| Cubic meters per second (m³/s) | Cubic feet per hour (ft³/h) |
|---|---|
| 1 Cubic meter per second | 127132.800206 ft³/h |
| 2 Cubic meters per second | 254265.600413 ft³/h |
| 3 Cubic meters per second | 381398.400619 ft³/h |
| 4 Cubic meters per second | 508531.200825 ft³/h |
| 5 Cubic meters per second | 635664.001032 ft³/h |
| 10 Cubic meters per second | 1271328.00206 ft³/h |
| 20 Cubic meters per second | 2542656.00413 ft³/h |
| 25 Cubic meters per second | 3178320.00516 ft³/h |
| 50 Cubic meters per second | 6356640.01032 ft³/h |
| 100 Cubic meters per second | 12713280.0206 ft³/h |
| Reference | Cubic meters per second (m³/s) | Cubic feet per hour (ft³/h) |
|---|---|---|
| A domestic shower | 0.00015 m³/s | 19.0699 ft³/h |
| A kitchen tap | 0.000166667 m³/s | 21.1888 ft³/h |
| A garden hose | 0.00025 m³/s | 31.7832 ft³/h |
| The Amazon river | 209000 m³/s | 2.65708 × 1010 ft³/h |
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.
The cubic foot per hour is a unit of volumetric flow rate equal to one cubic foot passing a point every hour. Its symbol is ft³/h, and in the gas trades it is abbreviated CFH. It is the unit of fuel gas supply throughout North America, where every appliance and every pipe is sized by it.
Natural gas is billed and metered in cubic feet, so the hourly rate is what a gas fitter works with. A domestic furnace consumes 80 to 150 cubic feet per hour, a water heater 40, a cooking range 65, and a whole house at design conditions perhaps 250. A residential meter is rated at 250 CFH and the service pipe at rather more.
Pipe sizing tables are built entirely around this unit. Given a length of pipe, a diameter and an allowable pressure drop, the table returns a capacity in cubic feet per hour, and the fitter compares it against the sum of the appliance ratings on that branch. Undersize the pipe and the burners starve when everything runs at once; oversize it and the material cost rises for no benefit.
The unit converts easily into heat. Natural gas carries roughly 1,000 British thermal units per cubic foot, so a rate in cubic feet per hour is very nearly the same number in thousands of British thermal units per hour. An appliance rated 100,000 British thermal units per hour therefore burns about 100 cubic feet per hour, which is why the two figures appear together on every rating plate.
Propane behaves differently and is worth stating separately. It carries about 2,500 British thermal units per cubic foot, so the same appliance needs less than half the volumetric flow, and a pipe sized for natural gas is generously large for propane. Confusing the two is a classic and dangerous error in the trade.
Beyond fuel gas, the unit describes slow air and gas flows in imperial practice: purge lines, instrument air, small blowers and laboratory gas supplies. Sixty cubic feet per hour is one cubic foot per minute, and 35.31 cubic feet per hour is one cubic metre per hour.
One cubic foot per hour equals about 0.02832 cubic metres per hour, about 28.32 litres per hour, or about 0.01667 cubic feet per minute.