Well Pump Calculator
A comprehensive engineering tool to calculate well pump sizing, peak capacity, required motor horsepower, vertical total dynamic head (TDH), water demand, system pressure, pressure tank size, run time schedules, and recovery limits.
Well Pump Sizing & Design Calculator
How to Size a Well Pump
Properly sizing a water well pump ensures consistent water pressure and prevents motor damage from short-cycling. A well pump must deliver appropriate GPM flow rates while overcoming elevation gravity lift and pipe friction resistance.
Follow this step-by-step engineering sizing method:
- 1Calculate Demand: Estimate peak flow rate requirements (GPM) based on household fixture count.
- 2Determine Drawdown Lift: Use dynamic pumping water levels rather than static water levels for elevation head.
- 3Compute Dynamic Head: Measure static lift heights, friction loss in pipelines, and tank switch pressures.
- 4Select Motor HP: Choose the standard horsepower (BHP) rating that matches these parameters.
Water Demand Calculation
Water demand describes the volume of water the system must supply during peak fixtures utilization. Daily demands are based on headcount consumption, while peak flow capacity is derived from fixture counts:
Peak requirements dictate GPM capacities, ensuring that when multiple taps are running, pressure does not drop below fixture minimums.
Pump Head Calculation
Total Dynamic Head (TDH) defines the total elevation lift resistance and pressure requirements a well pump must run against. This vertical lift is calculated from the dynamic pumping water level rather than the static resting water level:
Friction losses are calculated using Hazen-Williams models. Adding the pressure tank equivalent head (PSI × 2.31) ensures the pump can successfully push water into the bladder tank against system pressure thresholds.
Pressure Tank Sizing
A pressure tank stores water under air pressure to prevent short-cycling. A pump must run for a minimum duration (typically 60 to 120 seconds) on each cycle to cool its electrical windings:
Drawdown percentage is the proportion of water that drains between the cut-in and cut-out switch pressures, typically 22% to 31% of the total tank volume.
Pump Horsepower
Horsepower requirements are computed by matching fluid density, GPM flow, and TDH lift. Sizing determines both the hydraulic output (useful work) and the mechanical Brake Horsepower (shaft rating):
BHP requirements are matched to standard submersible motor configurations. Sizing up protects against high head demands or density changes.
Well Capacity
Well capacity describes the maximum safe water volume extractable from a well. It is dictated by recovery rates—the speed at which water refills the casing from surrounding aquifer fractures. Operating a pump above recovery limits risks empty casing conditions, causing cavitation and motor failure.
Pump Power
Evaluating power draw calculates mechanical shaft inputs, electrical inputs, and energy costs. The calculation determines kilowatts consumed per GPM and estimates daily run costs.
Engineering Formulas
Review the primary mathematical models used in our calculations:
- Hazen-Williams Pipe Loss: h_f = 0.002083 × L × (100/C)^1.852 × Q^1.852 / d^4.8655
- Velocity Head Equation: h_v = v² / 2g
- Hydraulic Power (HP): WHP = Q_GPM × TDH_ft / 3960
- Shaft Power (BHP): BHP = WHP / Efficiency
- Bladder Tank Drawdown: Drawdown = V_tank × Drawdown_pct
Worked Examples
Review these step-by-step sizing scenarios for well pumps and bladder pressure tank sizing calculations.
Example 1: Residential Well Pump
Sizing a well pump for a 4-person household with 2.5 bathrooms. The dynamic pumping level stabilizes at 120 ft depth. The pipe length is 150 ft of 1.25-inch PVC (C=150).
2. Static Head = 120 ft pumping level lift.
3. Pressure Tank Switch pressure = 40/60 PSI. Equivalent head = 50 PSI × 2.31 = 115.5 ft.
4. Pipe Friction loss = 1.2 ft (Hazen-Williams).
5. TDH = 120 + 115.5 + 1.2 = 236.7 ft.
6. BHP at 65% efficiency = (12 × 236.7) / (3960 × 0.65) = 1.10 HP. Select: 1.5 HP submersible pump.
Example 2: Deep Water Well
Sizing a deep bedrock well pump to deliver 25 GPM from a pumping water level depth of 350 ft. The drop pipe run length is 380 ft of 1.5-inch PVC. Tank cut-out pressure is 60 PSI (138.6 ft equivalent).
2. Friction Head Loss = 380 × (6.1 ft / 100) = 23.2 ft.
3. TDH = 350 + 23.2 + 138.6 = 511.8 ft.
4. BHP at 70% efficiency = (25 × 511.8) / (3960 × 0.70) = 4.61 HP. Select: 5.0 HP motor.
Example 3: Farm Irrigation Well
A livestock pasture well supplies water to automated feeders. The daily requirement is 4,800 Gallons per day. The pump capacity is 20 GPM.
2. Recommended schedule: 4 hours daily operation.
Example 4: Commercial Water Supply Well
A commercial greenhouse irrigation well has a casing recovery yield rate of 6 GPM. The target pumping flow capacity is 15 GPM, running for 60-minute increments.
2. Volume deficit over 60 min = 9 GPM × 60 min = 540 Gallons drawn from casing storage.
3. Sustainable long-run flow is capped at recovery limit of 6 GPM.
Example 5: Pressure Tank Sizing
Select the minimum bladder tank size for a well pump rated at 15 GPM flow capacity. Sizing for a minimum cycle time of 1 minute using a standard 40/60 PSI switch (26.3% drawdown percentage).
2. Minimum total tank size = 15 gallons ÷ 0.263 = 57.0 Gallons.
3. Selection: Select a standard 80-gallon bladder tank.
Reference Tables
Use these reference tables for lookup estimates during well pump design calculations.
