A customer may contact a pump supplier and ask for a submersible well pump capable of delivering 100 m³/h. Before selecting a model, however, the supplier should first ask how that figure was calculated. In many projects, the requested flow comes from an old pump nameplate, a previous quotation, or the maximum value shown in a product catalogue.
These figures can be useful references, but they do not always represent the actual water demand. The required flow should be based on how much water the project consumes, how many hours the pump can operate, how much water the well can sustainably produce, and whether a storage tank is available.
This calculation is also important when comparing suppliers. A reliable pump manufacturer should not recommend a model based only on horsepower or maximum catalogue flow. The supplier should review the required flow together with the Total Dynamic Head, dynamic water level, well yield, pipeline conditions, motor power, and operating schedule.
Quick Flow Rate Calculation
The basic flow-rate calculation is:
Required Flow Rate = Daily Water Demand ÷ Available Pumping Hours
For example, if a project requires 480 m³ of water per day and the pump can operate for eight hours:
480 m³/day ÷ 8 h/day = 60 m³/h
The preliminary required flow is therefore 60 m³/h. This value must still be checked against peak demand, storage capacity, sustainable well yield, dynamic water level and the required Total Dynamic Head.
A complete pump duty point must include both flow and head, for example:
60 m³/h at 120 m TDH
Flow Rate Calculation by Application
| Application | Basic calculation | Important checks |
|---|
| General water supply | Daily demand ÷ pumping hours | Peak demand, storage and well yield |
| Tank filling | Refill volume ÷ required filling time | Water consumed during filling |
| Irrigation | Area × water depth × 10 ÷ efficiency ÷ pumping hours | Well yield and irrigation pressure |
| Sprinkler system | Number of simultaneous sprinklers × flow per sprinkler | Pressure at the most distant sprinkler |
| Livestock supply | Number of animals × daily demand per animal | Cleaning water and storage |
| Municipal supply | Population × daily demand per person ÷ pumping hours | Peak factor, leakage, future growth and standby capacity |
| Industrial supply | Sum of water demands operating simultaneously | Production schedule and variable demand |
These calculations provide a preliminary flow requirement. The final pump model should not be selected until the flow has been compared with the sustainable well yield and combined with the required Total Dynamic Head.
What Does Submersible Well Pump Flow Rate Mean?
Pump flow rate is the volume of water delivered during a specified period. It is commonly expressed in cubic metres per hour, litres per second, litres per minute, or gallons per minute. A flow of 50 m³/h means that the pump delivers 50 cubic metres of water in one hour at a particular operating head.
The same pump will not necessarily deliver 50 m³/h in every installation. Actual flow changes with the dynamic water level, pipeline length, pipe diameter, discharge pressure, valve position, pump speed, and internal wear. A pump installed in a shallow irrigation well may deliver more water than the same model operating in a deep borehole with a long discharge pipeline.
For this reason, a quotation showing only “flow: 50 m³/h” is incomplete. A useful pump duty point should state both flow and head, such as 50 m³/h at 120 m Total Dynamic Head.
Start with the Actual Daily Water Demand
The first step is to determine how much water the project needs each day. This may include irrigation water, industrial process demand, municipal supply, livestock consumption, domestic water use, equipment cleaning, or storage-tank replenishment.
Once the daily demand is known, divide it by the number of hours available for pumping:
Required Flow Rate = Daily Water Demand ÷ Available Pumping Hours
If a project requires 480 m³ of water each day and the pump can operate for eight hours, the initial flow requirement is 60 m³/h. This is a preliminary duty flow, not the final pump selection. It must still be compared with peak demand, storage capacity, well yield, and the pump performance curve.
When requesting quotations, buyers should tell suppliers how the flow was calculated. This allows different manufacturers to evaluate the same project conditions instead of quoting pumps based on different assumptions.
