Feet to mm (ft to mm)

One foot is exactly 304.8 millimeters. 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: 4 ft x 304.8 = 1,219.2 mm.

Going the other way, one millimeter is 0.0032808399 feet.

For mental arithmetic: Multiply by 300 and add 1.6%. 10 ft is 3,000 plus 48, so 3,048 mm.

A foot is exactly 304.8 mm, so every foot-and-inch dimension converts to millimeters with at most one decimal place. No rounding is needed, which matters on a shop drawing. The exception is old land survey work. Until the end of 2022 the US also had a survey foot of 1200/3937 meters, or 304.8006 mm, used in state plane coordinates and property records. The difference is two parts per million, 0.61 mm per thousand feet, which is invisible on a building and is several feet on a coordinate measured in millions of feet. If a figure comes from a pre-2023 survey, which foot it used is a question worth asking.

Precision is worth matching to the input. A measurement given as 0.5 ft carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.