What Is an Engineer's Scale?

An engineer’s scale is an instrument that is similar to a ruler and is used to measure scale on technical drawings. It is shaped like a long triangular prism, about 12 inches (30.5 cm) long, with each side of each point marked with a different scale for measuring. A total of six scales are used. The engineer’s scale is an important tool for reading blueprints in construction and civil engineering.

When a large structure must be built, full-size drawings of the plan for the structure are rarely feasible. To overcome this problem, smaller drawings are created “to scale.” In other words, smaller measurements on the paper represent the larger measurements in reality. For example, an engineer might work from a scale drawing where a distance of 1 inch on paper represents 10 feet in reality, or a distance of 1 centimeter represents 10 meters, depending on the engineer's preferred measurement system.

An engineer’s scale allows engineers to read the real dimensions of objects from a blueprint drawing. The tool is placed on the drawing, and the measurement is read and interpreted based on the ratio of the scale, which is given at the side of each edge before the markings. For example, using a scale where 1 inch (2.54 cm) represents 10 feet (3.05 m), if the length of a wall on a blueprint measures 4 inches (10.16 cm), the wall is 40 feet (12.2 m) long. Subdivisions between marks can be used to indicate smaller increments.

Many different scale ratios can be used, depending on the need. A typical engineer’s scale uses gradations that translate into large measurements, with 10 feet (about 3.05 m) being the usual minimum. Some instruments might even have markings for translating distances that are miles or kilometers in length. Such measurements might be necessary in laying pipeline or other large-scale tasks.

An engineer’s scale measures long distances rather than shorter ones because it is mainly used in civil engineering. A similar instrument called an architect’s scale is used for measuring smaller distances — for example, in determining space and dimensions within a building. The architect’s scale might have a system of markings where 0.25 inches (about 6 mm) on the tool represents 1 foot (about 305 mm) in actual space.

Engineer’s scales are used worldwide and can be purchased for use with either metric or imperial units. Although they are often made out of white plastic, it is also possible to buy wood and metal versions. Most engineers prefer to use a straightedge or common ruler to draw straight lines in the process of drafting a blueprint, but the engineer’s scale can also be adapted for this purpose if necessary.