The following lesson provides a basic understanding of how to read scales, dials, and gauges. It will not provide any information on the actual use of the tools.

When you have completed this lesson, you will be able to:

Major sections of this unit


In one sense, the term “scale” means the scale of a drawing. In another sense, it means the succession of graduations on any graduated standard of linear measurement, such as the graduations on a steel tape or a thermometer. The more common type rules and tapes are divided into fractions, inches, and feet. Explained here are the scales on a 12-inch steel machinist’s rule (Figure 1).

Figure 1 — 12-inch machinist’s rule.

The rule is divided into twelve inches. The inches are further divided into eighths, sixteenths, thirtyseconds, and sixty-fourths.

Look at the rule. There is a small numeral marked on the end of the rule nearest the 1-inch mark. This numeral indicates the number of divisions per inch (Figure 2).

Figure 2 — Divisions per inch.

When referring to fractions, always use the reduced name. This is the smallest numerator (top number) and denominator (bottom number). For example, 3/6 can be reduced to 1/2 by dividing both the top and bottom by 3. Generally, fractions may be reduced to their lowest forms by repeated division by 2 or 3. Look at the section between the “2” and the “3” on the edge marked with an “8” for eighths (Figure 3).

Figure 3 — 1 /8-inch scale.

There are eight equally spaced lines. The lengths of these lines differ and indicate different fractions or parts of an inch.

The longest line is in the center and is equal to 4/8- or 1/2-inch.

Each half-inch is divided in half by a slightly shorter line indicating 2/8- or 1/4-inch on the left and 6/8- or 3/4-inch on the right.

Each 1/4-inch is divided in half by the shortest line which indicates 1/8-inch, and will indicate 1/8-, 3/8-, 5/8- and 7/8-inch.

Now turn the rule and look at the edge with a 16 marked on it (Figure 4).

Figure 4 — 1/16-inch scale.

There are now 16 equal divisions between each inch. Since 2/16-inch reduces to 1 /8-inch, divide each 1/8-inch into two equal parts producing 1/16-, 3/16-, 5/16-, 7 /16-, 9/16-, 11/16-, 13/16-, and 15/16-inch.

Common tapes and rules usually are not graduated smaller than sixteenths. However, precision measurements require smaller graduations.

Look at the back of the machinist’s rule. Find the edge marked 32 (Figure 5) and once again look between the numbers “2” and “3.”

Figure 5 — 1/32-inch scale.

To read this rule, remember:

1. Sixteen divisions (16/32) are equal to 1/2- inch.
2. Eight divisions (8/32) are equal to 1/4-inch.
3. Four divisions (4/32) are equal to 1/8-inch.
4. Two divisions (2/32) are equal to 1/16-inch.

To read 2 5/8-inches on the scale, first find the two inch mark, then determine the number of 32nds in 5/8-inch.

To determine the number of 32nds in 5 /8-inch, remember four divisions or 4/32 are equal to 1 /8-inch. If 1/8-inch is equal to 4/32-inch, then 5 /8-inch is equal to 20/32-inch as shown:

• If 1 /8 = 4/32, then 5/8 = 20/32-inch
• (4 x 5 = 20)
1. Find the 20/32-inch reading on the scale as shown above.
2. Write the new fraction 2 20/32-inches.

Finally, look at the edge marked 64 (Figure 6). Each inch is now divided into 64 equal parts.

Figure 6 — 1 /64-inch scale.

To read this rule, remember:

1. Thirty-two divisions (32/64) are equal to 1/2- inch.
2. Sixteen divisions (16/64) are equal to 1/4- inch.
3. Eight divisions (8/64) are equal to 1/8-inch.
4. Four divisions (4/64) are equal to 1/16-inch.
5. Two divisions (2/64) are equal to 1/32-inch.

To read 2 3/4 -inches on this scale, first find the two inch mark. Next, determine the number of 64ths in 3/4 -inch.

To determine the number of 64ths in  /4 -inch, remember every sixteen divisions or 16/64 are equal to 1/4-inch. If 1/4-inch is equal to 16/64-inch, then 3/4-inch is equal to 48/64-inch as shown:

1. Locate the number 48 between the 2- and 3- inch marks on the scale.
2. Write the new fraction 2 48/64-inches.

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The metric system is based upon multiples of ten. For example, there are 10 millimeters in a centimeter and 100 centimeters in a meter

Figure 7 — Metric rule.

The example provided will deal only with millimeters (mm).

The meter is the starting point. From that point, there are two scales for measuring. A meter divided by 100 equals a centimeter (cm), 1 /100 or 0.01 meter.

Next divide a centimeter (cm) by 10. This will equal a millimeter (mm), 1 /1000 or 0.001 meter.

Now let’s look at a section of the rule between 2 cm and 3 cm.

There are 10 equal divisions which are equal to 1 /10 cm or 1 mm.

To measure 26 mm, first locate the longest line designated 2 cm or 20 mm.

Next count 6 additional lines to find 26 mm


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Pressure, vacuum, compound, and duplex gauges are used to measure the difference between atmospheric pressure, pressure, and temperature in a system.

A typical pressure gauge is constructed of a Bourdon tube connected by mechanical linkages and gearing to a pointer. The movement of the pointer, with respect to a fixed dial, indicates pressure changes with graduated markings representing magnitudes of pressure.


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1. What term describes the succession of graduation on any graduated standard of linear measurement?

A. Dial
B. Unit
C. Scale
D. Gage

2. What measurements are common rules and tapes divided into?

A. Hours
B. Inches
C. Angles
D. Seconds

3. On a machinist’s rule, what does the small numeral marked on the end nearest the 1-inch mark indicate?

A. Manufactured date
B. Manufacturer’s identification
C. Number of inches of the rule
D. Number of divisions per inch

4. Common tapes and rules are usually not graduated below what minimum increment, in inches?

A. 1/64
B. 1/32
C. 1/16
D. 1/8

5. On a machinist’s rule edge marked 32, how many divisions are equal to a 1/2-inch?

A. 8
B. 16
C. 32
D. 64

6. On what multiple is the metric system based?

A. 2
B. 8
C. 10
D. 12

7. To find 26 millimeters, how many lines after the 2 centimeter line should you count?

A. 4
B. 6
C. 8
D. 10

8. What indicator is used to measure the temperature in a system?

A. Gauge
B. Pointer
C. Repeater
D. Rule

9. What type of tube is used in a typical pressure gauge?

A. Bourdon
B. Compound
C. Graduated
D. Metric


Answers to Exercises

1. C
2. B
3. D
4. C
5. B
6. C
7. B
8. A
9. A

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