Two ways to get the same flow rate
Volumetric flow rate is how much fluid passes a point per unit of time. Measure it directly as volume over time, or compute it from the pipe's cross-section area and the flow velocity. For a round pipe, A = π × (d∕2)², so a pipe of inner diameter d and velocity v gives Q straight away.
Flow-rate units, converted
The same flow reads very differently depending on the unit. GPM and CFM are US; m³/h and L/min are used everywhere else. These are the factors people actually look up.
| From | Equals |
|---|---|
| 1 GPM (US) | 3.785 L/min |
| 1 L/min | 0.264 GPM · 0.0167 L/s |
| 1 m³/h | 16.67 L/min · 4.40 GPM |
| 1 m³/s | 1000 L/s · 15,850 GPM |
| 1 CFM (ft³/min) | 28.32 L/min · 1.699 m³/h |
A typical home tap runs 5–15 L/min; office plumbing 20–60 L/min. HVAC airflow is quoted in CFM in the US and m³/h in Europe.
Volume flow vs mass flow, and where each matters
- Volume flow (Q). Cubic metres or litres per unit time. Independent of density, so it is the natural choice for water, plumbing and HVAC.
- Mass flow (ṁ). Mass per unit time, found by ṁ = ρ × Q. Use it for gases, combustion and process chemistry, where temperature and pressure change the density.
- Continuity. In a full pipe with no leaks, Q is the same at every cross-section: A₁v₁ = A₂v₂. Narrow the pipe and the velocity climbs.
Common questions
What is the difference between Q = volume / time and Q = area x velocity?
They give the same answer two ways. Volume divided by time uses what you actually collected; area times velocity works it out from the pipe geometry and how fast the fluid moves. For steady flow in a full pipe the two are equal.
How do I convert GPM to litres per minute?
Multiply US gallons per minute by 3.785. So 10 GPM is 37.85 L/min. To go back, divide L/min by 3.785.
Why does water speed up when a pipe narrows?
The same flow rate has to pass through a smaller cross-section, so the velocity rises to keep Q = area x velocity constant. That is why a thumb over a hose nozzle makes the water shoot further.
