A thou (also called a mil) is 1/1000 of an inch = 0.0254 mm. To convert millimetres to thou, multiply by 39.3701; to convert thou to mm, multiply by 0.0254. The thou is used in precision machining, spring manufacturing, wire gauges, and printed circuit board (PCB) design. A human hair is approximately 50–100 thou (1.27–2.54 mm) in diameter; a sheet of standard copier paper is about 4 thou (0.1 mm).
In PCB design, trace widths and spacings are specified in thou (mils): 6-thou traces are standard for consumer electronics; high-density interconnect (HDI) boards use 3–4 thou. Wire gauges (AWG) are defined in mils of diameter. Sheet metal thickness for HVAC ductwork uses gauge numbers that correspond to thou. Engineers working across metric and imperial systems regularly convert between mm and thou for tight-tolerance specifications.
Millimetres to Thou (Mils) Table
mm
Thou (mils)
Common Reference
0.0254
1
Minimum PCB trace (some)
0.1
3.94
Standard PCB trace min
0.15
5.91
HDI board trace
0.254
10
0.01 inch, precise milling
0.5
19.69
Drill bit tolerance
1.0
39.37
1 mm baseline
Thousandths of an Inch (Thou) in Precision Machining
One thou (short for thousandth) equals 0.001 inch or exactly 0.0254 mm. Machinists, toolmakers, and engineers in the US and UK use thou as their everyday precision unit because tolerances in machined parts, printed circuit boards, and precision instruments are typically specified in thousandths of an inch. Converting mm to thou: divide by 0.0254 (equivalently, multiply by 39.3701). A human hair is about 2–3 thou (0.05–0.08 mm) in diameter. Clearance fits in engine bearings are specified in tenths of a thou. PCB trace widths in electronics are given in mils (also called thou in this context). CNC machine tool positioning accuracy is often stated in sub-thou (tenths or hundredths of a thousandth). Vernier calipers, micrometers, and dial indicators in the US read in thousandths directly, making thou the intuitive precision language for US manufacturing shops.
