Tape Measures & Rules

Example Lecture Slides

Tape Measures

Fractional Inch

The most common tape measures are graduated in 1/16 inch increments.

Tape measures have limited use in the machine shop. The typical resolution of 1/16 or 1/32 of an inch is only useful for selecting stock and sawing to oversize dimensions.

Tape measures are designed to be used with the metal hook on the end. The hook is mounted on slots so that it can move in or out to compensate for its own thickness. When measuring from the end of the tape measure, it is necessary to use the hook.

It is common for a dropped tape measure to land on this hook and bend it, so be sure the hook is not bent before using the tape. A bent hook will affect the measurement by a significant amount.

Measuring tapes are designed to be used with the hook on the end. The hook is designed to slide in and out to compensate for its material thickness (mm shown for reference).
Here the hook is used in tension. The reading is 1 1/4 or 1.25 inches.

Decimal Inch

Tape measures are available in decimal, often in 1/50 or .02 increments. These have scales similar to the ones normally found on steel rules. These are discussed below.

Do not confuse decimal inch rules and tape measures with metric. These are not the same thing.

Steel Rules


Steel rules are useful for for limited applications in a machine shop. They often have graduations ranging from 1/8 to 1/100 of an inch.

Steel rule graduation styles can be categorized into different styles for quick reference. See wikipedia.org article.

A machinists rule graduated in 1/8 and 1/16 of an inch (mm shown for reference)
A machinists rule graduated in 1/32 and 1/64 of an inch (mm shown for reference)

Some are graduated in decimal parts of an inch, and are available with resolutions of the following:

  • 1/10 (.01)
  • 1/50 (.02)
  • 1/100 (.01)
A decimal machinists rule graduated in 10th and 50ths (mm shown for reference)
A decimal machinists rule (mm shown for reference)


Metric machinists rules and tape measures are typically graduated in 1 or 0.5 mm increments.

Metric machinists rule graduated in 1 and 0.5 mm increments (inch shown for reference)

Specialty Rules and Tapes

Pi Tape

Pi tapes are used to measure a large cylinder’s diameter indirectly by measuring the circumference. Since the circumference of a circle is equal to π times the diameter, the graduations on a pi tape have been enlarged by a factor of π (3.14159…). Many pi tapes have a vernier scale to improve precision.

Glass Rules

Glass rules can be used with optical equipment such as microscopes and optical comparators for checking magnification and movement accuracy.