Thou, Mil, Millimeter

A thou is shorthand for a thousandth of an inch in the imperial system.

Also known as a mil.

But don't confuse mil with a millimeter (mm), which is a thousandth of a meter in the metric system.

You can convert thou to mm by multiplying it (as a decimal of an inch) by 25.4 like this:

Example: 50 thousandths of an inch

50 thou as a decimal of an inch: 0.050 inch

Then: 0.050 inch x 25.4 = 1.27 mm

Here is a handy table for you:

inches								mm
164ths	132nds	116ths	18ths	14s	12s	inches (decimal)	thou (rounded)	mm
164						0.015625	16	0.397
264	132					0.03125	31	0.794
364						0.046875	47	1.191
464	232	116				0.0625	63	1.588
564						0.078125	78	1.984
664	332					0.09375	94	2.381
764						0.109375	109	2.778
864	432	216	18			0.125	125	3.175
964						0.140625	141	3.572
1064	532					0.15625	156	3.969
1164						0.171875	172	4.366
1264	632	316				0.1875	188	4.763
1364						0.203125	203	5.159
1464	732					0.21875	219	5.556
1564						0.234375	234	5.953
1664	832	416	28	14		0.25	250	6.35
1764						0.265625	266	6.747
1864	932					0.28125	281	7.144
1964						0.296875	297	7.541
2064	1032	516				0.3125	313	7.938
2164						0.328125	328	8.334
2264	1132					0.34375	344	8.731
2364						0.359375	359	9.128
2464	1232	616	38			0.375	375	9.525
2564						0.390625	391	9.922
2664	1332					0.40625	406	10.319
2764						0.421875	422	10.716
2864	1432	716				0.4375	438	11.113
2964						0.453125	453	11.509
3064	1532					0.46875	469	11.906
3164						0.484375	484	12.303
3264	1632	816	48	24	12	0.5	500	12.7
3364						0.515625	516	13.097
3464	1732					0.53125	531	13.494
3564						0.546875	547	13.891
3664	1832	916				0.5625	563	14.288
3764						0.578125	578	14.684
3864	1932					0.59375	594	15.081
3964						0.609375	609	15.478
4064	2032	1016	58			0.625	625	15.875
4164						0.640625	641	16.272
4264	2132					0.65625	656	16.669
4364						0.671875	672	17.066
4464	2232	1116				0.6875	688	17.463
4564						0.703125	703	17.859
4664	2332					0.71875	719	18.256
4764						0.734375	734	18.653
4864	2432	1216	68	34		0.75	750	19.05
4964						0.765625	766	19.447
5064	2532					0.78125	781	19.844
5164						0.796875	797	20.241
5264	2632	1316				0.8125	813	20.638
5364						0.828125	828	21.034
5464	2732					0.84375	844	21.431
5564						0.859375	859	21.828
5664	2832	1416	78			0.875	875	22.225
5764						0.890625	891	22.622
5864	2932					0.90625	906	23.019
5964						0.921875	922	23.416
6064	3032	1516				0.9375	938	23.813
6164						0.953125	953	24.209
6264	3132					0.96875	969	24.606
6364						0.984375	984	25.003
6464	3232	1616	88	44	22	1	1000	25.4

The simplest fraction is highlighted and is the one you should use.

Example: You would not talk about a ²/₁₆" drill bit, instead you would say ¹/₈"