Use the Real Pumping Hours
Daily demand is sometimes divided by 24 hours even though the pump cannot operate continuously. This produces a flow value that is too low. A project consuming 480 m³/day requires 20 m³/h if the pump runs for 24 hours, but the requirement increases to 60 m³/h if only eight hours of pumping are available.
Actual operating time may be limited by electricity supply, irrigation shifts, generator use, maintenance periods, noise restrictions, or the recovery rate of the well. The calculation should reflect the real operating schedule, not an ideal schedule that cannot be maintained at the site.
A supplier should also ask whether the stated pumping hours include maintenance and unexpected shutdowns. A system that depends on continuous operation every hour of the day may have little reserve capacity when demand increases or equipment requires inspection.
Flow Rate Calculation Examples by Application
How to Calculate Flow for Agricultural Irrigation
Irrigation flow depends on the irrigated area, crop-water requirement, climate, irrigation method, system efficiency, and available pumping time. A useful relationship is that one millimetre of water applied over one hectare equals ten cubic metres.
For a 20-hectare field requiring 5 mm of water per day, the theoretical daily demand is:
20 × 5 × 10 = 1,000 m³/day
If the irrigation system operates at 80% efficiency, the pump must supply:
1,000 ÷ 0.80 = 1,250 m³/day
With ten hours of available pumping time, the preliminary flow requirement becomes 125 m³/h. Before choosing a pump, this figure should be checked against the tested well yield and the pressure required by the sprinklers, drip lines, filters, and main pipeline.
When comparing irrigation-pump suppliers, ask whether the recommended model delivers 125 m³/h at the actual required head. A quotation that only highlights “maximum flow” does not confirm that the pump can operate the irrigation system correctly.
Calculate Sprinkler Flow from Simultaneous Operation
Where sprinkler information is available, the total flow can be calculated from the number of sprinklers operating at the same time. If 24 sprinklers each require 2.5 m³/h, the combined flow is 60 m³/h.
The important point is simultaneous operation. A project may have 60 sprinklers installed, but if the field is divided into three zones and only 20 sprinklers operate in each zone, the pump does not need to supply all 60 at once. Zoning can reduce pump flow, motor power, pipe diameter, cable size, and generator capacity.
The supplier should still verify the pressure required by the most distant sprinkler. Correct flow without sufficient pressure will result in uneven irrigation and poor distribution.
How to Calculate Flow for Tank Filling
For a tank-filling application, divide the volume to be replenished by the required filling time:
Required Flow = Refill Volume ÷ Filling Time
If a 500 m³ tank normally retains a 100 m³ reserve, the refill volume is 400 m³. To restore that volume within five hours, the pump must provide 80 m³/h. If the site continues consuming 20 m³/h during filling, the total pump requirement becomes 100 m³/h.
Tank level controls, reserve volume, overflow protection, and allowable pump starts should also be considered. A larger storage tank can sometimes reduce the required peak pump flow and allow the pump to operate closer to a stable, efficient duty point.
For commercial comparison, buyers should ask whether the supplier has considered simultaneous water consumption. A pump selected only from tank volume may be undersized if the system continues using water while the tank is being refilled.
Storage Capacity Can Reduce Peak Pump Flow
A storage tank separates water production from water consumption. Without sufficient storage, the pump may need to match the highest instantaneous demand. With storage, the pump can operate at a steadier flow while the tank supplies short-duration peaks.
Suppose a site normally consumes 40 m³/h but reaches 100 m³/h for one hour each day. Selecting a pump for the full 100 m³/h may leave it oversized during most operating periods. A correctly sized tank can supply part of the peak while a smaller pump continues operating at a stable rate.
When evaluating suppliers, compare the complete proposed system rather than only the pump price. A manufacturer that recommends a smaller pump combined with suitable storage may offer a lower total operating cost than a supplier quoting a much larger motor.
How to Calculate Flow for Livestock Water Supply
Livestock demand is generally calculated from the number of animals and the estimated daily consumption per animal. Climate, animal size, production conditions, cleaning requirements, and drinking-trough storage can all affect the total volume.
