Start with the Required Flow
TDH cannot be calculated in isolation because pipe losses change with flow. Before working on head, define how much water the system must deliver and over what period. An irrigation project may need 60 m³/h for eight hours, while a storage-tank application may accept a lower hourly flow over a longer filling period. A vague request such as “I need a 45 kW pump” gives the supplier no reliable basis for selection; “I need 60 m³/h at the final delivery point” does. Once the flow is fixed, water-level drawdown, pipe friction, outlet pressure, and the pump curve can all be assessed at the same operating condition.
Use a Simple TDH Formula
For most deep well systems, the working formula is TDH = elevation head + pressure head + friction head + equipment losses. Elevation head is the vertical lift from the pumping water level to the delivery elevation. Pressure head is the pressure that must remain at the outlet. Friction head covers the loss through the rising main and surface pipeline, while equipment losses cover items such as check valves, filters, flow meters, and control valves. Keep every item in the same unit—meters of water or feet of water—and calculate each loss at the required flow. A useful final statement should read like “60 m³/h at 118 m TDH,” not simply “118 m head.”
Use the Pumping Water Level, Not the Static Level
The static water level is measured when the pump is stopped. Once pumping begins, the water level normally drops until inflow from the aquifer balances the amount being pumped; this is the pumping water level, also called the dynamic water level or drawdown level. That operating level is the correct starting point for the lifting calculation. If the static level is 45 m below the wellhead, the pumping level is 72 m at 60 m³/h, and the pump is installed at 150 m, use 72 m—not 45 m and not 150 m. Seasonal variation also matters. A level measured after heavy rain may be useless for sizing a pump that must run through the dry season, so use pumping-test data and a realistic low-water condition whenever they are available.
Calculate the Elevation Head
Elevation head is the vertical distance from the pumping water level to the point where the water must arrive. Suppose the pumping level is 72 m below the wellhead and the inlet of a storage tank is 18 m above it. The elevation head is 72 + 18 = 90 m. The length of the horizontal pipeline is not added to this figure. A 500 m horizontal run contributes friction, but it does not create 500 m of vertical head. This distinction sounds basic, yet it is one of the quickest ways to spot an unreliable pump calculation.
Convert the Required Pressure into Head
If the system discharges into an open tank, the pressure at the water surface is normally zero gauge pressure. Irrigation networks, pressure tanks, filters, and process lines are different because useful pressure must remain after the water reaches its destination. For clean water, pressure head in meters is approximately pressure in bar × 10.2; in imperial units, pressure head in feet is pressure in psi × 2.31. A required outlet pressure of 2.0 bar therefore adds about 20.4 m of head. Use the pressure required at the design flow, and check whether a pressure switch or controller expects the pump to operate at a fixed set point or across a wider pressure range.
Calculate Pipe and Fitting Losses
Friction loss depends on the flow rate, actual pipe inside diameter, pipe material, length, condition, and fittings. A smaller pipe may reduce the purchase cost but increase the pump head and electricity cost for years. Use a friction chart or hydraulic calculation that matches the actual pipe rather than relying on nominal diameter alone. If the straight pipe is 220 m long, the elbows and valves add an equivalent 30 m, and the chart shows a loss of 2.4 m per 100 m at the design flow, the friction head is 2.4 × 250 ÷ 100 = 6.0 m. Partially closed valves, old pipes, and undersized sections can raise this number quickly, so the pipeline should be treated as part of the pump system rather than as an afterthought.
Add Losses from Inline Equipment
Filters, strainers, flow meters, backflow preventers, check valves, heat exchangers, and water-treatment equipment all consume pressure. Some manufacturers express the loss as meters of head, while others provide a pressure drop in bar, kPa, or psi. Convert the values into the same head unit used in the TDH sheet and add them at the design flow. Where a filter is involved, decide whether the pump must handle the pressure drop of a clean filter or the higher loss expected before cleaning. For the example below, the combined loss through the check valve, meter, and filter is taken as 2.0 m.
Work Through the Calculation
Consider a project requiring 60 m³/h from a 180 m well. The pump is planned at 150 m, the static level is 45 m below the wellhead, and the measured pumping level at the required flow is 72 m. Water enters a tank 18 m above the wellhead and must retain 2.0 bar at the inlet. Pipe and fitting loss is 6.0 m, while valves and other equipment add 2.0 m. The calculation is: 1. Elevation head: 72 + 18 = 90 m. 2. Pressure head: 2.0 × 10.2 = 20.4 m. 3. Friction and equipment loss: 6.0 + 2.0 = 8.0 m. 4. TDH: 90 + 20.4 + 8.0 = 118.4 m. The pump must therefore deliver 60 m³/h at approximately 118.4 m TDH. The 150 m pump setting keeps the unit submerged, but it is not added to the head calculation.
Match the Duty Point to the Pump Curve
The calculated duty point is only useful when it is checked against the manufacturer’s performance curve. Locate the required flow on the horizontal axis and the TDH on the vertical axis, then confirm that the intersection sits inside the pump’s recommended operating range and reasonably close to its efficient region. Also check motor power, stage count, discharge size, and the operating range expected as water levels change. A modest allowance may be sensible when seasonal drawdown or equipment losses are uncertain, but adding 20 or 30 percent without a defined reason often creates a different problem: an oversized pump producing too much pressure, drawing unnecessary power, and requiring a valve to waste the extra head.
Avoid the Usual Calculation Mistakes
Most incorrect selections can be traced to a short list of errors: 1. using total well depth as TDH; 2. using pump installation depth instead of pumping water level; 3. using static water level and ignoring drawdown; 4. adding horizontal pipe length as though it were vertical lift; 5. forgetting the pressure needed at the final outlet; 6. ignoring elbows, valves, filters, and meters; 7. calculating friction at a different flow from the design duty; and 8. adding an arbitrary safety margin large enough to push the pump away from its efficient operating range. If a sizing sheet starts with well depth and motor power but contains no pumping water level, outlet pressure, or pipe data, it is not yet a pump selection.
Check the Water Demand Before Final Selection
TDH tells you the resistance the pump must overcome, but it does not tell you whether the requested flow is appropriate for the application or sustainable for the well. Irrigation flow should be based on crop demand, irrigated area, irrigation method, operating zones, and available pumping hours. The target flow must also remain below the well’s reliable yield. A large pump cannot force an aquifer to supply water it does not have; it will only increase drawdown and the risk of low-water operation. For application-specific planning, see our agricultural irrigation pump selection guide.
Prepare the Data a Pump Supplier Actually Needs
A useful quotation request should include the figures that define both the well and the system:
1. required flow rate; 2. total well depth; 3. casing inside diameter; 4. static water level; 5. pumping water level at the required flow; 6. proposed pump setting depth; 7. delivery elevation; 8. required outlet pressure; 9. pipe material, diameter, and length; 10. major fittings, valves, filters, and meters; 11. water temperature and water quality; 12. voltage, phase, and frequency; and 13. expected operating hours. Mark estimates clearly rather than presenting them as measurements. A supplier can work with incomplete data, but the assumptions should be visible so they can be checked before the pump is manufactured.
Final Selection
A sound deep well pump calculation begins at the pumping water level and ends at the actual delivery condition. Define the required flow, add the vertical lift, convert the outlet pressure into head, calculate the pipe and fitting losses, include the pressure drop through equipment, and then place that duty point on the pump curve. Do not substitute well depth, pump setting depth, or motor power for this process. For a model recommendation, send us your operating data, including the flow, pumping water level, well diameter, delivery elevation, pressure, pipe details, and electrical supply.