| Liters per second (L/s) | Cubic feet per second (ft³/s) |
|---|---|
| 1 Liter per second | 0.035314666724 ft³/s |
| 2 Liters per second | 0.070629333448 ft³/s |
| 3 Liters per second | 0.105944000172 ft³/s |
| 4 Liters per second | 0.141258666896 ft³/s |
| 5 Liters per second | 0.17657333362 ft³/s |
| 10 Liters per second | 0.35314666724 ft³/s |
| 20 Liters per second | 0.70629333448 ft³/s |
| 25 Liters per second | 0.8828666681 ft³/s |
| 50 Liters per second | 1.7657333362 ft³/s |
| 100 Liters per second | 3.5314666724 ft³/s |
| Reference | Liters per second (L/s) | Cubic feet per second (ft³/s) |
|---|---|---|
| A domestic shower | 0.15 L/s | 0.0052972 ft³/s |
| A kitchen tap | 0.166667 L/s | 0.00588578 ft³/s |
| A garden hose | 0.25 L/s | 0.00882867 ft³/s |
| The Amazon river | 209000000 L/s | 7380765 ft³/s |
The litre per second is a unit of volumetric flow rate equal to one litre passing a point every second. Its symbol is L/s. It is the unit of pumps, drains and ventilation — the scale at which a flow is large enough to be a design problem but small enough to belong to a single building.
A litre per second is a substantial stream. A kitchen tap fully open delivers about a fifth of it, a bath fills at roughly a third, and a fire hose runs at ten to twenty times as much. In visual terms it is a jet about as thick as a thumb moving briskly, and it would fill a domestic bath in about two minutes.
Building services are specified in this unit throughout Europe. Ventilation rates are given as litres per second per person — around eight to ten in an office, more in a room where people cook or exercise — and a designer multiplies that figure by the number of occupants to size the fans and the ductwork. The same unit describes rainwater drainage, where a roof's area and the local rainfall intensity together determine the flow a downpipe must carry.
Pumps are rated the same way. A domestic circulating pump moves a fraction of a litre per second, a borehole pump a few, and a large sewage pump hundreds. Because the power a pump needs is the flow multiplied by the pressure it must overcome, this figure sits at the centre of every pump calculation.
For gases the unit describes compressors and blowers, though there the volume depends on pressure and temperature and so must be stated at defined conditions. A compressor rated at fifty litres per second of free air is measured with the air at atmospheric pressure, and the same machine moving compressed air is shifting a far smaller volume.
A litre per second is a thousandth of a cubic metre per second, which is why hydrologists and building engineers rarely use the same unit even when they are describing the same water. A river carrying a cubic metre per second is carrying a thousand litres per second, and both figures are correct.
One litre per second equals 1,000 millilitres per second, 60 litres per minute, or about 0.0353 cubic feet per second.
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.