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Diameter Measuring Tools

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When you first hear the term “diameter measuring tools,” you probably think of a machinist making precise measurements as he blueprints a racing engine, or perhaps a plumber measuring the inside and outside diameters of replacement plumbing pipes. But workers in other disciplines need to find the diameters of much larger items — foresters, for example, use special measuring tools to determine the diameter of trees. All of these tools can be used across different disciplines, depending on the size of the item to be measured.


Mid section view of a woman measuring her waist with a tape measure
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Rulers and measuring tapes are perhaps the most common tools used to measure diameters. When a round object has a flat surface where its cross-section can be clearly seen, these tools can be used to make a direct measurement.

Cloth Measuring Tape

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This dressmaker's tool lets you arrive at diameter the old fashioned way. After you measure an object's circumference by encircling the item with your cloth measuring tape, you can divide that number by pi (3.14) to determine its diameter. The simple mathematical formula is C/π=D.


penny in a micrometer guage to measure thickness
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Inside and outside micrometers are the most commonly used of the precision measuring instruments. Shaped like a half-circle with a rod that can be screwed in and out of one side, they provide a delicate adjustment that can be changed by a minute amount, providing exact measurements. Although they come in a variety of sizes, they are generally meant for smaller jobs.


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Calipers are generally made from a pair of curved metal arms hinged at one end. Precision calipers may have an adjustment screw that moves them together or apart in a precise way, while tree-sized calipers feature a fixed arm, a movable arm, and a scale to measure the distance between them. After rulers, calipers are perhaps the most easily recognized tool for diameter measurement.

Snap Gauge

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Snap gauges are most frequently seen in machine shops. Typically they are set for a specific diameter, and machinists will use them to check that a milled component is the correct size. It is not unusual to use them in pairs — one with the maximum allowable diameter, another with the minimum allowable diameter. Between the two, the machinist can confirm that the machined piece is within acceptable tolerances.

Combination Logger's Tape

Low angle view of man sitting on a huge tree trunk
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Also known as a girth tape, a logger's tape is used to estimate the diameter of a tree (and, similarly, columns in buildings and similar constructions). Typically made of fiberglass, a logger's tape is usually marked in both normal measuring units (for direct circumference measurements) and "PI" marks (a conversion to diameter). Logger's tapes are pretty accurate when the cross-section of the tree is perfectly circular, but tend to over-estimate the diameter slightly if the tree is not.

About the Author

North Carolina native Mike Southern has been writing since 1979. He is the author of the instructional golf book "Ruthless Putting" and edited a collection of swashbuckling novels. Southern was trained in electronics at Forsyth Technical Community College and is also an occasional woodworker.