The units of flow rate
Flow rate is volume divided by time, so it inherits the volume system's quirks along with a time unit. Plumbing and pumps are usually specified in litres per minute or cubic metres per hour, irrigation in litres per second, and American equipment in gallons per minute. Which unit a datasheet uses tells you roughly what industry it came from.
| Unit | Symbol | Equals |
|---|---|---|
| Litres per second | l/s | 1 l/s |
| Litres per minute | l/min | 0.01666667 l/s |
| Litres per hour | l/h | 0.00027778 l/s |
| Cubic metres per hour | m³/h | 0.27777778 l/s |
| Cubic metres per second | m³/s | 1,000 l/s |
| Gallons per minute | gpm | 0.0630902 l/s |
How to convert by hand
Every flow rate conversion follows the same procedure: take the amount to the base unit, then from there to the target unit. In practice it is a single multiplication, because the two steps combine into one factor.
Convert to a common base, usually litres per second, then out to the target. One cubic metre per hour is 16.67 litres per minute, and one US gallon per minute is 3.785 litres per minute.
The equivalents people look up most
| Conversion | Result |
|---|---|
| A kitchen tap | 12 l/min · 0.72 m³/h |
| A shower | 9 l/min · 0.54 m³/h |
| A garden hose | 15 l/min · 4 gpm |
| A small pump | 3 m³/h · 50 l/min |
| 1 m³/h | 16.67 l/min · 0.278 l/s |
Common mistakes
- Mixing US and imperial gallons per minute. The 20% gap in the gallon carries straight into the flow rate.
- Confusing flow rate with pressure. A pump's head and its flow are different specifications, and a pump can have plenty of one and little of the other.
- Reading m³/h as m³/min. The factor of 60 turns a modest pump into an impossible one.
- Comparing rated flow with real flow. Datasheet figures are at zero head; real installations always deliver less.