How to set scale on a scanned plan
Quick answer
Scanned or photographed plans have no reliable paper size, so scale presets can't be trusted. Either let AI detect the scale from a scale bar or labelled dimension on the sheet, or draw a line over a dimension you know the real length of and enter that length — both calibrate from pixels, so they stay accurate on a scan.
Ratio presets like 1:100 only work when the PDF is at its true paper size. A scan, a photo of a drawing, or a PDF that's been resized loses that guarantee — so measuring against a preset gives wrong numbers.
The fix is calibration: tell the tool one real-world distance and let it work out the rest. Here's how to do it accurately.
Step by step
- 1
Try ✨ Detect first
In the set-scale dialog, run AI detection. On a scan it will look for a graphic scale bar or a labelled dimension and calibrate from those — both are measured in pixels, so they stay correct even though the scan has no true paper size. It reports what it used and how confident it is; check the figure before applying.
- 2
Find a known dimension on the drawing
Look for a stated dimension line, a structural grid spacing, a door width, or a printed scale bar — anything where you know (or can read) the real length. The longer and clearer, the more accurate your scale.
- 3
Choose "draw a known dimension"
In the set-scale dialog, choose to draw a known dimension (calibrate) rather than a preset ratio.
- 4
Draw along that dimension
Click one end and then the other end of the known distance, as precisely as you can. Zoom in first — a few pixels of error at high zoom is far less than at fit-to-screen.
- 5
Enter the real length and units
Type the true length (e.g. 5.4) and pick the unit (m, mm, ft…). The tool converts your pixel measurement into a real-world scale for that page.
- 6
Verify against a second dimension
Measure a different known dimension and check it reads correctly. If it's off, recalibrate using a longer reference line.
- 7
Measure as normal
With the scale set, run your linear, area and count take-off — every measurement on the page is now in real units.
Tips & gotchas
- •Calibrate off the longest known dimension you can find — it minimises error.
- •Scale is per page: multi-sheet PDFs at different scales need calibrating per sheet.
- •Photos taken at an angle are distorted and won't measure true — use a straight-on scan where possible.
- •If the scan is skewed, calibrate along the same axis you'll mostly measure in.
Frequently asked questions
Why don't scale presets work on a scanned plan?
Presets assume the PDF is at its true paper size, so one PDF point equals a known real distance. A scan or resized PDF breaks that assumption, so presets give inaccurate measurements. Calibrating from a known dimension avoids the problem.
What if there's no dimension on the drawing?
Use any element whose real size you know — a standard door leaf (often ~0.9 m), a grid spacing, or a printed scale bar. Accuracy improves with longer references.
Do I have to set the scale on every page?
Yes if pages are at different scales. Scale is stored per page, so calibrate each sheet you measure.
Does AI scale detection work on a scan?
Often, yes. It prefers a graphic scale bar or a labelled dimension, which calibrate from pixels and so are unaffected by the scan having no true paper size. A stated ratio alone is flagged as lower confidence on a scan, because presets can't be trusted there. Always verify the proposed scale against a second known dimension.
Can I do this for free?
Yes — AI detection and draw-to-calibrate are both included on Solid Takeoff's free plan.
Try it on your own plan — free
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