Pumping Formulae

At Malcolm Thompson Pumps, we want to give our customers all the tools they need to properly select, operate and maintain their pumps, and that includes providing them with all the necessary Pumping Formulas.

Selecting the right pumps for your operations is key to achieving maximum efficiency and cost-effectiveness in your operations. There are various ways through which pump input power may be calculated. Below we’ve listed down a pump power formula for every given measurement.

To best ensure your pump choice meets your requirements, or for any questions regarding the formulas below, please do not hesitate to get in touch with one of our experienced customer service team members on 1800 439 607. Our friendly team will be happy to assist you with pump selection, specifications and getting a competitive quote.

Useful Terms

Term Notation Unit
Kilowatt kW
Volumetric flow rate Q L/m or L/s or m³/h
Head H m
Velocity v m/s
Pressure p kPa
Density ɛ kg/m³
Rotational speed N r/min
Specific gravity S.G. SG
Gravitational acceleration g m/s² (9.8m/s)
Electrical current A ampere
Temperature °C
Pump efficiency Ƞ Pump efficiency, decimal

Pumping Formulae for any liquid

Volumetric flow from mass flow & density
For any liquid
Vol flow (l/s) = 1000 * Mass Flow (kg/s)

Density (kg/m3)

Head of liquid from pressure & density
For any liquid
Head (m) = 1000 * Pressure (kPa)

Density (kg/m3) x g

Pump input power from pump performance
For any liquid
Power (kW) = Density (kg/m3) * Volumetric Flow (L/s) * Head (m)

1000 * 102.0 * Efficiency (decimal)

Pump input power from kilowatt – hour meter
Power (kW) = 36 * Disc.Revs. * Meter Ratio Constant * Motor Efficiency (dec)

Time (s) * Meter Disc. Constant (rev/kW-h)

Pump input power from current and voltage
Power (kW) = Current (A) * Voltage (V) * Motor Efficiency (dec) * Pump Efficiency (dec)

57 735

Motor input energy per volume of liquid pumped
For any liquid
Energy/ Volume (kW-h/kl) = Density (kg/m3) * Head (m)

367 347 * Motor Efficiency (dec) * Pump Efficiency (dec)

Pump input power from engine
Power (kW) = Pi * Torque (N.m) * Speed (rpm)

30 000

= Torque (N.m) * speed (rpm)

9 549

Pumping Formulae for water at 20°C

Volumetric flow from mass flow
Vol flow (L/s) = 1.002 * Mass Flow (kg/s)
Head of liquid from pressure
Head (m) = 0.1022 * Pressue (kPa)
Head (ft) = 0.3354 * Pressure (kPa)
Pump input power from pump performance
Power (kW) = volumetric flow (L/s) * Head (m)

102.2 * Efficiency (decimal)

Motor input energy per volume of liquid pumped
energy/volume (kW-h/kl) = Head (m)

368.0 * Motor efficiency (dec) * pump efficiency (dec)

Velocity of flow
Through a circular section
Velocity of flow (m/s) = 1273 * Volumetric Flow (L/s)

( Diameter (mm) )²

Velocity head
Velocity head (m) = 1000 * Volumetric Flow (L/s)

Area (mm2)

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