Drawing scales

Reading a scale

An architect's scale is written as a paper length that equals one foot: 1/8" = 1'-0" means an eighth of an inch on the paper is a foot of building. Turn that into the number you actually want, feet per inch of paper, by asking how many of those paper lengths fit in one inch. Eight eighths fit in an inch, so an inch is eight feet. Four quarters fit in an inch, so 1/4" = 1'-0" is four feet to the inch, which is exactly twice as large a drawing as the eighth inch one.

An engineer's scale is written the other way round, as one inch equals a round number of feet: 1" = 20'. Site plans, roads and utilities use it because a lot is measured in hundreds of feet, not in inches. The two families are not interchangeable and a set uses both, which is why the scale is stated on every drawing and not just once on the cover.

Two habits keep you out of trouble. First, read the scale of the drawing you are looking at, not the scale of the sheet, because one sheet often carries drawings at different scales and each one states its own. Second, when you are holding a print, check the bar scale before you measure anything. The words in the title block survive photocopying and resizing; the bar scale does not, and that is the whole reason it is drawn.