Help · Circle tool
How to Measure the Diameter or Radius of a Circle on an Image
Round objects are hard to measure with a straight line: you never know if you crossed the exact centre. The MetricCanvas circle tool draws the full circle while you measure, so you can match it to the edge of the object and read its diameter or radius in real units.
Choose the circle tool and the mode
Press C or click the circle icon in the toolbar. Two buttons appear next to it: Ø for diameter and R for radius. Press C again at any time to switch between both modes; your choice is remembered next time.
As with every measurement, calibrate the image first so the result is shown in metres, centimetres or inches. Press L to go back to the straight line tool.
Measure by diameter
In Ø mode, click one edge of the round object and then the opposite edge, or click and drag across it. A circle preview follows the cursor so you can see whether it fits the outline. The label shows the diameter, for example “Ø 1.20 m”.
Hold Cmd (Ctrl on Windows) to keep the diameter horizontal or vertical, which helps when the object is aligned with the plan grid.
Measure by radius
In R mode, the first click sets the centre and the second click sets a point on the edge. This is ideal when the centre is marked — the axis of a column, the centre of a roundabout, a compass point on a drawing — or when only part of the circle is visible.
The label shows the radius, for example “R 0.60 m”, and a small dot marks the centre. Drag either handle to adjust the circle; select it and press K to lock it when it is final.
Practical uses
Check whether a round dining table fits a room, measure the diameter of a pipe or duct on a technical drawing, estimate a tree canopy or a roundabout on a satellite image, or size a round rug or window. Circle measurements are included in the CSV export with their type (diameter or radius).
If you need the area, use the diameter in your spreadsheet: area = π × (diameter ÷ 2)².