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How big is a Thou?

25.4 micrometers!

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Thou

A thou is a unit of length equal to one thousandth of an inch: exactly 25.4 micrometers, or 0.0254 millimeters. A micrometer is a millionth of a meter. The same unit is also called a mil. In the United States it is used to measure the thickness of thin materials, such as plastic sheeting, and the width of wires.

Definition

Since 1959, the inch has been exactly 25.4 millimeters. That year, a notice in the U.S. Federal Register fixed the foot at exactly 0.3048 meters, which is 12 inches of 25.4 millimeters each. A thou is a thousandth of an inch, so it is exact too: 25.4 micrometers.

The National Institute of Standards and Technology (NIST), which keeps the measurement standards of the United States, lists the unit under the name mil. It defines 1 mil as exactly 0.001 inch, which is 0.0254 millimeters or 25.4 micrometers. The name mil comes from the Latin word millesimus, meaning "thousandth", and has been recorded in English since the early 1700s. A mil is not a millimeter, which is about 39 thou.

Size comparisons

  • There are 1,000 thou in an inch, so a 12-inch ruler is 12,000 thou long.
  • A human hair, 80–100 micrometers wide, is about 3–4 thou wide.
  • A thou is 1.44 twips. The twip is a unit that computer programs use to position text and drawings on a page, and 1,440 of them make an inch.
  • A red blood cell, 7–8 micrometers across, is about three-tenths of a thou wide.

Where the thou is used

NIST notes that the mil is traditionally used by some U.S. industries to measure thickness. Plastic sheeting is one example.

Guidelines from the U.S. Department of Housing and Urban Development for working safely around old lead paint recommend plastic sheeting 6 mil thick, about 0.15 millimeters. The U.S. Environmental Protection Agency sets no thickness of its own for that work. It requires only that the sheeting keep dust in and not tear.

Wires are measured in thousandths of an inch too. The area of a wire's cut end, its cross-section, is given in circular mils. One circular mil is the area of a circle 1 mil across, about 0.0005 square millimeters.

Sources

Scale of the Universe > Thou
Twip
A twip is a twentieth of a typographical point. That means that 12-point font is approximately 240 twips high. Also, TUIP can stand for "The Universe in Perspective”, an old name for the Scale of the Universe. TUIP and twip are pronounced the same.
Infrared Wavelength
Things that are extremely rare are "inf-rare". (infinitely rare). Inf-rare is also a verb, meaning "to make inf-rare". (e.g., "John inf-rared the precious food by eating the last of it.") Infrared light used to be quite common, but one day, a scientist somehow "infrared" it.
Width of a Silk Fiber
Silk is valuable and can be woven into clothing, which usually turns out quite soft. Silk is made by silkworm larvae, which I think are pretty gross. But there's also synthetic silk, which is not made from disgusting creepy-crawlies.
Skin Cell
The outermost layer of skin, which is also the only visible layer of skin, is made up of dead cells. That's because dead cells are the strongest, and stronger skin provides more protection! These cells continuously fall off. In fact, dust is 95% dead skin cells!
Silt Particle
According to the Udden-Wentworth scale, silt particles vary from the smallest of 1/256 mm to the largest of 1/16 mm. This silt particle to the right falls into that range quite well.
Smallest Object Visible to the Naked Eye
Under optimal conditions, the naked eye can see objects as small as 0.1 millimeters. That means that anything smaller than this circle is impossible for you to see without a microscope. You're missing out quite a lot, you know!

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