Help · Calibration
How to Calibrate the Scale of an Image to Measure It
A photo or a scanned plan has no real units — only pixels. Calibration tells MetricCanvas how many pixels equal one metre, centimetre or inch, so every line you draw afterwards shows a real-world length. This guide explains how to calibrate an image correctly and how to avoid the most common scale errors.
What image scale calibration does
Calibrating an image means pairing a distance you can see in the picture with its known real length. MetricCanvas divides the pixel length of that reference by the value you type and stores the result as the scale (for example, 1 m = 84.20 px). You can see the active scale at any moment in the Scale panel of the sidebar.
Without calibration the measure tool has nothing to convert pixels into, so you always calibrate first. If you calibrate again later, every existing measurement is recalculated automatically with the new scale.
Step by step: calibrate with a known length
1. Upload your image by dragging it anywhere onto the page or by clicking the upload box. 2. Select the Calibrate tool (ruler icon) in the toolbar. 3. Click the first end of a dimension you know — a dimension line on a plan, a door width, a scale bar or a tape measure in the photo. 4. Click the second end, or simply click and drag between both ends. 5. Type the real length, choose the unit (m, cm, mm, ft or in) and press Enter or Save.
Hold Cmd (Ctrl on Windows) while drawing to keep the reference perfectly horizontal or vertical, or Alt to snap it to 45° steps. This makes it much easier to follow straight dimension lines on technical drawings. Press Esc at any time to cancel.
Calibrate with a circle diameter
Some references are round: a coin, a pipe, a manhole cover or a round table on a plan. In the calibration dialog choose Diameter; MetricCanvas draws the circle so you can check that it matches the object edge before saving.
Mark the diameter through the centre of the circle. Measuring a chord that misses the centre gives a shorter reference and makes every later measurement too large.
Tips for an accurate scale
Use the longest reliable reference you can find: an error of a few pixels matters much less on a 5 m wall than on a 20 cm tile. Zoom in (pinch, Ctrl/Cmd + scroll or the + key) before placing the points, and drag the round handles afterwards to fine-tune the ends — they highlight when you hover over them.
Keep the reference in the same plane as the things you want to measure. In photos, perspective makes distant objects look smaller, so a reference on another wall gives a wrong scale. Once the calibration line is right, select it and click the small padlock (or press K) so you cannot move it by accident.