A note on scales 

This table lists the common metric scales and their approximate imperial equivalents.

Metric units are based on ratios of the same unit of measure. A scale of 1:500 means 1 unit of measure in the drawing or model equals 500 units of the same measure in full scale. The base unit of measurement (m, cm, mm) is always provided on the drawing or model in addition to the scale ratio. For example: 1:500 means 1 cm = 500 cm (5m), or 1:20 means 1 mm = 20 mm.

Imperial units are based on ratios of inches to feet. A scale of 1”:40’ means 1 inch in the drawing or model equals 40 feet in full scale. For example: 1”: 40’ means 1 inch = 40 feet.