Guides

Text height for scale 1:100: the sum, the chart, and how to stop doing it

The text height for scale 1:100 is the height you want on paper times 100. So 2.5 mm lettering on the printed sheet is 250 mm tall in the drawing, and 3.5 mm is 350 mm. At 1:50 you multiply by 50 instead: 2.5 mm is 125 mm, and 3.5 mm is 175 mm.

That is the whole rule. Some of the charts you'll find online divide where they should multiply and give you 0.025 for 1:100. That number is for text drawn the wrong way round. If you draw at full size, which you should, the text in the drawing has to be big, because the plot shrinks everything a hundred times.

The rule, stated once

Model-space text height is the paper height times the scale denominator. The denominator is the second number in the scale: the 100 in 1:100, the 50 in 1:50. That number is also what AutoCAD users call the text height scale factor.

• 2.5 mm at 1:100 is 2.5 × 100 = 250 mm in the drawing.
• 3.5 mm at 1:50 is 3.5 × 50 = 175 mm in the drawing.
• 5 mm at 1:20 is 5 × 20 = 100 mm in the drawing.

These figures assume you draw in millimetres. If your drawing unit is the metre, move the decimal point three places: 250 mm becomes 0.25. That is probably where the 0.025 figures come from, someone mixing up units with scale. Check your units before you trust any chart, this one included.

For an enlargement the sum goes the other way, because the drawing is smaller than the print. At 2:1, 2.5 mm on paper is 1.25 mm in the drawing.

Text height scale chart

Paper heights across the top, scale down the side. Every figure in the body of the table is the height to type into a full-size drawing in millimetres.

The scales in the left column are all from the permitted list in ISO 5455. A scale there is full size, a reduction of 1:2, 1:5, 1:10 and their decimal multiples, or an enlargement of 2:1, 5:1, 10:1 and theirs. Nothing in between, so you'll never need a row for 1:75.

Which paper height to pick

The chart only helps once you know the paper height. ISO 3098-1 gives a series: 1.8, 2.5, 3.5, 5, 7, 10, 14 and 20 mm. Each step is √2 larger than the last, the same ratio as one A size to the next. A drawing made for A1 and reduced to A3 drops each line of text neatly to a smaller standard size, and it still reads.

The usual choices:

• Dimension figures: 2.5 mm.
• Notes: 3.5 mm.
• Titles: 5 or 7 mm.

The common convention ties this to the sheet: dimension figures and notes at 2.5 mm on A4 to A2, and at 3.5 mm on A1 and A0, with titles a size or two larger. Don't go under 1.8 mm. Nothing smaller is legible on a plot.

The proportions behind the height

The height is the capital height, h. For lettering B, the one CAD uses, the rest follows from it:

• Stroke width is h/10, so 0.25 mm for 2.5 mm text. (Lettering A uses h/14.)
• Lower-case letters are 0.7h.
• The gap between characters is 0.2h.
• Lines of text sit at least 1.4h apart, so 3.5 mm between lines of 2.5 mm text.

Upright is the default. Sloped lettering leans 15° from vertical. Whichever you use, ISO 129-1 asks that text reads from the bottom or the right-hand edge of the sheet, never upside down, and that no line crosses it.

Which scale for which drawing

The usual convention for building work: site plans at 1:500 to 1:1250, floor plans and elevations at 1:100 or 1:50, sections at 1:50 or 1:20, details at 1:10 to 1:1. Mechanical parts go at 1:1 or 1:2, and small parts at 2:1 or 5:1. Choose the scale so the detail reads, not so the text fits.

Whatever the scale, the dimension figure is always the true size. A wall 3600 mm long is dimensioned 3600 at 1:100 and at 1:50. And the drawing is never scaled off with a rule. The scale is written in the title block as SCALE 1:50; where a sheet holds views at more than one scale, the main one goes in the title block and every other view is titled with its own.

Where the sum goes wrong

Multiplying 2.5 by 100 isn't hard. The trouble is what happens after.

• Mixed scales on one sheet. A 1:50 plan with a 1:10 detail beside it needs two text heights for the same printed size: 125 mm and 25 mm. Put the wrong one in the detail and it prints at 12.5 mm.
• A change of scale late on. The client wants the plan at 1:50 instead of 1:100. Every note, every dimension style, every title now has to halve.
• Units. A drawing in metres and a chart in millimetres give you text a thousand times wrong, and you don't find out until the plot.
• Blocks and title blocks that were drawn for one scale and inserted at another.

AutoCAD's own answer is annotative text. You set the text at its paper height and give it an annotation scale, and the text sizes itself to the viewport's scale. Autodesk documents it as the way to handle exactly this problem, and it works well once a template is set up for it. Many offices still use the older way of typing the multiplied height into each text style, which is why the charts exist.

Setting lettering in paper millimetres instead

The cleaner fix is to stop treating text height as a size in the drawing at all. It is a size on paper. That is how ISO 3098 defines it, and that is what you actually care about.

This is how isogram handles it. You set text height as the height on the printed sheet, in millimetres, and the app works out its size on the drawing from the scale. Pick 2.5 mm and it prints at 2.5 mm, whether the view is 1:100, 1:50 or 1:5. There's no factor to multiply, so there's nothing to get wrong, and a scale change late in the job doesn't mean editing every note.

The rest follows the same standards: lettering to ISO 3098, dimensions to ISO 129-1, scales to ISO 5455 and sheets to ISO 5457 (or ASME, if your office works to that). You draw at full size in Draft, lay out the sheets with viewports, title blocks and revisions in Sheets, and publish the set as one PDF. Before the set goes out, isogram reads every sheet for what a senior drafter would catch, including lettering too small to print.

If you're coming from AutoCAD, the habits carry over. The command line takes the familiar aliases, F3, F8 and F10 do what you expect, and the AutoCAD commands guide lists the rest. Your existing DWG files open, from R13 to the 2018 format; the guide to opening DWG files covers what comes across, and dimensions keep the figure AutoCAD wrote.

The short version

• Text height in the drawing = paper height × scale denominator.
• At 1:100: 2.5 mm is 250 mm, 3.5 mm is 350 mm, 5 mm is 500 mm.
• At 1:50: 2.5 mm is 125 mm, 3.5 mm is 175 mm, 5 mm is 250 mm.
• Pick paper heights from 1.8, 2.5, 3.5, 5, 7, 10, 14, 20 mm, and never go below 1.8 mm.
• Or set heights in paper millimetres and let the software do the sum.

isogram's public beta opens in December 2026 on the Mac and Windows, with Linux following in an update, and it's free to test. Join the list and you'll be emailed the download link.

Questions

31 October 2026

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