For example, 500 cattle using an average of 50 litres per animal per day require 25,000 litres, or 25 m³/day. If cleaning and other farm activities require another 10 m³/day, the total demand becomes 35 m³/day. With five pumping hours, the initial flow requirement is 7 m³/h.
A storage tank can supply short drinking peaks, allowing the well pump to operate more steadily. The supplier should confirm whether the pump fills a tank or supplies the drinking system directly, because these two arrangements may require different flow and pressure conditions.
How to Calculate Flow for Residential Water Supply
Residential projects should consider both average daily consumption and peak-hour demand. The number of houses, residents, bathrooms, kitchens, gardens, pressure tanks, and storage tanks all affect the final requirement.
If 40 houses each consume an average of 600 litres per day, the total daily demand is 24 m³. With a storage tank and eight pumping hours, the average replenishment flow is 3 m³/h. This value may be suitable for filling the tank, but it may not be sufficient for a direct-pressure system supplying several homes at the same time.
A supplier should therefore ask whether the pump is used for tank filling, direct pressure boosting, or both. Buyers comparing proposals should make sure each manufacturer is quoting for the same system arrangement.
How to Calculate Flow for Municipal Water Supply
Municipal calculations usually begin with population and average daily consumption per person. Commercial use, public facilities, industrial demand, leakage, peak-day demand, fire-water reserves, and future growth may then be added.
For a population of 5,000 people using 150 litres per person each day, domestic demand is 750 m³/day. Adding 15% for public use and distribution losses increases the requirement to approximately 862.5 m³/day. If the station operates for 16 hours, the average pumping flow is approximately 54 m³/h.
Municipal stations are rarely selected from average flow alone. Peak demand, redundancy, maintenance access, and standby capacity are equally important. A manufacturer may recommend several smaller pumps instead of one large unit so that the station can adjust to changing demand and continue operating during maintenance.
How to Calculate Flow for Industrial Water Supply
Industrial flow should be calculated from the processes that actually use water. Cooling, washing, production lines, boiler-water treatment, dust suppression, cleaning, and general factory use may all operate on different schedules.
The total pump flow should equal the sum of the demands that occur simultaneously. If production line A requires 25 m³/h, line B requires 20 m³/h, and general use requires 5 m³/h, the normal combined flow is 50 m³/h. A cleaning system requiring another 15 m³/h should only be added if it operates at the same time.
When requesting a quotation, provide suppliers with a process-demand schedule rather than only a final total. This allows the manufacturer to determine whether one fixed-speed pump, several staged pumps, or a variable-frequency system is more suitable.
Check the Calculated Flow Against the Well
The calculated project demand must be compared with the sustainable output of the well. Well yield is normally established through a pumping test that records discharge rate, dynamic water level, pumping duration, and recovery after shutdown.
If a project requires 70 m³/h but the tested well yield is only 50 m³/h, installing a 70 m³/h pump will not create additional groundwater. The dynamic water level may continue to fall until the pump loses sufficient submergence, begins drawing sand, or reaches a dry-running condition.
A responsible supplier should identify this conflict before recommending a pump. Possible solutions include increasing the daily pumping period, installing additional storage, reducing peak demand, improving the well, or using another water source.
Dynamic Water Level Must Be Connected to Flow
A dynamic water-level figure is incomplete unless the corresponding pumping rate is also known. “Dynamic water level: 65 m” does not show how the well behaves under different flow conditions.If these measurements are unclear, review the difference between static and dynamic water level before calculating the final pump duty point.
A more useful statement is: “Dynamic water level: 65 m while pumping 40 m³/h for eight hours.” If the flow increases to 60 m³/h, the water level may fall significantly farther. This changes the required head, installation depth, motor-cooling conditions, and dry-run risk.
When comparing technical proposals, check whether the supplier has used the dynamic water level at the intended flow. A quotation based only on static water level may underestimate the required pump head.
