How to Read a Metric Pi Tape


"Pi tape" is a registered trademark for a tool called periphery tape. Periphery tape is a metal measuring tape used to measure the diameter of a round object by wrapping the tape around the circumference and aligning a Vernier scale with the main body scale, which converts the circumference to diameter. Periphery tapes are useful for measuring large objects, thin walled parts, or going where typical tools cannot reach. Periphery tape is practical because alternatives such as micrometers are more expensive and less portable. Some manufacturers claim an accuracy of 0.03 mm (0.001 inch).

Things You'll Need

  • Periphery tape
  • Clean rag
  • Adhesive tape (optional)
  • Calculator
  • Clean the tape and the object to be measured. Even very small foreign objects can spoil the reading.

  • Wrap the tape around the object. The tape must be kept parallel to the cross-section of the measured object for the most accurate measurement. For larger objects, use adhesive tape to hold the periphery tape in place as you align it. Pull snug on the nub on the end with about 5 pounds of pulling force.

  • Align the two scales. The scale on the main body is labeled with a numeral for each 10 mm. Each line between numerals represents 0.5 mm. The scale on the end of the tape is the Vernier scale. It is marked with lines from 0 - 50. Each line on the Vernier scale represents 0.01 mm.

  • Find the line on the main body scale that is directly to the left of the zero on the Vernier scale. For example: the first line to the right of the 100 mm line on the main body is closest to the left of the zero line on the Vernier scale, (100 mm + 0.5 mm), thus 100.5 mm.

  • Find the line on the Vernier scale that aligns the most closely with any line on the main body. All of the Vernier lines will be near a line on the main body. Find the one that lines up the best of all. For example: if the second line on the Vernier scale lines up perfectly with any line on the main body, this represents (2 X 0.01 mm) thus 0.02 mm.

  • Add the two values together. Using the previous examples, the measurement for the diameter is (100.5 mm + 0.02 mm), thus 100.52 mm.

Tips & Warnings

  • Measure objects of known diameter to get used to using the Vernier scale.
  • Measure objects of known diameter to calibrate the periphery tape.
  • Periphery tape uses thus geometric formula: (circumference / pi) = diameter.
  • Ensure that the tape is as parallel as possible for most accurate results.

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  • Photo Credit measure tape image by Karol Grzegorek from
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