How to find the scale of a drawing

Scale bars go missing, PDFs get printed on the wrong paper size, and screenshots are any size at all. As long as one length on the drawing is known, you can still measure it accurately.

The idea

Scale is just a ratio between a length on the drawing and the same length in real life. Find one length you know for certain, measure it on the drawing, and the ratio follows. Every other line on the same drawing then converts the same way.

Good known lengths

• A written dimension on the drawing, such as an overall width or a room size.
• A standard item: a single bed is 90 cm wide in the UK; internal doors are usually 76 to 84 cm; a brick is 215 mm long.
• A scale bar, even a small one: calibrate along its whole length.
• Something you can measure on site with a tape.

Doing it in ScaleDraw

1. Add the drawing

   Drag it onto the board.

2. Calibrate

   Press Calibrate, drag along your known length and type it in. ScaleDraw works out the scale and remembers it for that picture.

3. Check

   Measure a second thing you know. If it's off, recalibrate on a longer dimension.

For a PDF that was produced at a stated scale such as 1:100, you can type the ratio instead. See drawing scales explained.