3.0: Prelude to Scales

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Learning Task 3

Use scale rulers to determine actual dimensions from drawings

Scale drawings are accurate and convenient visual representations made and used by engineers, architects, and people in the construction trades. The accuracy is achieved because the drawing is proportional to the real thing. The convenience comes from the size of the drawing. It is large enough to provide the desired detail but small enough to be handy.

The flexibility to draw proportionally in different sizes is provided by scales. For the purposes of representation, we will only be concerned with reduction scales. Reduction scales make the drawing smaller than the object. The kinds of rulers we will be discussing for making scaled drawings are the architect’s scale and the metric scale, both shown in Figure 1.

Architect’s and metric rulers

The scale of the drawing is always written on the drawing, unless the drawing is not drawn to scale. In the latter case, this will be indicated by the “not to scale” abbreviation (NTS). The scale is the ratio of the size of the drawing to the object. For drawings smaller than the object, the ratio is that of a smaller distance to a larger one.

The architect’s scales use ratios of inches to a foot. The most common architect’s scale used is 1/4 inch to the foot, written on drawings as:

Scale 1/4" = 1'-0"

This means that a line 1/4" long on the drawing represents an object that is one foot long. At the same scale, a line 1½" long represents an object 6' long, because 1½" contains 6 quarter-inches.

Metric scale ratios use the same units in both ratio terms, resulting in an expression of how many times smaller than the object the drawing is. For example, the standard metric scale ratio that corresponds approximately to ¼" = 1'-0" is written on drawings as "Scale 1:50."

This means that the object is 50 times as large as the drawing, so that 50 mm on the object is represented by 1 mm on the drawing. For another example, 30 mm on the drawing represents 50 × 30 mm = 1500 mm (or 1.5 metres) on the object.

Figure 2 lists the scale ratios used for building plans and construction drawings in both metric and the approximate equivalent architectural scale ratios.

Type of Drawing	Common Metric Ratios	Imperial Equivalents and Ratios		Use
Site plan	1:500 1:200	1" = 40'-0" 1/16" =1'-0"	1:480 1:192	To locate the building, services and reference points on the site
Sketch plans	1:200	1/16" =1'-0"	1:192	To show the overall design of the building To indicate the juxtaposition of the rooms and locate the positions of piping systems and components
General locations	1:100	1/8" =1'-0"	1:96
Drawings	1:50	1/4" =1'-0"	1:48
Construction details	1:20 1:10 1:5 1:1	1/2" =1'-0" 1" = 1'-0" 3" =1'-0" Full size	1:24 1:12 1:4 1:1	To show the detail of system components and assemblies

Preferred scales for building drawings