Convert mass flow rate between lb/min, kg/s, GPM, L/min, and m³/h. Includes pipe velocity estimation and quick conversion reference table.
Pounds per minute (lb/min) is a mass flow rate unit commonly used in US engineering, particularly for fuel systems, HVAC water loops, steam systems, and chemical processes. Converting between lb/min, kg/s, GPM, and L/min requires knowing the fluid density. That is why the same mass flow can map to very different volume flow values depending on the fluid.
This calculator converts mass flow rates between all common units and also computes volumetric flow rates (GPM, L/min, m³/h, CFM) using the specified fluid density. If a pipe diameter is given, it estimates the flow velocity — critical for checking erosion limits and pressure drop.
Preset buttons load typical values for automotive fuel systems, HVAC water loops, steam boilers, chemical feeds, and compressed air. A quick conversion table provides reference values for water at standard conditions.
The tool bridges the gap between US customary and metric systems, which is the most common source of flow rate calculation errors in international engineering projects.
Flow rate unit conversion errors between US customary and SI systems are a common source of engineering mistakes. This calculator eliminates that risk.
Having mass flow, volume flow, and pipe velocity all in one tool streamlines the design process for piping, HVAC, and process systems.
1 lb/min = 0.007560 kg/s = 0.4536 kg/min. Volume flow: Q_vol = ṁ / ρ. 1 GPM = 3.785 L/min = 6.309×10⁻⁵ m³/s. Velocity: v = Q_vol / A_pipe.
Result: 0.907 kg/s, 14.4 GPM, 0.91 L/s, velocity = 0.46 m/s
120 lb/min × 0.00756 = 0.907 kg/s. Volume = 0.907/998 = 9.09×10⁻⁴ m³/s = 0.909 L/s = 14.4 GPM. Velocity = 9.09×10⁻⁴ / (π×0.025²) = 0.46 m/s.
The same lb/min value can represent a very different volume flow rate depending on fluid density. A pound per minute of water, for example, occupies far less volume than a pound per minute of air.
If velocity looks unrealistically high, verify the density, pipe diameter, and whether you meant mass flow or volumetric flow. That is usually where the mismatch comes from.
Use this calculator to compare pump sizing, line sizing, and process flow notes before finalizing a spec sheet.
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lb/min is preferred when the fluid density varies (steam, gases, slurries) or when mass balance is important (chemical reactors, combustion). GPM is convenient for water at constant temperature.
Divide by the fluid's weight density in lb/gal. For water: 1 gal = 8.33 lb, so GPM = lb/min ÷ 8.33. For other fluids, adjust by specific gravity.
For water systems: 1-3 m/s for general piping, <1 m/s for gravity drain, 3-6 m/s maximum for short runs. Higher velocities increase erosion and noise.
CFM (cubic feet per minute) is a volume flow rate. For gases, always specify conditions (SCFM = standard, ACFM = actual). Converting to mass flow requires knowing gas density at those conditions.
Temperature changes density. Hot water at 80°C has ρ = 972 kg/m³ instead of 998. This means the same mass flow gives higher volume flow through the pipe.
Steam density varies enormously with pressure: 0.6 kg/m³ at 1 atm to 40+ kg/m³ at high pressure. Always use the correct steam table density for conversions.