liminfoPump Sizing Calculator
Free web tool: Pump Sizing Calculator
NPSH Calculation
Results
Formulas
About Pump Sizing Calculator
The Pump Sizing Calculator performs the core hydraulic calculations needed to select and size a centrifugal pump for a piping system. Given a flow rate (m³/h), static head (m), and friction head loss (m), it computes the Total Dynamic Head (TDH = static head + friction head). From TDH, fluid density (kg/m³), and pump efficiency, it calculates hydraulic power (Ph = ρgQH / 1000 kW), brake power or shaft power (Pb = Ph / η_pump), and electrical (input) power (Pe = Pb / η_motor). These three power values form the chain from fluid energy to motor nameplate rating.
The tool also calculates the Available Net Positive Suction Head (NPSHa), which determines whether the pump will cavitate. NPSHa = (Patm − Pvapor) × 1000 / (ρg) + suction head − suction friction loss. If NPSHa is less than the pump's required NPSHr (from the pump curve), cavitation will occur. This is a critical safety and reliability check for any installation. Atmospheric pressure defaults to 101.325 kPa and vapor pressure to 2.339 kPa (water at 20°C), but both can be adjusted for different fluids, temperatures, or elevations.
The specific speed (Ns = N√Q / H^0.75, where N is assumed 1450 RPM) is calculated and used to recommend the most suitable pump impeller type: positive displacement (Ns < 20), radial/centrifugal (20–80), mixed flow (80–160), or axial flow (Ns > 160). A recommended motor size is also provided from the IEC/NEMA standard motor series, selected as the smallest standard motor that exceeds Pe × 1.15 (a 15% safety margin). This single tool covers the primary calculations an engineer needs for preliminary pump selection.
Key Features
- Calculates Total Dynamic Head (TDH) from static head and friction head loss
- Computes hydraulic power: Ph = ρgQH / 1000 (kW)
- Computes brake (shaft) power: Pb = Ph / pump efficiency
- Computes electrical (motor input) power: Pe = Pb / motor efficiency
- Calculates NPSHa from atmospheric pressure, vapor pressure, suction head, and suction friction loss
- Computes specific speed (Ns) and recommends pump type: positive displacement, centrifugal, mixed flow, or axial
- Recommends the smallest standard IEC/NEMA motor size at Pe × 1.15 safety margin
- Displays all formulas inline (TDH, Ph, Pb, NPSHa) for full engineering transparency
Frequently Asked Questions
What is Total Dynamic Head (TDH)?
TDH is the total energy per unit weight that the pump must add to the fluid, expressed in meters of fluid column. It is the sum of static head (elevation difference between suction and discharge) and friction head loss in the piping and fittings. TDH = static head + friction head loss. This is the primary sizing parameter for pump selection.
What is the difference between hydraulic power, brake power, and electrical power?
Hydraulic power (Ph) is the power actually delivered to the fluid: Ph = ρgQH/1000 kW. Brake power (Pb) is the shaft power the pump requires from the motor, accounting for pump inefficiency: Pb = Ph / η_pump. Electrical power (Pe) is the actual power drawn from the supply, accounting for motor losses: Pe = Pb / η_motor. Pe is always the largest of the three values.
What is NPSHa and why does it matter?
NPSHa (Available Net Positive Suction Head) is the pressure margin at the pump inlet above the fluid's vapor pressure. If NPSHa falls below the pump's required NPSHr (from the pump curve), the fluid will locally vaporize, forming bubbles that collapse violently inside the impeller — a condition called cavitation. Cavitation causes noise, vibration, erosion, and impeller damage. NPSHa must always exceed NPSHr by at least 0.5–1.0 m margin.
What is specific speed and which pump type should I choose?
Specific speed (Ns = N√Q / H^0.75) is a dimensionless index that characterizes the impeller geometry best suited for a given flow/head combination. Low Ns (< 20): positive displacement pumps for very high head and low flow. Ns 20–80: radial centrifugal pumps, the most common type. Ns 80–160: mixed-flow pumps for medium head and higher flow. Ns > 160: axial-flow (propeller) pumps for very high flow and low head.
Why is the motor size 15% larger than the calculated electrical power?
The 1.15 safety factor accounts for motor derating due to voltage variations, ambient temperature, motor aging, and unexpected process variations that may cause the pump to operate beyond its design point. Selecting a motor that is too close to the maximum calculated power risks overloading and tripping. Standard practice is to select the next standard motor size above Pe × 1.15.
What fluid density should I use for water?
Water at 20°C has a density of approximately 998 kg/m³ (the tool's default). Hot water at 80°C is about 972 kg/m³; seawater is about 1025 kg/m³; light oils range from 800–870 kg/m³. Using the correct density is important because hydraulic power is directly proportional to ρ — a 10% difference in density produces a 10% difference in required power.
How do I estimate friction head loss for my system?
Friction head loss depends on pipe diameter, length, material (roughness), fluid velocity, and fittings. For a rough estimate, use 1–3 m per 100 m of pipe for low-velocity water systems. For more accurate results, use the Darcy-Weisbach equation or the Hazen-Williams formula with your pipe parameters. The total system friction head loss should include both straight pipe losses and equivalent lengths for fittings, valves, and bends.
What flow rate units does this calculator use?
The calculator accepts flow rate in m³/h (cubic meters per hour). Internally it converts to m³/s (÷ 3600) for power calculations. Common conversions: 1 m³/h = 4.40 US gallons per minute (GPM) = 0.278 liters per second (L/s). If you have a flow in GPM, divide by 4.40 to get m³/h.