Conversion from Cubic meters per second to Gallons per second

Information about the Cubic meter per second (m³/s)

The cubic metre per second is the SI unit of volumetric flow rate, equal to one cubic metre passing a point every second. Its symbol is m³/s, and hydrologists call it the cumec. As the coherent SI unit it needs no conversion factor: a velocity in metres per second multiplied by an area in square metres gives a flow directly in cubic metres per second.

Rivers are its natural subject. A small stream runs at a fraction of a cumec, the Thames at Kingston averages about 65, the Rhine at the Dutch border around 2,200, and the Amazon roughly 209,000 — a fifth of all the fresh water reaching the world's oceans. Flood warnings, abstraction licences and reservoir operating rules are all written in these numbers.

Hydroelectric power follows from the same figure. The power available is the flow multiplied by the head, by the density of water and by gravity, so 10 cubic metres per second falling 50 metres yields about 4.9 megawatts before losses. A turbine's rating and a river's flow-duration curve together determine how much of the year a station can run at full output.

Large ventilation and process equipment is rated here too. A road-tunnel fan moves tens of cubic metres per second, a power-station cooling-water pump tens more, and a blast furnace draws hundreds of cubic metres of air per second. Once the numbers reach this scale the cubic metre per second is more readable than any smaller unit.

The unit is also convenient because a cubic metre of water is a tonne. One cubic metre per second is therefore one tonne of water per second, which turns a flow into a force and a mass loading without further arithmetic — useful when sizing a spillway, a screen or a bridge pier.

Building services and hydrology sit awkwardly on either side of it. Ventilation engineers work in litres per second because their flows are a thousandth of a cumec, while river engineers would need six digits to express theirs in litres. The factor of a thousand between the two units is what keeps both trades in comfortable numbers.

One cubic metre per second equals 1,000 litres per second, 60 cubic metres per minute, or about 35.31 cubic feet per second.

Information about the Gallon per second (gal/s)

The gallon per second is a unit of volumetric flow rate equal to one imperial gallon passing a point every second. Its symbol is gal/s. The imperial gallon is 4.54609 litres, defined in 1824 as the volume of ten pounds of water at a stated temperature, so one gallon per second moves about four and a half litres — and, conveniently, about ten pounds — every second.

Firefighting is the discipline that works at this scale. A hose reel delivers a fraction of a gallon per second, a standard hose line 1 to 2, and a major pumping appliance 10 or more. British fire service practice quoted pump capacities in gallons per minute for most of the twentieth century, and the per-second figure is what a hydraulic calculation of pressure loss in the hose actually uses.

Large pumps and water mains reach it easily. A borehole pump on a farm might deliver half a gallon per second, a district water main several tens, and a river intake for a power station hundreds. Since the gallon of water weighs ten pounds, a flow in gallons per second converts to a mass flow in stones per second by dividing by 1.4, an arithmetic that older British engineering handbooks used freely.

Flood and drainage engineering in Britain used the unit until metrication. Culvert capacities, pumping station ratings and land drainage schemes were designed in gallons per second or per minute, and much of the surviving infrastructure still carries plates giving its capacity in those terms, which anyone assessing an old asset must convert with care.

The imperial gallon must be distinguished from the American one, which is 3.78541 litres — about twenty per cent smaller. A pump rated 10 gallons per second delivers 45.5 litres per second in Britain and 37.9 in the United States, a difference far too large to ignore in any calculation of capacity or of cost.

For scale, one gallon per second fills a domestic bath in about twenty-five seconds, and 0.22 gallons per second is one litre per second. A flow of ten gallons per second is a small stream, running at about 45 litres per second.

One gallon per second equals about 4.546 litres per second, about 0.004546 cubic metres per second, or about 1.201 US gallons per second.