because both dimensions shrink. At 1:100 a square metre becomes a square centimetre — 10,000 times smaller.
What if my drawing has been printed at the wrong size?
Measure something you know — a door at 900 mm — and derive the true scale from it. Never trust a scale bar on a photocopy.
Does this apply to BIM?
Models are 1:1; scale only appears on the sheet. This page is for the sheet, and for reading paper drawings that were never modelled.
Where these figures come from
- Sabine (1922) — Collected Papers on Acoustics — the reverberation-time equation
- CIBSE Guide A — Environmental design — daylight, comfort and ventilation criteria
- Neufert — Architects' Data — the standard reference for space and dimension
- International Code Council — International Building Code — occupant load and egress factors
- Building Regulations — Approved Documents — the UK building regulations
Last checked: September 2026. Formulas are the standard building-science and code relationships; default thresholds are common international values and should be replaced with the ones in your code.