| US Gallons per second (gal US/s) | Cubic feet per second (ft³/s) |
|---|---|
| 1 US Gallon per second | 0.133680555565 ft³/s |
| 2 US Gallons per second | 0.26736111113 ft³/s |
| 3 US Gallons per second | 0.401041666695 ft³/s |
| 4 US Gallons per second | 0.53472222226 ft³/s |
| 5 US Gallons per second | 0.668402777825 ft³/s |
| 10 US Gallons per second | 1.33680555565 ft³/s |
| 20 US Gallons per second | 2.6736111113 ft³/s |
| 25 US Gallons per second | 3.34201388912 ft³/s |
| 50 US Gallons per second | 6.68402777825 ft³/s |
| 100 US Gallons per second | 13.3680555565 ft³/s |
| Reference | US Gallons per second (gal US/s) | Cubic feet per second (ft³/s) |
|---|---|---|
| A domestic shower | 0.0396258 gal US/s | 0.0052972 ft³/s |
| A kitchen tap | 0.0440287 gal US/s | 0.00588578 ft³/s |
| A garden hose | 0.066043 gal US/s | 0.00882867 ft³/s |
| The Amazon river | 55211959 gal US/s | 7380765 ft³/s |
The US gallon per second is a unit of volumetric flow rate equal to one United States gallon passing a point every second. Its symbol is gal US/s. The US gallon is exactly 3.785411784 litres, descended from the English wine gallon of 231 cubic inches, so one US gallon per second is a little under four litres every second.
Fire protection in the United States works at this scale. A fire hydrant flows several gallons per second, a pumper truck delivers 20 or more, and a large sprinkler system's design demand runs to tens. Fire codes state these figures in gallons per minute, but hydraulic calculations of friction loss in the mains use the per-second value, and the two are the same number divided by sixty.
Municipal water and wastewater engineering also reaches it. A neighbourhood water main carries a few gallons per second at peak hour, a lift station handles tens, and a treatment plant's influent hundreds. American practice usually states plant capacity in million gallons per day, and one gallon per second is 0.0864 million gallons per day — a conversion any water engineer performs from memory.
Industrial cooling and process water are described here as well. Condenser cooling water, quench flows and washdown systems all reach gallons per second, and the pump power required follows from the flow multiplied by the head, so an increase in flow of ten per cent shows up directly on the electricity bill.
The difference from the imperial gallon deserves a clear statement. The US gallon is about 17 per cent smaller than the imperial one, so the same numerical rate is a materially different flow on either side of the Atlantic. Equipment documentation should say which gallon it means, and prudent engineers convert to litres before comparing anything.
For scale, one US gallon per second is about 3.79 litres per second, and 0.264 US gallons per second is one litre per second. A flow of ten US gallons per second is nearly 38 litres per second, enough to fill a domestic bath in three seconds and comparable to a small stream.
One US gallon per second equals about 3.785 litres per second, about 0.003785 cubic metres per second, or about 0.8327 imperial gallons 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.