Well Pump Sizing Guide: How to Choose the Right Size
Three Key Sizing Factors
1. Water Demand (GPM Needed)
How much water does your household use at peak times? This determines the minimum flow rate.
2. Total Dynamic Head (TDH)
The total height the pump must push water, including:
- Depth to water (pumping level)
- Height above ground to pressure tank
- Pressure required (typically 40-60 PSI)
- Friction loss in pipes
3. Well Recovery Rate
How fast does your well refill? The pump should NOT exceed the well's production rate, or you'll pump the well dry.
Calculating Water Demand
Quick Method: Fixture Count
Add up fixtures and use this formula:
Peak GPM = Number of fixtures × 0.75 to 1.0
Fixture Flow Rates
| Fixture | GPM |
|---|---|
| Shower | 2.0-2.5 |
| Kitchen faucet | 2.0-3.0 |
| Bathroom faucet | 1.0-2.0 |
| Toilet (filling) | 2.0-3.0 |
| Dishwasher | 1.5-2.0 |
| Washing machine | 3.0-5.0 |
| Outdoor hose | 3.0-5.0 |
Typical Residential Needs
- 1-2 bathrooms: 6-8 GPM
- 2-3 bathrooms: 8-12 GPM
- 3-4 bathrooms: 12-16 GPM
- Large home/irrigation: 15-25+ GPM
Example Calculation
Home with: 2 showers, 3 faucets, 2 toilets, dishwasher, washing machine
Peak scenario: 1 shower + 1 faucet + washing machine running
Demand: 2.5 + 2.0 + 4.0 = 8.5 GPM
Recommendation: 10 GPM pump (allows some margin)
Depth and Head Pressure
Understanding Total Dynamic Head
TDH is measured in feet. Each 2.31 feet = 1 PSI of pressure.
TDH Calculation
- Pumping water level: Depth to water when pump is running
- Elevation rise: Height from well to highest outlet
- Pressure requirement: 50 PSI × 2.31 = 115 feet
- Friction loss: Typically 5-15% of pipe length
Example TDH
Well: 300 ft deep, pumping level at 200 ft
Elevation: 20 ft rise to house
Pressure: 50 PSI (115 ft equivalent)
Friction: ~15 ft
TDH: 200 + 20 + 115 + 15 = 350 feet
Pump Performance Curves
Every pump has a performance curve showing GPM at various head pressures. The pump must deliver your required GPM at your TDH.
HP Guidelines by Depth
| Well Depth | Typical HP | Typical GPM Range |
|---|---|---|
| 50-100 ft | 1/2 HP | 8-12 GPM |
| 100-200 ft | 3/4 HP | 8-15 GPM |
| 200-300 ft | 1 HP | 10-20 GPM |
| 300-400 ft | 1.5 HP | 10-25 GPM |
| 400-600 ft | 2 HP | 15-30 GPM |
| 600+ ft | 3+ HP | Varies |
Note: These are rough guidelines. Actual sizing requires pump curves and well-specific data.
Pump Sizing Problems
Undersized Pump
- Low water pressure at fixtures
- Can't run multiple fixtures
- Pump runs constantly
- Motor overheats and fails early
Oversized Pump
- Exceeds well recovery rate
- Pumps well dry (damages pump)
- Short cycling (rapid on/off)
- Higher electricity costs
- Excessive pressure (stresses plumbing)
The Goldilocks Zone
The right pump delivers enough GPM for peak demand without exceeding well capacity. It runs long enough per cycle to stay cool but not so long it drains the well.
Frequently Asked Questions
What size well pump do I need?
Most homes need 8-12 GPM. Calculate your peak demand, determine your TDH, and don't exceed your well's recovery rate. A professional can help match pump to conditions.
What HP well pump do I need?
HP depends on depth and GPM. Rough guide: 1/2 HP for shallow (<100 ft), 3/4-1 HP for medium (100-300 ft), 1.5-2 HP for deep (300-500 ft).
Is a bigger pump always better?
No. Oversized pumps can pump the well dry, short cycle, and waste electricity. Match pump capacity to both demand AND well production.
How do I know my well's recovery rate?
A professional can perform a pump test. Or check your original well report if available. Recovery rate is crucial for proper sizing.
Can I install a bigger pump if my well is low-producing?
No—a bigger pump won't create more water. For low-yield wells, use a pump matched to the well's rate combined with storage tanks.
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