Why calibrate
A preset assumes the sheet was plotted at exactly its stated ratio. Often it was not: PDFs get re-plotted to fit a different paper size, drawings arrive from clients at odd ratios, and some sheets carry a scale bar but no ratio at all. Calibration sidesteps the guesswork. You tell ZeroCount how long a feature really is, draw a line along it, and the app works out a correction factor for the whole drawing.
When to calibrate, and when to pick a preset
| Situation | Do this |
|---|---|
| The title block states a standard ratio and a test measurement reads right | Pick the preset. See Setting the Drawing Scale. |
| A test measurement is out by a consistent few per cent | Calibrate against a scale bar or a long dimension. |
| The sheet has a scale bar but no stated ratio | Calibrate. |
| The drawing was uploaded before ZeroCount detected paper sizes (no paper pill in the header) | Calibrate, or re-upload. See Paper Size and Measurement Accuracy. |
| The ratio is known but not in the list | Type it on the Manual Entry tab instead. |
Calibration is only reachable under the Metric and Both scale systems, because the Architectural list has no Custom entry. If you work in Architectural, switch to Both to calibrate (answer No, Keep Existing if prompted), calibrate, then switch back to Architectural. Under Both the dialog asks for the known distance in metres.
Calibrating against a known distance
You will draw one line, so first find a feature whose real length you trust: the scale bar, a dimensioned wall, or a labelled grid spacing.
- 1
Zoom in so the feature fills a good part of the screen. See Navigating the Canvas.
- 2
In the header, click the scale select 1 : 100 and choose Custom. The Custom Scale dialog opens.
- 3
Click the Draw & Calibrate tab.
- 4
In Expected (m), type the real length of the feature (e.g. 10 for a 10 m scale bar).
- 5
Click Draw & Calibrate. The dialog closes and the line tool is armed.
- 6
CTRL+CLICK at one end of the feature.
- 7
CTRL+CLICK at the other end.
- 8
Right-click to finish the line.
As soon as the line is finished, a toast confirms Calibration applied with the factor beneath it, for example Factor: 1.0526x. The temporary line is removed automatically; it never becomes a marker in your takeoff.
Calibrate against the longest trustworthy distance on the sheet. A click that lands 1 mm off on a 10 m bar is a 1 per cent error; the same click on a 1 m dimension is 10 per cent. Zooming in before you click reduces the error further.
What you see afterwards
- The scale select shows the effective ratio, for example
1 : 105, in place of the preset name. This is the preset multiplied by the calibration factor, rounded. - Every Linear and Area group on the drawing is corrected immediately, whatever scale it uses. Lengths are multiplied by the factor once; areas by the factor squared. There is no Update Existing Groups? prompt for a calibration.
- Reopening the Custom Scale dialog shows Current calibration: 1.0526x on the Draw & Calibrate tab.
- Coverage discs in Coverage View are drawn to the drawing's scale, so they resize to match.
- The Excel export and the Live Legend use the corrected figures.
The calibration is saved with the drawing.
Resetting a calibration
If you typed the wrong expected distance, or drew the line badly, clear the calibration and go again:
- 1
Click the scale select in the header and choose the custom entry (it shows the current ratio, e.g.
1 : 105). - 2
Click Reset Calibration at the bottom of the dialog. The button only appears while a calibration is active.
The factor returns to 1 and the drawing measures at its preset ratio again. Picking any preset from the scale select also clears the calibration.
Common issues
The factor is wildly off, such as 0.02x or 48x
The expected distance was entered in the wrong unit, or the scale select was on None (ratio 0) or the wrong preset. Reset the calibration, pick the preset closest to the title block, then calibrate again with the distance in metres.
I cannot find Custom in the scale list
The drawing is in the Architectural scale system. Switch to Both as described above.
The line will not start
Plain clicks pan the canvas. Hold Ctrl (or Cmd on a Mac) while clicking each point, then right-click to finish. If you use Legacy Controls, plain clicks place points instead.
Next steps
- Paper Size and Measurement Accuracy — why some drawings need calibrating and others do not.
- Setting the Drawing Scale — presets, None and Manual Entry.
- Drawing Linear Measurements — measure a run once the drawing is calibrated.
- Coverage View — coverage discs follow the calibrated scale.