Plans for a building cannot conveniently be made of the full size of the building, so they are made much smaller but drawn to scale, so that the proportions on the drawings all bear the same relation to one another that the same portions will on the completed building. The general drawings consisting of floor plans and elevations are generally drawn to a scale of 1/4-inch to 1 foot, and the details made to a scale of 3/4-inch to 1 foot. Sometimes the general plans are drawn to the smaller scale of 1/8-inch to the foot. By 1/4-inch to the foot is meant that 1/4-inch in length on the plan will equal 1 foot on the building, and that, consequently, a pipe 3 inches long on the plans, when drawn to a scale of 1/4-inch per foot, will be 12 feet long in the building. If on the other hand the scale were 1/8-inch to the foot 3 inches on the drawing would be equal to 24 feet in the building. The scale to which drawings are made is always marked on the plans, and having this scale furnished, an estimator possesses the key for determining the amount of pipe in the building.

The process of measuring materials on plans is known as the process of "scaling." The manner of scaling pipe by means of a 2-foot rule is shown at (a) in Fig. 1.

In the example, assume that the scale is 1/4-inch to the foot, and it is desired to know the length of pipe shown in the illustration. By measuring, it will be found that the length of the pipe over all is just 6} inches. As each -1/4-inch on the plan equals 1 foot of pipe, and there are 6 1/2 x4 = 26 quarters, or 6 1/2 inches, of pipe on the plan, there must be 26 feet of pipe required for this installation.

As was just shown, an ordinary 2-foot rule can be used for scaling plans, but when a great amount of scaling is to be done, much better time can be made, without the like liability to error, by using a regular scaling rule. One type of a scaling rule is shown at (6) in Fig. 1 It will be noticed that the 1/4-inch marks are numbered consecutively, so that by reading the number opposite any mark will give in feet the length of pipe scaled. For example, opposite the end of the pipe is the quarter mark, numbered 26, which indicates that the section of the pipe measured is 26 feet long.

A very convenient rule for scaling is made triangular in shape and has 3/32, 1/8, 3/16, 1/4, 3/8, 1/2, 3/4, 1, 1 1/2, 2 and 3 inch scales marked on its various sides, which enable it to be conveniently used when taking off quantities from plans having any of the foregoing scales per foot.

Plumbing details are usually drawn to a different scale than the plans and elevations. In the details the intention is to show more clearly than can be indicated on the plans just how certain portions of the work are to be done. In order that the drawings will be large enough to show all necessary detail, they are generally made to scales of from 1/2 inch per foot to 1 inch per foot or even larger if occasion requires.