Thou

US & ImperialCurrent unit

One thousandth of an inch (0.0254 mm), also called a mil. Used in machining, plastics and the thickness of films and wire.

1 thou = 0.0000254 meters (exactly).

About the thou

Machinists say a part is out by a few thou, and the word is a clipping of thousandth, keeping only the first syllable. In North America the same length is often called a mil, which is easy to confuse with a millimetre in international drawings. One thou is a thousandth of an inch, which is exactly 0.0254 mm or 25.4 micrometres, so about 39 of them fit in a millimetre. The word is both singular and plural.

Joseph Whitworth pressed in 1857 for engineers to think in decimal thousandths instead of fractions of an inch, a habit that needed vernier calipers and micrometers to put into practice. Today the unit describes the thickness of paper, foil, wire, plastic sheet and paint films, with a plastic ID card about 30 thou thick. It also appears in older spark-plug gaps, gasket thicknesses and circuit-board geometry. For finer work, a tenth, meaning 0.0001 inch or 2.54 micrometres, is common, and gauge blocks are often given in millionths or in micrometres.

Sources: Wikipedia: Thou (unit); Wiktionary: thou

Thou conversion chart

1 thou expressed in every other length unit:

More US & Imperial units

• inch (length)
• foot (length)
• yard (length)
• mile (length)
• league (length)
• furlong (length)
• chain (length)
• rod (length)
• link (length)
• hand (length)
• pound (weight & mass)
• ounce (weight & mass)
• grain (weight & mass)
• dram (weight & mass)
• cubic inch (volume)
• cubic foot (volume)

All US & Imperial units

Browse all length units.