Add a Controlled Design Allowance
A calculated average flow may require a small allowance for demand variation, measurement uncertainty, minor leakage, gradual pump wear, or limited future expansion. The allowance should be justified by the project.
If the calculated flow is 60 m³/h and a 10% allowance is appropriate, the design flow becomes 66 m³/h. The selected pump should then deliver approximately 65–66 m³/h at the required head within its recommended operating range.
Excessive allowances can create more problems than they solve. Doubling the required flow may increase drawdown, motor power, cable size, pipe cost, energy consumption, and the risk of operating outside the efficient range.
Other Factors That Affect the Final Flow Requirement
Required Flow Is Not Maximum Catalogue Flow
Pump catalogues often show rated flow, a recommended operating range, and maximum flow. These values are not interchangeable.
Maximum flow is generally located near the far-right side of the performance curve, where available head is lower. It is not necessarily suitable for continuous operation. Operating too far to the right can increase motor load, reduce efficiency, and increase hydraulic wear.
When comparing manufacturers, ask each supplier to mark the proposed duty point on the pump curve. The quotation should show the flow, head, efficiency, motor power, and recommended operating range at the actual project condition.
When Parallel Pumps Are More Suitable
Projects with variable demand may benefit from two or more pumps instead of one large unit. Common arrangements include one duty pump with one standby pump, two pumps operating in parallel, or several pumps staged according to demand.
Multiple pumps can improve reliability and allow more efficient operation during periods of lower demand. They also make maintenance easier because one unit can be inspected while another continues supplying water.
However, two identical pumps operating in parallel do not necessarily produce twice the flow of one pump. As total flow increases, pipeline friction rises and the system operating point changes. A supplier proposing parallel operation should provide the combined pump curve and explain the control strategy.
A Variable-Frequency Drive Does Not Replace Correct Sizing
A variable-frequency drive can adjust motor speed and help a pump respond to changing demand. It is often useful for constant-pressure systems, soft starting, staged operation, and applications where flow varies throughout the day.
A VFD does not make an incorrectly selected pump correct. The pump must still deliver the maximum required flow and head while operating safely at reduced speeds. Minimum frequency, motor cooling, stable operating range, and manufacturer limits must be reviewed.
When comparing proposals, ask whether the pump was selected to match the main duty point before the VFD was added. A large pump controlled at very low frequency may not be the most efficient or reliable solution.
Flow Unit Conversions
Flow-unit errors can cause serious selection problems. The following conversions are commonly used:
① 1 L/s = 3.6 m³/h
② 1 m³/h = 16.67 L/min
③ 1 m³/h ≈ 4.40 US GPM
④ 1 US GPM ≈ 0.227 m³/h
For example, 20 L/s equals 72 m³/h, while 220 US GPM is approximately 50 m³/h. Quotations, drawings, data sheets, and purchase orders should always state the unit clearly.
Buyers comparing international suppliers should pay particular attention to units. Two quotations may appear different simply because one uses m³/h and the other uses US GPM.
A Practical Irrigation Flow Calculation
Consider an irrigation project with 30 hectares of cultivated land, a crop-water requirement of 4.5 mm/day, an overall irrigation efficiency of 75%, and 12 available pumping hours per day.
The theoretical daily crop demand is:
30 × 4.5 × 10 = 1,350 m³/day
After adjusting for irrigation efficiency:
1,350 ÷ 0.75 = 1,800 m³/day
The required flow is:
1,800 ÷ 12 = 150 m³/h
If the tested well yield is 170 m³/h, the water source appears capable of supporting the proposed duty, subject to the pumping-test conditions. The next step is to calculate the Total Dynamic Head at approximately 150 m³/h.
A quotation for this project should not only say “150 m³/h pump.” It should identify the selected duty point, required head, model, number of stages, motor power, efficiency, materials, voltage, and pump-curve position.
