The architect's scale and engineer's scale are quite similar in appearance but they have different scale markings and are used for different purposes. Here's how they compare to each other.
The architect's scale is used by architects and builders for obtaining measurements from scaled drawings of buildings and other structures. It is typically three-sided with 11 scales or flat with 4 scales. It is available in lengths ranging from 4" to 36" although 12" models are most common. Once made primarily from wood, most architect's scales are now made from plastic, aluminum, and stainless steel.
Common scales for a triangular architect's scale are: 16 (full), 3, 1-1/2, 1, 3/4, 1/2, 3/8, 1/4, 1/16, 1/8, 3/32. Major divisions of each scale represent feet which are further subdivided into 12ths or 16ths. A scale labeled 1/8 translates to 1/8" = 1' or 1/96 scale. The 16 or full scale is a standard ruler with each mark representing 1/16". An architect's scale is read from the left or right side depending on the scale being used.
The engineer's scale, also known as a civil scale, is used for measuring length and transferring length measurements at different scales or proportions of actual length. It is triangular in cross-section with each of the three faces containing two different scale markings for a total of six scales. In North America, the scales are labeled 10, 20, 30, 40, 50 and 60, with the graduations representing decimal fractions of an inch. Thus, one inch on a drawing equals between 10 to 60 feet.
The engineer's scale is usually made from plastic and is slightly over 12 inches in length, but with only 12 inches of markings, leaving the ends unmarked. The extra length accounts for wear and tear over time and prevents the end ticks from wearing off.
Although similar in appearance to an architect’s scale, the engineering scale is designed to be more precise and has a decimal scaling scheme whereas an architect's scale uses fractional scaling. It is designed to only be read from left to right. An architect's scale can be read from either the left or right side.
The 1/4 scale of an architect's scale is a common scale for floor plans of houses and average size structures. This scale makes drawings manageable in size while allowing for a fair amount of information to be recorded. Larger structures such as shopping malls would require a smaller scale such as 1/8 or 3/32 to still fit the drawing on a single sheet of paper. Features such as doorways would require a larger scale, say 1-1/2, to show sufficient detail.
The engineer's scale is used by civil engineers for designing bridges, roads, water mains, and topographical features. Typical scales in these applications are 1" = 100' for plan views of highways, and 1" = 5' vertical and 1" = 100' horizontal for profile views. It is also used when a greater precision is required than that afforded by an architect's scale. An example is laying out printed circuit boards with the spacing of integrated circuit leads set to one-tenth of an inch.
To take a measurement with an architect's scale, first find the scale that matches the scale of the drawing. If the drawing scale is 1/4 inch equals 1 foot, the 1/4 scale would be used. Align the zero mark of the architect's scale with one side of the object being measured and obtain the foot measurement on the other side. This will be the closest large mark that the line passes. Shift the tool slightly to align the end of the line with the closest foot mark with the other end protruding into the end section of the scale that contains the smaller divisions. Each of these divisions represents one inch. Find the inch mark that lines up with the end of the line being measured.
To take measurements with an engineer's scale, first obtain the scale in the legend of the drawing. It will be something like 1" = 20' or perhaps as a ratio like 1:20. Locate the matching scale on the engineer's scale and line up the zero on the left side with the object being measured. Find the tick mark that lines up with the other end. Read the major mark and add the minor mark as tenths. Multiply this value by 10 to get the actual length.
Example: Suppose you're measuring a power line and the drawing shows a scale of 1:20 (1 inch equals 20 feet). Using the scale marked "20", you obtain a length measurement of 7.4. In this scenario, the actual length is 74 feet (7.4 times 10).