| Cubic feet per minute (ft³/min) | Milliliters per hour (mL/h) |
|---|---|
| 1 Cubic foot per minute | 1699010.7954 mL/h |
| 2 Cubic feet per minute | 3398021.5908 mL/h |
| 3 Cubic feet per minute | 5097032.3862 mL/h |
| 4 Cubic feet per minute | 6796043.1816 mL/h |
| 5 Cubic feet per minute | 8495053.977 mL/h |
| 10 Cubic feet per minute | 16990107.954 mL/h |
| 20 Cubic feet per minute | 33980215.908 mL/h |
| 25 Cubic feet per minute | 42475269.885 mL/h |
| 50 Cubic feet per minute | 84950539.77 mL/h |
| 100 Cubic feet per minute | 169901079.54 mL/h |
| Reference | Cubic feet per minute (ft³/min) | Milliliters per hour (mL/h) |
|---|---|---|
| A domestic shower | 0.317832 ft³/min | 540000 mL/h |
| A kitchen tap | 0.353147 ft³/min | 600000 mL/h |
| A garden hose | 0.52972 ft³/min | 900000 mL/h |
| The Amazon river | 442845921 ft³/min | 7.524 × 1014 mL/h |
The cubic foot per minute is a unit of volumetric flow rate equal to one cubic foot passing a point every minute. Its symbol is ft³/min, universally abbreviated CFM. It is the working unit of air movement in the English-speaking world: fans, compressors, extractors and air conditioners are all sold by it.
Ventilation is specified in it throughout North America. An extractor fan over a domestic cooker is rated at 200 to 400 CFM, a bathroom fan at 50 to 110, a large commercial rooftop unit at several thousand, and a whole-house fan at 3,000 or more. Building codes give minimum rates per person or per square foot of floor, and the designer multiplies to reach the fan size.
Compressed air uses it with a qualifier that matters. A compressor is rated in standard cubic feet per minute, referred to a fixed temperature, pressure and humidity, because a cubic foot of air at 100 pounds per square inch contains far more air than one at atmospheric pressure. A tool that consumes 5 SCFM will starve on a compressor rated 5 CFM of displacement, since displacement and delivery are not the same number.
Dust extraction in woodworking is a familiar case. A table saw needs about 350 CFM at the port to capture chips, a planer 400 to 800, and a shop with several machines running needs the sum plus an allowance for duct losses. Since a blast gate closes off unused branches, the same collector can serve the whole shop one machine at a time.
Engine and vehicle work uses it too. A carburettor is rated in CFM, an engine's airflow at peak power is quoted in CFM, and a radiator fan moves 1,500 to 3,000. The rating is what determines whether the airflow matches the engine's demand at the speed it is designed to work hardest.
The conversions are worth learning: 1 CFM is about 0.4719 litres per second, about 1.699 cubic metres per hour, and 2,119 CFM is one cubic metre per second. European and American catalogues describe the same equipment in these different units, and the numbers only match once one of them is converted.
One cubic foot per minute equals about 0.4719 litres per second, about 1.699 cubic metres per hour, or about 7.481 US gallons per minute.
The millilitre per hour is a unit of volumetric flow rate equal to one millilitre passing a point every hour. Its symbol is mL/h. It is the unit of slow, sustained delivery, and above all of the infusion pumps that keep hospital patients supplied with fluid, nutrition and medication over many hours.
An intravenous line is almost always programmed in millilitres per hour. Maintenance fluid for an adult runs at 80 to 125, a slow drug infusion at 5 to 20, and a paediatric or neonatal line at 1 or 2. Those are the numbers a nurse enters, and the pump converts them into the mechanical rate of a screw driving a syringe plunger.
At the bottom of that range the precision required is remarkable. A syringe driver set to one millilitre per hour is advancing the plunger by a fraction of a millimetre per minute, and it must do so smoothly enough that the drug arrives at a steady concentration rather than in pulses. The engineering of these devices is largely about eliminating the stiction that would otherwise make the delivery uneven.
Insulin pumps work below this again, in tenths of a millilitre per hour, and deliver in tiny discrete pulses rather than continuously. Implanted pumps for pain medication may run at a fraction of a millilitre per day, which is a thousandth of a millilitre per hour, and are refilled at intervals of months.
Outside medicine the unit describes laboratory perfusion, slow chemical dosing, and the leakage rates that a seal is designed to stay below. A seal specified to leak less than a millilitre per hour is losing about a cupful a week, which for many purposes is entirely acceptable and for others is a failure.
For scale, a millilitre per hour would take a full day to fill a tablespoon and about a month to fill a small cup. It is slow enough that the total delivered over a shift is the meaningful quantity, and every pump displays that alongside the rate.
One millilitre per hour equals about 0.01667 millilitres per minute, 0.000278 millilitres per second, or 0.001 litres per hour.