A customer may say:
“My well is 150 metres deep, so I need a pump with 150 metres of head.”
This is not always correct. A submersible well pump normally lifts water from the dynamic water level, not from the bottom of the well. The pump must also overcome the elevation above ground, the pressure required at the outlet, and the resistance created by pipes, valves, fittings, and filters.
These values are combined to calculate the Total Dynamic Head, usually abbreviated as TDH. Once the required flow and TDH are known, the pump duty point can be checked against the performance curve.
Before calculating the head, the project flow should already be confirmed. Read our guide on how to calculate the required flow rate for a submersible well pump if the flow has not yet been determined.
What Is Total Dynamic Head?
Total Dynamic Head is the total pressure the pump must produce while delivering the required flow. It is normally expressed in metres of water head or feet of head.
For most deep-well systems, TDH contains four main parts:
① Vertical lift
② Required outlet pressure
③ Pipeline friction loss
④ Losses through valves, fittings, and equipment
The basic formula is:
TDH = Vertical Lift + Pressure Head + Pipe Friction Loss + Equipment Losses
A complete pump requirement should therefore be written as both flow and head, for example:
60 m³/h at 120 m TDH
A statement such as “60 m³/h pump” or “120 m head pump” is incomplete because the supplier needs both values to select the correct model.
Why Well Depth Is Not the Required Pump Head
The total well depth shows how far the borehole was drilled. It does not show the actual distance from which water is lifted during operation.
Consider the following well:
① Total well depth: 180 m
② Static water level: 35 m
③ Dynamic water level: 70 m
④ Pump installation depth: 110 m
The head calculation normally begins from the 70 m dynamic water level. It does not begin from the 180 m well bottom or the 110 m pump installation depth.
The difference between static and dynamic water levels is explained in our guide to static water level vs. dynamic water level.
Selecting a pump based on total well depth may result in too many pump stages, excessive pressure, a larger motor, and unnecessary energy consumption.
Start with the Dynamic Water Level
Static water level is measured when the pump is stopped. Dynamic water level is measured while the pump is running at a specific flow.
The dynamic level is normally used because it represents the actual pumping condition. For example, a well may have a static level of 30 m but a dynamic level of 65 m while pumping 50 m³/h. The vertical-lift calculation should begin from approximately 65 m.
The test flow should always be recorded together with the dynamic water level. A statement such as “dynamic water level: 65 m” is not complete unless the pumping rate and test duration are also known.
Where the water level changes significantly between wet and dry seasons, use the lowest expected operating level when checking the required head.
Add the Discharge Elevation
If water is delivered above ground level, that elevation must be added to the vertical lift.
Suppose the dynamic water level is 65 m below ground and the inlet of the storage tank is 20 m above ground. The total vertical lift is:
65 m + 20 m = 85 m
Measure the elevation to the actual discharge point. For a storage tank, this is normally the inlet level rather than the base of the tank.
For irrigation systems, the highest point in the field may determine the required elevation. For municipal or industrial systems, the calculation may need to include the height of a building, reservoir, or process tank.
Convert the Required Pressure into Head
Many systems need pressure at the outlet. Sprinklers, filters, pressure tanks, industrial equipment, and water networks cannot operate correctly from flow alone.
For water, a useful conversion is:
1 bar ≈ 10.2 m of water head
If a sprinkler system requires 3 bar at the inlet:
3 × 10.2 = 30.6 m head
This 30.6 m must be added to the vertical lift and friction losses.
Do not confuse outlet pressure with pump maximum pressure. The pump must provide the required pressure while also delivering the design flow.
Include Pipe Friction Loss
Water loses pressure while moving through a pipe. The amount of loss depends mainly on flow rate, pipe internal diameter, pipe length, material, and internal roughness.
A small pipe creates higher water velocity and greater friction loss. Over a long pipeline, increasing the pipe diameter can reduce the required pump head and lower long-term electricity costs.
Friction loss must be calculated at the actual design flow. If the required flow increases, the friction loss also increases, often significantly.
The rising pipe inside the well and the horizontal discharge pipeline should both be included.
Include Valves, Fittings, and Filters
Elbows, tees, reducers, check valves, isolation valves, flow meters, filters, and control valves all create additional resistance.
These losses may be small in a simple system, but they can become important where the pipeline contains many fittings or restrictive equipment. Filters can produce especially high losses as they become dirty.
For preliminary pump selection, estimated losses may be acceptable. For final project design, actual manufacturer data should be used wherever possible.
A supplier quotation that ignores filters and valves may recommend a pump that cannot maintain the required outlet pressure.
