Kotelys
This guide follows French practice: French standards (NF DTU), French regulations and French bill conventions. Most sources are in French.
What 1:50 means
The scale of a drawing is the ratio between a length measured on the drawing and the corresponding real length, both in the same unit. “1:50”, written “1/50” or “1/50e” on French drawings, means that one unit on the drawing represents 50 units in reality: 1 cm on the sheet is 50 cm, 1 mm is 5 cm. The number after the colon is the denominator; the larger it is, the more the drawing is reduced.
A large scale (1:20, 1:10) shows details; a small scale (1:500, 1:1000) shows an overall view, because the fraction 1/20 is larger than 1/500.
Three formulas are enough, with E the denominator:
real length = length on the drawing × E
length on the drawing = real length ÷ E
E = real length ÷ length on the drawing
The third one is used to calculate the scale of a drawing that does not state it: a 6.00 m dimension that measures 6 cm on the sheet gives E = 600 cm ÷ 6 cm = 100, so 1:100, provided you divide two lengths in the same unit.
The conversions to know
| Scale | 1 mm on the drawing | 1 cm on the drawing | 1 real metre on the drawing | 1 cm² on the drawing |
|---|---|---|---|---|
| 1:20 | 2 cm | 0.20 m | 50 mm | 0.04 m² |
| 1:50 | 5 cm | 0.50 m | 20 mm | 0.25 m² |
| 1:100 | 10 cm | 1.00 m | 10 mm | 1 m² |
| 1:200 | 20 cm | 2.00 m | 5 mm | 4 m² |
| 1:500 | 50 cm | 5.00 m | 2 mm | 25 m² |
| 1:1000 | 1 m | 10.00 m | 1 mm | 100 m² |
Areas follow the square of the scale: at 1:100, 1 cm² of sheet represents 1 m²; at 1:50, only 0.25 m². This is also why a scale error weighs almost twice as heavily on an area as on a length: a 2% error on lengths makes about 4% on areas.
The usual scales for each type of drawing
The NF EN ISO 5455 standard sets a range of recommended scales for technical drawings, architecture included; for French building projects, drawings break down as follows:
| Drawing | Usual scale | What it can be measured for |
|---|---|---|
| Location plan | 1:2000 to 1:25,000 | No quantities: it locates the plot |
| Site plan | 1:200 or 1:500 | Roads, services, fencing, green spaces |
| Preliminary design drawings (French APS, then APD stages) | 1:200, then 1:100 | First areas and quantities |
| Project drawings (PRO stage), basis of the tender documents | generally 1:50 | Detailed take-off, item by item |
| Details | 1:20 to 1:2 | Sections, thicknesses, profiles |
These values come from the French order of 22 March 2019 on public design team duties (preliminary design at 1:200, developed design at 1:100, project drawings “generally” at 1:50, details “from 1:20 to 1:2”) and from a practical guide to French building permit applications (location plan from 1:2000 to 1:25,000 depending on the setting, site plan at 1:200 or 1:500). A set of drawings therefore mixes several scales, sometimes on the same sheet: set the scale drawing by drawing.
The scale bar
The scale bar (or graphic scale) is a small graduated ruler drawn on the drawing: a segment divided into metres (0, 1, 2, 5, 10 m), used as a standard for measuring distances. Its advantage: it is reduced or enlarged together with the drawing. On a drawing photocopied at 71% or printed with “Fit to page”, the written scale (“1:100”) becomes wrong, while the scale bar stays right. That is why the building permit guide cited below asks for the scale of the site plan and the section to be shown in both forms, written and graphic.
On a file, you can calibrate the software on the longest segment of the scale bar.
Scaling a drawing from a known dimension
When the written scale is not enough (reduced file, scan, drawing without a title block), you scale the drawing by calibration: click both ends of a known distance, type its true length, and the software works out the ratio.
- Take the longest dimension: over 20 cm of sheet, a 0.5 mm click error makes 0.25%; over 2 cm, 2.5%.
- Click the extension lines or the intersections of grid lines, zoomed in, never the text.
- Type the dimension with its unit: on a drawing dimensioned in centimetres, “1250” does not mean 1,250 m.
- A drawing with no dimensions at all is calibrated on its scale bar, or on a length measured on site.
Why a reduced PDF or a scan distorts the scale
The title block scale assumes that the sheet is at its original size. Yet a drawing gets printed, reduced and scanned, and a reduction can be applied at each step without anything showing it.
- Change of paper size. A sizes are obtained by cutting the sheet in half: their sides are in the ratio √2, and going from one size to the next reduces lengths to 71%. An A1 printed on A3 is halved.
- “Fit to page”. When printing, or “printing” to a PDF, this option shrinks the page to fit the printable area. On the same paper size, the reduction comes only from the margins: a few per cent, invisible to the eye.