How to Evaluate a Pump Supplier and Quotation
What a Reliable Pump Supplier Should Confirm
A reliable submersible well pump supplier should review more than the requested flow. Before recommending a model, the supplier should confirm the daily water demand, available pumping hours, well yield, static and dynamic water levels, pipeline size, discharge elevation, required pressure, electrical supply, water quality, and operating schedule.
The supplier should also explain how the proposed duty point was selected. Buyers should expect to receive a performance curve, technical data sheet, material list, motor information, pump dimensions, and clarification of the recommended operating range.
A supplier who immediately recommends a pump based only on horsepower or well depth may not have reviewed the complete system. This does not necessarily mean the product is poor, but it increases the risk of an unsuitable selection.
How to Compare Submersible Well Pump Quotations
The lowest-priced quotation is not always the lowest-cost solution. Two pumps with the same motor power may have different flow, head, efficiency, materials, stage numbers, cable requirements, and operating ranges.
When comparing quotations, check:
① Flow and head at the actual duty point
② Pump efficiency at the duty point
③ Motor power and rated current
④ Pump and impeller materials
⑤ Maximum allowable sand content
⑥ Voltage, frequency, and starting method
⑦ Cable length and cable specification
⑧ Check valve and accessory scope
⑨ Warranty and spare-parts availability
⑩ Engineering and after-sales support
Comparisons should be made at the same operating condition. A cheaper pump quoted at a lower head is not equivalent to a pump that delivers the required flow at the project’s full Total Dynamic Head.
Information to Send When Requesting a Quotation
A supplier can provide a more accurate selection when the project data are complete. Before requesting a quotation, prepare:
① Application
② Required daily water volume
③ Available pumping hours
④ Required flow rate
⑤ Total well depth
⑥ Static water level
⑦ Dynamic water level and corresponding test flow
⑧ Sustainable well yield
⑨ Pipe diameter and total length
⑩ Discharge elevation and required pressure
⑪ Water quality and sand content
⑫ Voltage, frequency, and phase
⑬ Daily operating hours
⑭ Preferred pump material
⑮ Required accessories and control system
Providing this information also makes it easier to compare suppliers because each manufacturer is working from the same project conditions.
Common Flow-Calculation and Purchasing Mistakes
Many pump problems begin with an incorrect flow requirement rather than a defective product. Common mistakes include copying the old pump nameplate, choosing maximum catalogue flow, using unrealistic pumping hours, and adding an excessive safety margin.
Buyers also make mistakes by ignoring storage capacity, adding demands that never occur simultaneously, overlooking well yield, or comparing quotations based only on horsepower and price.
The final mistake is treating flow and head as separate decisions. The pump must deliver the required flow at the required head. A model that provides 100 m³/h at low pressure may be unable to deliver the same flow from a deep well.
Flow Is Only Half of the Pump Duty Point
A flow requirement such as 80 m³/h is not enough to select a submersible well pump. The supplier must also know the head required to lift the water, overcome pipeline friction, and provide the necessary outlet pressure.
A complete duty point would be written as:
80 m³/h at 140 m Total Dynamic Head
Once both values are known, the operating point can be plotted on the pump performance curve. The selected pump should operate within its recommended range with suitable efficiency, motor power, materials, and protection.
The objective is not to choose the pump with the largest possible flow. It is to choose a pump that delivers the amount of water the project needs, at a rate the well can sustain, and at the pressure the complete system requires.
Once the required flow and Total Dynamic Head are known, read the submersible well pump performance curve to confirm the operating point, efficiency and recommended flow range.
Request a Pump Selection from SLAPK
To request a submersible well pump recommendation, send SLAPK your application, daily water demand, available pumping hours, well-test data, pipeline information, required outlet pressure, water quality, and electrical supply.
Our engineering team can review the required flow together with the Total Dynamic Head, dynamic water level, well yield, material requirements, and motor conditions. The recommended model can then be supported by a pump curve and technical data for the actual project duty point.
This approach helps buyers avoid oversized pumps, excessive well drawdown, unnecessary energy consumption, unsuitable materials, and pump operation outside the recommended performance range.