Total Dynamic Head Calculation Example
Consider a project with the following conditions:
① Required flow: 50 m³/h
② Dynamic water level: 60 m below ground
③ Storage tank inlet: 15 m above ground
④ Required pressure: 1 bar
⑤ Pipe and fitting losses: 12 m
First calculate the vertical lift:
60 + 15 = 75 m
Convert the required pressure:
1 bar × 10.2 = 10.2 m
Add the friction losses:
TDH = 75 + 10.2 + 12
TDH = 97.2 m
The preliminary pump duty point is therefore:
50 m³/h at approximately 97 m TDH
The next step is to plot this point on the pump curve and confirm the pump model, motor power, number of stages, and efficiency.
Irrigation Pump Head Example
Consider an irrigation project with:
① Required flow: 100 m³/h
② Dynamic water level: 55 m
③ Highest field elevation: 18 m above the well
④ Required sprinkler pressure: 3.5 bar
⑤ Pipeline friction loss: 22 m
⑥ Filter and valve losses: 6 m
The vertical lift is:
55 + 18 = 73 m
The pressure head is:
3.5 × 10.2 = 35.7 m
The Total Dynamic Head is:
73 + 35.7 + 22 + 6 = 136.7 m
The required duty point is approximately:
100 m³/h at 137 m TDH
This does not mean any pump with a maximum head above 137 m will be suitable. The pump must deliver 100 m³/h at that head within its recommended operating range.
Maximum Head Is Not Operating Head
Maximum head is normally the pressure a pump produces near zero flow. It is also known as shutoff head.
If a catalogue shows:
① Maximum flow: 100 m³/h
② Maximum head: 180 m
This does not mean the pump delivers 100 m³/h at 180 m head. These values occur at different points on the performance curve.
The actual project duty point should be marked on the curve. Our guide on how to read a submersible pump performance curve explains how to check flow, head, efficiency, and motor power.
When comparing quotations, ask suppliers to show the selected duty point instead of providing only maximum catalogue values.
How TDH Affects Motor Power
Higher flow and higher head require more hydraulic power. The motor must have enough capacity at the actual operating point.
Motor selection depends on:
① Required flow and TDH
② Pump efficiency
③ Motor efficiency
④ Voltage and frequency
⑤ Starting method
⑥ Cable voltage drop
⑦ Daily operating hours
A larger motor does not correct an incorrect pump selection. The impeller stages and pump curve must still match the required flow and head.
After TDH is calculated, the required motor can be reviewed using our guide on how many HP a submersible well pump needs.
Common TDH Calculation Mistakes
The most common mistake is treating total well depth as required pump head. Other frequent errors include using static water level instead of dynamic water level, forgetting discharge elevation, and excluding required outlet pressure.
Pipeline friction is also often underestimated. This happens when the pipe is too small, the wrong internal diameter is used, or valves, fittings, and filters are left out of the calculation.
Another problem is adding an excessive safety margin. Too much additional head can result in excessive pressure, higher energy consumption, unnecessary pump stages, and operation too far to the left of the pump curve.
A reasonable allowance may be added where measurements are uncertain, but it should not replace accurate project data.
What a Reliable Pump Supplier Should Provide
A reliable supplier should review the complete system before recommending a pump. The supplier should confirm the required flow, dynamic water level, discharge elevation, pipe size, pipeline length, outlet pressure, water quality, electrical supply, and operating schedule.
The technical proposal should include:
① Selected duty point
② Pump performance curve
③ Pump efficiency
④ Motor power and rated current
⑤ Number of stages
⑥ Pump and impeller materials
⑦ Voltage and frequency
⑧ Recommended operating range
Buyers can also compare the calculated duty point with available SLAPK submersible well pump models.
Two quotations should always be compared at the same flow and head. A lower-priced pump quoted at a lower head is not technically equivalent to a pump that meets the complete project requirement.
Information Required for a Pump Head Calculation
Before requesting a pump selection or quotation, prepare:
① Required flow rate
② Total well depth
③ Static water level
④ Dynamic water level and test flow
⑤ Discharge-point elevation
⑥ Required outlet pressure
⑦ Rising-pipe diameter and length
⑧ Horizontal-pipe diameter and length
⑨ Pipe material
⑩ Valves, fittings, and filters
⑪ Water quality and sand content
⑫ Voltage, frequency, and phase
Providing the same data to each supplier makes technical proposals easier to compare and reduces the risk of different manufacturers working from different assumptions.
Request a Pump Selection from SLAPK
A complete submersible well pump duty point should include both the required flow and Total Dynamic Head.
For example:
60 m³/h at 125 m TDH
Send SLAPK your flow requirement, dynamic water level, discharge elevation, pipe information, outlet pressure, water quality, and electrical supply. Our engineering team can review the hydraulic conditions and recommend a suitable model supported by a performance curve and technical data.
For a wider review of well conditions, materials, motor selection, and installation requirements, read our submersible well pump selection guide.