1. Typical Residential Water Demand & Peak Flow
| Bedrooms Count | Bathrooms Count | Average Peak GPM Flow |
|---|---|---|
| 2 Bedroom | 1.0 Bathroom | 6 − 8 GPM |
| 3 Bedroom | 2.0 Bathroom | 10 − 12 GPM |
| 4 Bedroom | 2.5 Bathroom | 13 − 15 GPM |
| 5+ Bedroom | 3.0+ Bathroom | 16 − 20+ GPM |
2. Pump Capacity Table (Well Yield vs. Pump Sizing)
| Well Casing Yield (GPM) | Recommended Pump Capacity (GPM) | Required Casing Storage Buffer |
|---|---|---|
| Under 3 GPM | 5 GPM | High Casing volume / Storage Tank |
| 4 − 8 GPM | 10 GPM | Standard bladder storage pressure tank |
| 10+ GPM | 15 − 25 GPM | Direct delivery option with standard tank |
3. Pump Head Table (PVC Friction Losses per 100ft)
| Flow Rate (GPM) | 1" PVC (ft Head Loss) | 1.25" PVC (ft Head Loss) | 1.5" PVC (ft Head Loss) |
|---|---|---|---|
| 10 GPM | 3.2 ft | 0.8 ft | 0.4 ft |
| 20 GPM | 11.4 ft | 2.9 ft | 1.4 ft |
| 30 GPM | 23.8 ft | 6.1 ft | 2.9 ft |
4. Motor HP Table (FLA Current Draw at 230V, Single-Ph)
| Motor Power (HP) | Running Current FLA (Amps) | Standard Wire Size (AWG) | Est. Power (kW) |
|---|---|---|---|
| 0.5 HP | 4.8 A | #14 AWG | 0.6 kW |
| 1.0 HP | 8.0 A | #12 AWG | 1.1 kW |
| 1.5 HP | 10.0 A | #12 AWG | 1.5 kW |
| 2.0 HP | 13.2 A | #10 AWG | 2.1 kW |
5. Standard Pressure Tank Sizes
| Total Tank Volume | 30/50 PSI Drawdown | 40/60 PSI Drawdown | 50/70 PSI Drawdown |
|---|---|---|---|
| 20 Gallons | 6.2 Gal | 5.2 Gal | 4.4 Gal |
| 44 Gallons | 13.6 Gal | 11.4 Gal | 9.7 Gal |
| 86 Gallons | 26.7 Gal | 22.4 Gal | 18.9 Gal |
| 119 Gallons | 36.9 Gal | 30.9 Gal | 26.2 Gal |
6. Well Recovery Rate Table
| Borehole Recovery (GPM) | Daily Yield Capacity | Pump Duty Cycle Constraint |
|---|---|---|
| 1 GPM | 1,440 Gal/day | Low flow sizer restriction / Tank needed |
| 5 GPM | 7,200 Gal/day | Standard household irrigation usage |
| 10+ GPM | 14,400+ Gal/day | Unrestricted continuous supply |
7. Flow Rate Conversion Table
| GPM (US) | LPM (L/min) | m³/hr (Cubic Meters) | LPS (L/sec) |
|---|---|---|---|
| 5 GPM | 18.9 LPM | 1.14 m³/hr | 0.32 L/s |
| 10 GPM | 37.9 LPM | 2.27 m³/hr | 0.63 L/s |
| 25 GPM | 94.6 LPM | 5.68 m³/hr | 1.58 L/s |
8. Pressure Conversion Table
| PSI (lb/in²) | Bar (Atmosphere) | Feet of Water Head (ft) | Meters of Water (m) |
|---|---|---|---|
| 30 PSI | 2.07 Bar | 69.3 ft Head | 21.1 m Head |
| 50 PSI | 3.45 Bar | 115.5 ft Head | 35.2 m Head |
| 70 PSI | 4.83 Bar | 161.7 ft Head | 49.3 m Head |
9. Typical Well Depths by Aquifer
| Aquifer Category | Borehole Depth (ft) | Typical Water Quality |
|---|---|---|
| Sand & Gravel aquifer | 50 − 150 ft | Soft water, potential surface runoff |
| Limestone Bedrock | 150 − 400 ft | Hard mineral water, safe recovery |
| Deep Aquifer Strata | 400 − 1000+ ft | High mineral content, excellent purity |
Applications
Well pumps supply ground water matching diverse application profiles:
- Residential Water Wells: Transporting tap water to home appliances.
- Agricultural Irrigation: Supplying water to pastures and crops.
- Industrial Groundwater Sizing: Large flow extraction for plant facilities.
- Livestock Operations: Supplying drinking water to troughs.
Frequently Asked Questions
Determine your peak household flow rate in GPM based on bathrooms and fixtures. Next, calculate the dynamic pumping depth in the well, and sum friction head losses to select a pump that delivers that flow at that total head.
Use the formula BHP = (Flow Rate GPM × Head ft × Specific Gravity) / (3960 × Efficiency). Select the next standard larger motor horsepower size to avoid overloading.
For household indoor water demand, multiply headcount by 75 gallons per person daily. For flow capacity demand, count bathroom groups (allow 3 GPM per bathroom) or use fixture unit guidelines.
A pressure tank is sized based on the pump flow capacity. Multiply the GPM capacity by the target cycle run time (e.g. 1 minute) and divide by the drawdown percentage associated with your switch settings (typically 26.3% for 40/60 switches).
Position the pump casing at least 10 to 20 feet below the dynamic drawdown pumping water level, but keep it 10 feet above the bottom of the casing to avoid sand suction.
This is determined by the casing diameter storage capacity and dynamic recovery rate (yield rate). Standard well yields are measured during test drillings and recorded on well drillers logs.
