Guide · Drawing scale
How to set and check the scale on a PDF drawing
A wrong scale is the most expensive mistake in a takeoff, because every quantity on the sheet inherits it. Here is how scale works on a PDF, how it goes wrong, and how to set it so you can trust the numbers.
Don't trust the scale in the title block on its own. Calibrate each sheet by clicking the two ends of the longest known dimension you can find and entering its value, then check the result against a second dimension somewhere else on the sheet. If the two disagree, the PDF has probably been resized or the scan has been stretched.
How scale works on a PDF
A drawing scale is a promise that holds only at one paper size. A floor plan marked "1:100 at A1" is true when printed on A1 paper. Save or print that same sheet onto A3 and every line is about half as long, but the title block still says 1:100.
| Drawn at | Printed or saved as | Real scale on the page | Areas if you trust the title block |
|---|---|---|---|
| 1:100 at A1 | A1 | 1:100 | Correct |
| 1:100 at A1 | A3 | about 1:200 | About 4 times too small |
| 1:50 at A1 | A3 | about 1:100 | About 4 times too small |
| 1:100 at A3 | A1 | about 1:50 | About 4 times too large |
Each step down the A series (A1 to A2 to A3) halves the sheet's area. Two steps, A1 to A3, halve every length and quarter every area. Use the drawing scale calculator to work out any combination.
How to calibrate a sheet
- Find a long, reliable dimension.An overall building length or a grid line spacing is better than a door width. The longer the dimension, the smaller the effect of an imprecise click.
- Zoom in on each end.Click exactly on the dimension's extension lines or arrowheads, not the text.
- Enter the real length.Type the value written on the drawing, in the unit you will measure in.
- Check a second dimension.Measure a different dimension, ideally running the other way on the sheet. It should agree to within a fraction of a per cent.
- Repeat on every sheet.Never assume sheet A-102 is at the same scale as A-101, even in one set.
In Project Blueprint, step 3 is Measure › Calibrate, and step 4 is Measure › Verify Scale: draw along a dimension line and Blueprint reads the figure beside it, then offers the right scale if yours is out. Calibrating one sheet changes only that sheet.
Four things that break the scale
1. Resized prints and exports
The most common cause. Someone printed to PDF with "fit to page", or the consultant exported A1 sheets onto A3 for email. Calibrate from a dimension and the problem disappears.
2. Stretched scans
Scanners and photocopiers sometimes stretch a drawing more in one direction than the other. The giveaway: a horizontal dimension checks out, but a vertical one is off by a few per cent. The fix is separate horizontal and vertical scales. Blueprint lets you set separate X and Y scales on a sheet for exactly this.
3. Mixed scales on one sheet
A sheet might carry a 1:100 plan and several 1:20 details. Measure the details at the plan's scale and every length is five times too big. Give each detail its own scale. In Blueprint, Measure › Viewport Region marks part of the sheet with its own scale.
4. Dimensions that are wrong on the drawing
Occasionally a figure is edited by hand and no longer matches the geometry. If two dimensions disagree with each other but the third agrees with the first, trust the pair and raise an RFI about the odd one out.
Checks worth building into every takeoff
- Verify the scale on every sheet before measuring anything on it.
- Sense-check one room you can calculate by hand, such as a rectangular bedroom.
- Run an automatic check before quantities go out. Blueprint's Measure › Check Takeoff flags measurements on sheets with no scale, and scales that disagree with the one printed on the sheet.
- Note the scale and how you set it at the top of your takeoff, so the next person can check it.
Working in imperial? The same method applies. Blueprint measures in mm, cm, m, ft and in, with metric as the default.
Frequently asked questions
What does 1:100 mean on a drawing?
One unit on the paper equals 100 of the same unit in reality. At 1:100, 10 mm on the printed sheet is 1,000 mm, or 1 m, on site. The scale only holds when the sheet is printed at its stated paper size.
Why is my PDF measuring wrong?
Usually because it was printed or exported to a different paper size than the one in the title block, such as an A1 drawing saved as A3. The drawing shrinks but the printed scale doesn't change. Calibrate from a known dimension instead of trusting the title block.
How accurate is calibrating from a dimension?
It depends on the length you calibrate from and how precisely you click. Calibrating from the longest dimension you can find, then checking a second one elsewhere, keeps the error small. If the two disagree by more than about one per cent, look for a stretched scan or a dimension that is wrong on the drawing.
What if a sheet has details at different scales?
Give each detail its own scale. In Project Blueprint, Measure › Viewport Region marks part of a sheet with its own scale, so a 1:20 detail and a 1:100 plan on the same sheet both measure correctly.
Can I measure a drawing marked NTS?
No. NTS means not to scale. Use the written dimensions instead, or ask for a scaled drawing.
Sources
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