Calibrate scale

Pick two points a known distance apart, then type that distance. Everything you measure afterwards uses it. A printed dimension makes the best reference, and a long one beats a short one.

Why you would use it

Nothing on a drawing has a real size until you tell the software what one known line means. Set it once per sheet and everything you measure afterwards comes out in millimetres, metres or feet.

For example

What to do

1. Pick the tool, then click one end of a dimension you already know.
2. Click the other end.
3. Type the real distance and pick its unit.

Going further

The panel stays open while you work, so you can zoom out to read a faint dimension and back in to place the points. Pick the longest known line on the sheet: an error in a short reference is multiplied across everything you measure with it.

If you already use another program

In a CAD program: You draw at true size, so there’s nothing to set. The nearest thing is scaling a pasted drawing until a known distance reads right.

In a PDF markup program: Calibrate, or Set scale: click two points a known distance apart and say how far it is. Here it’s kept per page, and nothing is measured until it’s set.