| Cubic feet per second (ft³/s) | Liters per minute (L/min) |
|---|---|
| 1 Cubic foot per second | 1699.0107954 L/min |
| 2 Cubic feet per second | 3398.0215908 L/min |
| 3 Cubic feet per second | 5097.0323862 L/min |
| 4 Cubic feet per second | 6796.0431816 L/min |
| 5 Cubic feet per second | 8495.053977 L/min |
| 10 Cubic feet per second | 16990.107954 L/min |
| 20 Cubic feet per second | 33980.215908 L/min |
| 25 Cubic feet per second | 42475.269885 L/min |
| 50 Cubic feet per second | 84950.53977 L/min |
| 100 Cubic feet per second | 169901.07954 L/min |
| Reference | Cubic feet per second (ft³/s) | Liters per minute (L/min) |
|---|---|---|
| A domestic shower | 0.0052972 ft³/s | 9 L/min |
| A kitchen tap | 0.00588578 ft³/s | 10 L/min |
| A garden hose | 0.00882867 ft³/s | 15 L/min |
| The Amazon river | 7380765 ft³/s | 1.254 × 1010 L/min |
The cubic foot per second is a unit of volumetric flow rate equal to one cubic foot passing a point every second. Its symbol is ft³/s, and in hydrology it has a name of its own: the cusec. One cusec is about 28.32 litres per second, roughly the flow of a vigorous garden hose or a small brook.
American river gauging is entirely built on it. The United States Geological Survey publishes stream flow in cubic feet per second at thousands of gauges, water rights in the western states are granted in cusecs, and the Colorado's allocations between states rest on figures first written in this unit a century ago. The Mississippi at its mouth averages about 600,000 cubic feet per second.
Irrigation districts inherited it from the same tradition. A miner's inch, an older western measure, is defined in most states as a fixed fraction of a cusec, and a canal headgate is set to deliver a farmer's entitlement in cubic feet per second for a stated number of hours. The product of the two is the acre-foot, the volume unit that follows naturally from the cusec.
An easy approximation makes the unit memorable: one cubic foot per second flowing for twenty-four hours delivers almost exactly two acre-feet — 1.983, to be precise. Irrigation engineers use the round figure daily, since it converts a rate into a seasonal water budget in a single step.
Flood hydrology is written in it too. A design storm on a small catchment might produce a few hundred cubic feet per second, a major river flood tens or hundreds of thousands, and culverts, bridges and levees in the United States are sized against a peak discharge expressed exactly this way.
Its metric counterpart is the cubic metre per second, which is 35.31 times larger. Hydrological literature moves constantly between the two, and the ratio is worth remembering: a hundred cusecs is a little under three cubic metres per second, and a thousand cubic metres per second is a large river in anyone's units.
One cubic foot per second equals about 28.32 litres per second, about 0.02832 cubic metres per second, or about 448.8 US gallons per minute.
The litre per minute is a unit of volumetric flow rate equal to one litre passing a point every minute. Its symbol is L/min. It is the unit of taps, showers and small pumps — the flows a person meets directly, at a rate slow enough that a minute is the natural interval to count over.
Household plumbing is specified in it almost everywhere. A modern shower head delivers 6 to 9 litres per minute, an older one 15 or more, a kitchen tap 5 to 10, and a bath filler 15 to 20. Water-efficiency regulations in many countries set maximum figures in exactly these terms, because the flow rate multiplied by a typical shower length gives the volume of water and the energy needed to heat it.
That calculation is the reason the unit matters beyond plumbing. Reducing a shower from 12 to 8 litres per minute cuts both the water and the heating energy by a third, and heating water is one of the largest energy uses in a house. A restrictor costing very little changes a household's energy bill measurably.
Medicine uses the unit for gases. Oxygen therapy is prescribed in litres per minute — 2 through a nasal cannula for mild supplementation, 15 through a mask with a reservoir in an emergency — and the flowmeter on the wall of a hospital room is calibrated in exactly this unit. Anaesthetic machines are set the same way.
Engines and compressors also appear here. A small air compressor delivers 100 to 200 litres per minute of free air; a car's cooling system circulates tens of litres per minute; a garden pump moves 20 to 60. In each case the number is the useful one because the equipment runs for minutes at a time rather than seconds.
Sixty litres per minute is one litre per second, so the two units differ by the same factor as the two time units. That makes the conversion easy to do mentally, and it explains why the same equipment is often described in litres per minute by its manufacturer and litres per second by the engineer designing the system it goes into.
One litre per minute equals about 0.01667 litres per second, 1,000 millilitres per minute, or about 0.22 imperial gallons per minute.