Flow rate is the volume of liquid or gas passing through a pipe, duct or opening per unit time, usually expressed in litres per second (l/s), cubic metres per hour (m³/h) or litres per minute (l/min). It is a key quantity in building services and building physics calculations, both for water and for air systems.
Units and conversion
| Unit | Symbol | Typical use |
|---|---|---|
| Litres per second | l/s | Standard for ventilation flow rates |
| Cubic metres per hour | m³/h | Common on fans and in water demand calculations |
| Litres per minute | l/min | Practical for taps and pumps |
Conversion: 1 l/s = 3.6 m³/h = 60 l/min
Application in construction
Flow rate appears in many calculations:
- Ventilation — minimum air change rates required by building regulations
- Domestic hot and cold water — peak demand based on fixture units and simultaneity
- Drainage and rainwater discharge — capacity of downpipes and gutters
- Heating systems — circulating flow through radiators and underfloor pipes
- Dewatering — pump capacity needed to keep an excavation dry
Calculation
For a given pipe: Q = A × v, where Q is the flow rate, A the cross-section and v the flow velocity. In practice, flow rate is determined using charts or software that include resistances, restrictions and simultaneity.
Why getting it right matters
Too low a flow rate causes poor ventilation, unhygienic water systems or cold radiators. Too high a flow rate creates noise, energy loss and accelerated wear. Correct commissioning of flow rates — particularly for ventilation and heating — is therefore essential for comfort and energy use.
Related terms
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