How to Use an Engineer's Scale Ruler

Written by don patton
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Drawings for large projects must be greatly reduced when compared to the objects they depict. The scale of the drawing defines the size relationship between the drawing and the objects. To measure something on a drawing, you use a special tool that is also called a scale. Scale tools are triangular rulers with two independent graduated surfaces on each face. There are architect scales and engineer scales. The rulers of an engineer's scale are labelled 10, 20, 30, 40, 50 and 60. They are sized such that one inch on the drawing equals between 10 to 60 feet.

Skill level:

Things you need

  • Engineer scale
  • Engineering drawing

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  1. 1

    Determine the scale of the drawing. Look in the drawing's title block legend for the specification of its scale. Title blocks are at the bottom right hand corner of at least the first page of the drawing, and sometimes on every sheet.

    The drawing scale may be given graphically; that is, a line may be shown and dimensioned with a specified length. In this case find the correct scale tool to use by trial and error measurements. Align each scale tool ruler with the line until you find one that matches.

  2. 2

    Position the scale tool so that the ruler that matches the drawing scale is against the paper. Align the zero at the left side with one end of the object you want to measure.

  3. 3

    Find the tick mark on the scale tool that lines up with the other end of the object you are measuring. Read the major mark value and add the minor marks as tenths. Multiply the composite reading by ten to get the length.

    The minor divisions on the ruler represent individual feet and the major tick marks are each ten feet. The markings at the major divisions represent one tenth of their actual value.

    Example: Find the length of a water line on plans for a factory that have a scale of 1 inch equals 20 feet.

    Use the 20 scale to measure on this drawing. Line up the zero mark on the scale that is labelled "20" to the left end of the water line. If the right end of the water line aligns with the tick mark that represents 8 major divisions and 2 small divisions, the reading is 8.2. Multiply 8.2 by 10 to get the length of 82 feet.

Tips and warnings

  • In order to use an engineer scale, the drawing scale must be 1 inch to a multiple of 10 feet. If one inch does not equal 10 to 60 feet, the engineer's scale cannot be used. If the drawing scale is given as a fraction of an inch per foot (1/4" = 1 foot), an architect scale is the appropriate tool to use.
  • Never use scales to draw lines; they should be used only for measuring.
  • When a drawing is reproduced, the image size might be modified. Always verify that the drawing scale is correct by measuring a dimensioned feature on the drawing to see that you arrive at the correct length.

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