Architectural scales explained
A drawing scale like 1:50 means every unit on paper represents 50 of the same unit in reality — 1 cm on the sheet is 50 cm of building. This guide covers the scales you'll actually meet, what each is used for, the conversion maths, and the catch that bites everyone: stated scales stop being true on a screen.
Common metric scales
| Scale | 1 cm equals | Typically used for |
|---|---|---|
| 1:5 | 5 cm | Construction details — junctions, joinery sections |
| 1:10 | 10 cm | Details, furniture, cabinetry |
| 1:20 | 20 cm | Room layouts, interior elevations, kitchen plans |
| 1:50 | 50 cm | Floor plans and sections of a single dwelling |
| 1:100 | 1 m | General-arrangement floor plans, planning drawings |
| 1:200 | 2 m | Whole-building plans, small site plans |
| 1:500 | 5 m | Site/block plans |
| 1:1250 | 12.5 m | Location plans (the standard UK planning-application scale) |
Common imperial (US) scales
US architectural scales are written as inches per foot. Because a foot is 12 inches, they convert to slightly odd ratios:
| Scale | Ratio | Typically used for |
|---|---|---|
| 3″ = 1′-0″ | 1:4 | Full construction details |
| 1½″ = 1′-0″ | 1:8 | Details, millwork |
| 1″ = 1′-0″ | 1:12 | Details |
| ½″ = 1′-0″ | 1:24 | Room-scale plans, interior elevations |
| ¼″ = 1′-0″ | 1:48 | The classic US floor-plan scale (≈ metric 1:50) |
| ⅛″ = 1′-0″ | 1:96 | Whole-building plans (≈ metric 1:100) |
| 1″ = 20′ | 1:240 | Civil/site plans (engineer's scale) |
The conversion maths
Real length = measured length on the drawing × the scale factor.
- You measure 84 mm on a 1:50 plan → 84 × 50 = 4200 mm = 4.2 m in reality.
- You measure 2½″ on a ¼″ = 1′-0″ plan → 2.5 ÷ 0.25 = 10 ft.
- Going the other way, a 3.6 m wall on a 1:100 drawing should measure 3600 ÷ 100 = 36 mm on the sheet.
Areas scale by the square of the factor: 1 cm² on a 1:50 drawing is 50² = 2500 cm² = 0.25 m² of real floor. This is why a small tracing error matters twice as much for areas as for lengths.
The catch: stated scales die on screens
All of the above assumes the drawing is at its original printed size. In practice you're looking at a PDF, zoomed to whatever fits your monitor, possibly resized when it was exported, or printed "fit to page" (which silently rescales by a few percent). The title block still says 1:100, but a centimetre on your screen is no longer a metre of building.
The robust fix is to ignore the stated scale entirely and calibrate against a dimension printed on the drawing: tell a tool the real length of one labelled wall, and the true scale of your copy at your zoom is known exactly. That's precisely what Scale Rule does — see how to measure a PDF floor plan.
Reading a scale bar
Many plans carry a graphic scale bar — a ruler drawn on the sheet, marked 0–1–2–5 m or 0–10–20 ft. Because the bar is part of the drawing, it resizes with it, which makes it the one scale indicator that survives resizing. If your plan has one, it's the ideal calibration target: draw your calibration line across the bar's full marked length and enter that value.
Skip the mental arithmetic. Calibrate once and read real dimensions straight off any plan — free, in your browser.
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