Cubic millimeter per hour (mm³/hour)

The cubic millimetre per hour is a unit of volumetric flow rate equal to one cubic millimetre passing a point every hour. Its symbol is mm³/h. It is one of the smallest flow rates in ordinary use: a full hour delivers a volume the size of a grain of coarse salt, so it belongs to processes measured in days rather than minutes.

Corrosion is described in these terms whenever the loss is treated as a volume rather than a depth. A steel surface corroding at 0.1 millimetres a year loses, over a square centimetre, roughly one cubic millimetre a year — and the hourly rate that produces it is a ten-thousandth of that. Cathodic-protection engineers work with such numbers because the whole point of the discipline is to make a rate small enough to ignore for decades.

Permeation and leakage testing lives here too. A sealed package, a fuel line or a medical device is tested for how much fluid crosses its wall, and the answer is often a few cubic millimetres per hour or less. A leak too small to see over a working day becomes, at this rate, a measurable volume after a month in a heated test chamber.

Slow drug delivery uses the unit directly. An implanted osmotic pump may release a few cubic millimetres of solution per hour for weeks, and an intrathecal pump can be programmed in fractions of one. Because the reservoir holds only a few millilitres, the hourly rate determines how long the implant lasts before it must be refilled.

Botany borrows it for sap and exudates. Xylem flow in a single small vessel, latex from a tapped rubber tree between collections, and phloem exudate sampled from an aphid stylet are all of this order, and researchers report them per hour because the sampling period is measured in hours.

The scale is worth stating plainly. A litre is a million cubic millimetres, so a flow of one cubic millimetre per hour would take about a hundred and fourteen years to fill a one-litre bottle. That is why the unit almost never describes a bulk transfer — it describes a process one hopes will stay slow.

One cubic millimetre per hour equals 0.001 millilitres per hour, about 0.01667 cubic millimetres per minute, or one microlitre per hour.

Cubic millimeter per hour conversion table