Why is the tape measure marked in 16ths?

Because halving is the natural way to divide a length by eye and by fold: half, quarter, eighth, sixteenth. A 16th of an inch is about 1.6 mm — fine enough for carpentry, coarse enough to read at arm’s length.

How big is 1/16 of an inch in millimetres?

1/16″ is exactly 1.5875 mm. Every mm value on this chart is exact, at 25.4 mm to the inch.

16ths of an inch

The tape-measure rule: sixteen marks, each with its reduced name, exact decimal and exact millimetres.

16ths	Reduced	Decimal	mm
1/16	—	0.0625	1.5875
2/16	1/8	0.125	3.175
3/16	—	0.1875	4.7625
4/16	1/4	0.25	6.35
5/16	—	0.3125	7.9375
6/16	3/8	0.375	9.525
7/16	—	0.4375	11.1125
8/16	1/2	0.5	12.7

16ths	Reduced	Decimal	mm
9/16	—	0.5625	14.2875
10/16	5/8	0.625	15.875
11/16	—	0.6875	17.4625
12/16	3/4	0.75	19.05
13/16	—	0.8125	20.6375
14/16	7/8	0.875	22.225
15/16	—	0.9375	23.8125
16/16	1	1	25.4

Finer rules: 32nds · 64ths · 128ths

Common questions

Why is the tape measure marked in 16ths?: Because halving is the natural way to divide a length by eye and by fold: half, quarter, eighth, sixteenth. A 16th of an inch is about 1.6 mm — fine enough for carpentry, coarse enough to read at arm’s length.
How big is 1/16 of an inch in millimetres?: 1/16″ is exactly 1.5875 mm. Every mm value on this chart is exact, at 25.4 mm to the inch.

Where this comes from. Exact rational arithmetic on the values shown — fractions are pairs of integers until display, and inch–metric conversion uses 25.4 mm to the inch, exact by definition since 1959. See the full methodology.