- Scanning. A scan has no scale, only a resolution. If the scanned sheet was already reduced, or if the image was resized afterwards, the stored resolution no longer matches the drawing.
The true scale of the file is the title block denominator divided by the reduction factor: a 1:50 drawing reduced to 71% is actually at 1:70.7; halved, at 1:100. Areas, for their part, are multiplied by the square of the factor.
Checking on two dimensions, in two directions
A scale is always checked, even when read in the title block: measure a dimension that was not used for calibration, one horizontal and one vertical, if possible far apart on the sheet.
- Same error on both: the scale is wrong, the drawing is sound. Recalibrate, then check again.
- Different errors: the document is distorted (old print, scan that slipped, photo taken at an angle) and a single scale will not correct everything; see take-off from a scanned drawing.
- Only one dimension is off: perhaps that dimension is wrong (altered by hand, or from an earlier revision), not the scale.
No acceptable error is universal: a few millimetres on a 10 m dimension come from the click, several per cent from a wrong scale.
Worked example
A floor plan at 1:100, drawn on A1 (594 × 841 mm), arrives as an A3 PDF (297 × 420 mm): it was “printed to PDF” on the office's usual paper size. An overall dimension of 12.50 m appears on the drawing.
With the title block scale
| Line | Value |
|---|---|
| Reduction from A1 to A3: 420 ÷ 841 | 0.499 |
| 12.50 m dimension on the A1 sheet: 12,500 mm ÷ 100 | 125 mm |
| Same dimension on the A3 sheet: 125 × 0.499 | 62.4 mm |
| Measured with the title block scale: 62.4 mm × 100 | 6.24 m |
| 68.00 m² flat, measured: 68.00 × 0.499² | 16.93 m² |
True scale of the A3 sheet: 100 ÷ 0.499 = 200.4, practically 1:200. Calibrating on the dimension gives the same: 12,500 mm ÷ 62.4 mm = 200.3.
A more discreet reduction
The same drawing, this time at the right A1 size, but “printed to PDF” with “Fit to page” and a 10 mm margin on each side (taken for the example): the 594 mm height must fit into 574 mm, the 841 mm width into 821 mm. The drawing is reduced by the smaller of the two ratios.
| Line | Value |
|---|---|
| Reduction: 574 ÷ 594 (821 ÷ 841 = 0.976 is larger) | 0.966 |
| 12.50 m dimension, measured: 12.50 × 0.966 | 12.08 m |
| Error on lengths: 1 − 0.966 | − 3.4% |
| 68.00 m² flat, measured: 68.00 × 0.966² | 63.45 m² |
| Area lost: 68.00 − 63.45 | 4.55 m² |
That is 6.7% less area, with nothing visible on screen. Two checked dimensions show it at once; without checking, the error goes straight into the bill of quantities.
The most common mistakes
- Measuring with a single scale a sheet that carries drawings at several scales (plan at 1:100, details at 1:20).
- Applying the title block scale to a printed, reduced or scanned PDF without measuring a single dimension.
- Forgetting the unit: centimetres entered as metres give lengths a hundred times too large.
- Checking only one direction on a document that may be distorted.
- Believing that a 3% error on lengths makes 3% on areas: it makes almost twice that.
Doing it in Kotelys
In Kotelys, the scale is set drawing by drawing (Home tab, Scale button).
- A scale from the list (1:50, 1:100…), another scale (1:133…) or a calibration: two clicks on a dimension, then its length with its unit (“12.50 m”, “1250 cm”); an alert flags an unusual scale.
- When a PDF is imported, Kotelys looks for the scale written on the page (“Éch. 1/100”, “1:200”) and proposes it, “read on the drawing”.
- Check the scale on a dimension of the drawing gives the difference in %: under 1%, the scale is right; above that, Kotelys offers to correct it with that dimension.
- For a multi-page PDF, a tickbox applies the scale to all its pages. Without a scale, lengths and areas show “Scale to be set”.
- The scale bar placed on an exported drawing stays right at any print size.
Kotelys does not guess a scale that the drawing does not state, and a misread dimension would give a wrong scale: checking remains your job.
Further reading: the measuring tools in Kotelys, and the other guides.
Sources
- Office québécois de la langue française, terminology dictionary: « échelle numérique » (definition, scale bar)
- AFNOR, NF EN ISO 5455 (October 1994) « Dessins techniques - Échelles »: range of recommended scales, architecture included
- French order of 22 March 2019 on the technical performance of design team duties (drawing scales by stage), text reproduced by marche-public.fr
- Flers Agglo, practical guide « Permis de construire, déclaration préalable » (scales of location, site and section drawings; written and graphic scale)
- Markus Kuhn, University of Cambridge, “International standard paper sizes” (ISO 216 A sizes, √2 ratio, reduction to 71%)
Thresholds and values described as “taken for the example” are not rules: those of your contract (specification, bill of quantities) always prevail.