| Milliliters per second (mL/s) | Gallons per minute (gal/min) |
|---|---|
| 1 Milliliter per second | 0.0131981548979 gal/min |
| 2 Milliliters per second | 0.0263963097959 gal/min |
| 3 Milliliters per second | 0.0395944646938 gal/min |
| 4 Milliliters per second | 0.0527926195918 gal/min |
| 5 Milliliters per second | 0.0659907744897 gal/min |
| 10 Milliliters per second | 0.131981548979 gal/min |
| 20 Milliliters per second | 0.263963097959 gal/min |
| 25 Milliliters per second | 0.329953872449 gal/min |
| 50 Milliliters per second | 0.659907744897 gal/min |
| 100 Milliliters per second | 1.31981548979 gal/min |
| Reference | Milliliters per second (mL/s) | Gallons per minute (gal/min) |
|---|---|---|
| A domestic shower | 150 mL/s | 1.97972 gal/min |
| A kitchen tap | 166.667 mL/s | 2.19969 gal/min |
| A garden hose | 250 mL/s | 3.29954 gal/min |
| The Amazon river | 2.09 × 1011 mL/s | 2.75841 × 109 gal/min |
The millilitre per second is a unit of volumetric flow rate equal to one millilitre of fluid passing a point every second. Its symbol is mL/s. Since a millilitre is a cubic centimetre, the unit is identical to the cubic centimetre per second, and the two names are used interchangeably depending on whether the fluid is thought of as a liquid or as a volume of space.
Volumetric flow rate is a volume divided by a time, so it has the dimensions of length cubed per second. That makes it convertible to any other flow unit by a single multiplication, and it also means that a flow rate multiplied by a duration gives the total volume delivered — the arithmetic that underlies every dosing calculation and every water bill.
A millilitre per second is a small but visible flow. A tap dripping steadily produces perhaps a tenth of this; a thin stream from a partly opened tap is a few millilitres per second; a garden hose at full flow is several hundred. In household terms it is the rate at which a teaspoon fills in about five seconds.
The unit belongs to the laboratory and to small machinery. Peristaltic pumps, dosing systems, fuel injectors and analytical instruments all work in this range, and their specifications are written in millilitres per second when the process is fast and in millilitres per minute when it is slow. Where a pump must deliver a precise quantity, the rate and the running time together define the dose.
Human physiology gives useful comparisons. Resting cardiac output is about eighty millilitres per second, urine production averages under a fiftieth of that, and saliva production is around a hundredth. The flow through a single capillary is far smaller still and is measured in cubic millimetres per hour.
Converting to the SI base unit requires care with the powers of ten. One millilitre per second is a millionth of a cubic metre per second, because a millilitre is a millionth of a cubic metre. That factor of a million between the practical unit and the base unit is why flow rates are almost never quoted in cubic metres per second outside hydrology and heavy engineering.
One millilitre per second equals 0.001 litres per second, 60 millilitres per minute, or about 0.0000353 cubic feet per second.
The gallon per minute is a unit of volumetric flow rate equal to one imperial gallon passing a point every minute. Its symbol is gal/min, usually abbreviated GPM. The imperial gallon is 4.54609 litres, so this unit sits comfortably in the range of pumps, wells and building services, which is where it did most of its work before metrication.
Water wells and boreholes are still described by it in Britain and Ireland. A domestic borehole yields 2 to 10 gallons per minute, an agricultural one 20 to 60, and a public supply borehole several hundred. The yield test that establishes the figure runs the pump for hours while watching the water level, and the sustainable yield in gallons per minute is what determines whether a licence can be granted.
Pumps and pumping stations were rated in it throughout the industrial era. Boiler feed pumps, sewage pumps and fire pumps all carried plates in gallons per minute, and older British pump curves plot head against this unit rather than against cubic metres per hour. Anyone matching a replacement pump to an old installation has to convert the duty point carefully.
Building services used it for hot and cold water services, heating circuits and cooling towers. A heating circuit's flow follows from the heat it must carry and the temperature drop across the circuit, and in imperial terms the arithmetic came out in gallons per minute against a drop in degrees Fahrenheit — a combination that survives in many older plant rooms.
The unit also has a firefighting history. British fire brigade pumps were classified as light, medium and heavy by their output in gallons per minute, and hydrant flow tests, hose reel ratings and sprinkler system design all rested on the same figure before European standards moved the trade to litres per minute.
For scale, one gallon per minute is about 4.5 litres per minute, and 13.2 gallons per minute is one litre per second. A domestic shower running at 9 litres per minute is just under 2 gallons per minute, which puts everyday plumbing at the low end of the unit's range.
One gallon per minute equals about 4.546 litres per minute, about 0.2728 cubic metres per hour, or about 1.201 US gallons per minute.