How to read a metric scale rule
Read the blue line off the scale rule. Get 10 right to finish. The box below also lets you switch to an architect's scale.
The blue line is laid on the 1:100 edge of a metric scale rule, starting at zero. How long is it in real size?
How to read it
A metric scale rule has a different ratio on each edge. Zero is at the end, and the numbers are usually in metres. Pick the edge that matches the scale on the drawing, put zero on one end of the line, and read where the other end falls.
| Edge | 1 cm on paper is | Each small mark (1 mm) is |
|---|---|---|
| 1:20 | 0.2 m | 20 mm |
| 1:50 | 0.5 m | 50 mm |
| 1:100 | 1 m | 100 mm |
| 1:200 | 2 m | 200 mm |
Common mistakes
- Using the wrong edge. A reading on the 1:100 edge of a 1:50 drawing is twice too long.
- Treating a small mark as 1 mm real. It is 1 mm on paper, which is 50 mm real at 1:50.
- Measuring an imperial drawing with a metric rule. 1/4" = 1'-0" is 1:48, not 1:50.
Questions
- What scales are on a metric scale rule?
- Most show 1:1, 1:5, 1:10, 1:20, 1:50, 1:100, 1:200, 1:500 and sometimes 1:1250 or 1:2500. Building plans use 1:50 and 1:100, details use 1:20 down to 1:5, and site plans use 1:200 or 1:500.
- Are the numbers on a scale rule in metres or millimetres?
- Usually metres on the building scales. On the 1:100 edge each numbered step is 1 m, on 1:50 it is 0.5 m and on 1:200 it is 2 m. Always check the labels on your own rule.
- Can I read a 1:100 drawing with the 1:50 edge?
- Yes, if you double what you read. A 1:100 drawing is half the size of a 1:50 drawing, so every reading on the 1:50 edge is half the real length. It is easier and safer to use the matching edge.
Explained in our own words and checked against trade school textbooks. Metric scales follow the ISO 5455 series.