An engineer's scale is a ruler, a tool for measuring distances, used in countries which employ Imperial units or the U.S. customary units instead of the Metric System. It is commonly made of plastic and is just over twelve inches long, so that the measuring ticks at the edges do not become unusable by wearage. It is used in making engineering drawings, commonly called blueprints, in scale. For example, "one-tenth size" would appear on a drawing to indicate a part larger than the paper itself. It is not to be used to measure the machined parts to see if these meet the specifications.

This scale is divided into decimalized fractions of an inch, but has a cross-section like an equilateral triangle, which enables the scale to have six edges indexed for measurement. One edge is divided into tenths of an inch, and the subsequent ones are directly marked for twentieths, thirtieths, fortieths, fiftieths, and finally sixtieths of an inch.

The engineer's scale came into existence when machining parts required a greater precision the usual, binary fractionalization of the inch as in the Architect's scale for houses and furniture. Since Britain has become metricized, as Canada and Australia before it, only the USA has practical need for these instruments. They were used, for example, in laying out printed circuit boards with the spacing of leads from integrated circuit chips as one-tenth of an inch. In the twenty-first century, those which are commonly purchased in the US are actually made in Germany.