Introduction
Flow rate is the volume of fluid passing a point per unit time. US plumbing uses gallons per minute (GPM); industrial systems use liters per second (L/s) or cubic meters per hour (m³/h); medical IVs use mL/hr. Garden hoses deliver 10-20 GPM, shower heads 1.5-2.5 GPM (US post-1992 regulation), fire hydrants 500-1,500 GPM.
Why flow rate units exist and how they diverged
Flow rate is volume divided by time. The SI unit is cubic meters per second (m³/s), but this is inconvenient for most household and medical flows — m³/s is a huge rate. Liters per minute, gallons per minute, mL/hour are more common in practice. Industrial fluid transport uses bigger units: m³/h, gallons per hour, barrels per day (oil industry).
How to convert flow rate
GPM (US) to L/min: multiply by 3.785. L/min to m³/h: multiply by 0.06. GPM to L/s: multiply by 0.0631. Flow × time = volume, so GPM × minutes = gallons delivered.
Units supported by this flow rate calculator
- m³/s
- m³/h
- L/s
- L/min
- L/h
- gal/min (US, GPM)
- gal/min (UK)
- gal/hr (US)
- cubic ft/min (CFM)
- cubic ft/s
Common flow rate conversion mistakes
- US vs UK gallons. A UK GPM is 20% bigger. Plumbing spec sheets usually specify.
- CFM vs GPM. Cubic feet per minute (CFM) for gas/air; GPM for liquids. Different units for different fluids.
- Max rated vs actual flow. A '2.5 GPM shower head' caps at that flow; actual depends on supply pressure. Low water pressure = less flow.
- Head vs flow. A pump can push high head (pressure, elevation) or high flow, but tradeoffs exist. A pump curve shows the relationship.
Real-world flow rate examples
- Garden hose: 10-20 GPM (38-76 L/min).
- Kitchen faucet: 1.5-2.2 GPM (5.7-8.3 L/min).
- Shower head (US post-1992): 2.5 GPM max (9.5 L/min); low-flow 1.5-1.8 GPM.
- Bathtub spigot: 4-7 GPM.
- Washing machine fill: 5-15 GPM for 1-2 minutes.
- Toilet flush: 1.28-1.6 gallons per flush (US WaterSense).
- Residential water main: 8-20 GPM typical max.
- Fire hydrant: 500-1,500 GPM.
- Large river: thousands of m³/s.
- Mississippi River at New Orleans: ~17,000 m³/s average.
- Niagara Falls: 2,800 m³/s summer flow.
- Medical IV drip (normal): 1-3 mL/min.
Tips for accurate flow rate conversion
- For irrigation, measure flow with a bucket and stopwatch: fill a 5-gallon bucket, note time. 60 sec = 5 GPM.
- For showers, low-flow heads (1.8 GPM) save significant hot water without feeling weak.
- For fire protection design, always work in GPM or L/s as specified by code.
Related: Volume Converter · Time Converter · Viscosity Converter.