4 mm to inches

Formula: inches = millimeters ÷ 25.4 (exact). Or multiply by 0.0393701 for a quick estimate of the same definition.

1. Start with 4 mm.
2. Divide by 25.4: 4 ÷ 25.4 = 0.1575 in.
3. Reverse check: 0.1575 × 25.4 = 4 — the conversion round-trips cleanly.

Dividing by 25.4 is exact: 4 ÷ 25.4 = 0.157480 in, shown as 0.1575 in. The 5/32 in reading is the nearest 1/64, 0.03125 mm smaller than 4 mm, not a second exact size.

The decimal column is exact (shown to four places). The fraction is the nearest 1/64 approximation used on rulers and bit charts — so the two can disagree. Here, 4 mm = 0.1575 in exactly, while 5/32 in differs by -0.031 mm. The converter disclaimer (“rounded to the nearest 1/64 fraction”) exists so that difference is never mistaken for a data error.

No. 5/32 in equals 3.96875 mm, so 4 mm is 0.03125 mm larger. The exact decimal is 0.1575 in, and 5/32 in is the nearest 1/64 tick (0.15625 in). A 4 mm hex key, a 4 mm bead, or 4 mm OD bowden tube should stay metric when the fit is tight; 5/32 in is the closest imperial stand-in, not an equal.

Compared with 5/32 in (3.97 mm): 4 mm is +0.031 mm (metric size is larger).

This page is for specification checks — drill bits, sockets, tubing, and stock — not body measurements. For shop picks across many metric diameters, see the drill bit & socket mm to inches chart.

A 4 mm hex key is the standard metric size for M5 socket-head screws and many furniture cams. Across the flats, 5/32 in is 3.96875 mm — 0.03125 mm undersize — so it may turn a fresh 4 mm socket and still cam out on a worn or high-torque screw. On 1.75 mm filament printers, the usual PTFE bowden tube is 4 mm outside diameter (often 2 mm inside); that OD is 0.1575 in, not a 5/32 in tube. A 4 mm round jewelry bead is the same 0.1575 in across.