Mm to feet (mm to ft)

One millimeter is exactly 0.0032808399 feet. That is a definition rather than a measurement: the imperial and US customary units were fixed against metric ones by the 1959 international yard and pound agreement, so this factor does not drift and will not be revised.

Worked through: 1,200 mm x 0.0032808399 = 3.93701 ft.

Going the other way, one foot is 304.8 millimeters.

For mental arithmetic: Divide by 300 and take off 1.6%. Or divide by 304.8 exactly.

A foot is exactly 304.8 mm, which is close enough to 300 that metric and imperial building materials look interchangeable and are not. Metric sheet goods are 1,200 by 2,400 mm; the imperial 4-by-8 sheet is 1,219.2 by 2,438.4 mm. The metric sheet is 1.6% smaller on both sides, so it does not land on studs set at the imperial 16-inch spacing (406.4 mm). Metric framing uses 400 or 600 mm centers for that reason. A 2,400 mm ceiling is 7 ft 10.5 in, not 8 ft, and that inch and a half is enough to stop an 8-foot imperial panel standing up in the room.

Precision is worth matching to the input. A measurement given as 100 mm carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.