A properly sized submersible well pump matches your flow rate and total dynamic head to the pump’s performance curve — not horsepower or depth alone.
The well pump that fails first on a property is rarely the cheapest one. To size a submersible well pump the right way, you match two numbers — the flow your home needs and the total dynamic head the pump must push against — to the pump’s published performance curve.
Horsepower and well depth alone do not set the size, and skipping that math is the root of most bad installs. This guide covers the flow calculation, the head calculation, the well data you need, and what the pump curve actually tells you.
Start With the Flow Rate the House Needs
Flow rate tells you how many gallons per minute the pump must deliver, and most homes need fewer than homeowners assume.
Add the taps, showers, and appliances that could run at once, plus any irrigation the same pump will feed. Before locking in a flow, confirm the well can yield it — a pump that outruns the aquifer will suck air and run hot.
How Do You Calculate Total Dynamic Head?
Total dynamic head (TDH) is the total height, in feet, that the pump must push water: vertical lift plus required pressure converted to head plus friction losses.
Lift runs from the pumping water level — where the water actually sits while the pump runs — up to the discharge point, never from the static level or total well depth. Friction comes from pipe distance, pipe diameter, and fittings; long or narrow runs add head that many installers forget.
Each part of the calculation needs one piece of well data, so gather these before you start:
| Well Data You Need | Why It Matters for Sizing |
|---|---|
| Total well depth | Sets the outer limit for pump depth and drop cable. |
| Static water level | Resting water level before the pump starts. |
| Pumping (dynamic) water level | Water level while pumping; this sets the true lift distance. |
| Well yield | Confirms the well can supply the flow you size for. |
| Casing or borehole diameter | Limits pump diameter; the nominal sizes should match. |
| Pipe size and run length | Drives the friction-loss part of the head calculation. |
| Required delivery pressure | Converts to head at 2.31 feet per PSI. |
Add the three components — lift feet plus pressure feet plus friction feet — and you have the TDH half of the duty point. Using the static water level instead of the dynamic pumping level is the classic error here: it understates lift and sends you to an undersized pump.
Read the Pump Curve, Not the Motor Label
A pump is the right size when your GPM and TDH meet inside the recommended band of that model’s performance curve. The motor’s rated horsepower has no place in that decision.
Grundfos’s domestic well pump sizing course walks through curve-matching for a home system, and Red Lion’s guide covers the same method with residential examples.
- Plot your operating point. Find your GPM on the horizontal axis and your head in feet on the vertical axis; where the two cross should sit inside the pump’s recommended efficiency band.
- Size the pump to the casing.
- Confirm well yield before you trust the curve. Oversizing flow against well yield is the fastest way to burn out a motor, so verify what the well recovers before you order.
- Place the intake correctly. Intake ports belong below the minimum pumping level and above the bottom of the casing, so the pump stays submerged without pulling sediment.
- Check the electrical side. Sizing is not complete until voltage, phase, and ampacity are verified against NEC motor-circuit requirements.
That process fits domestic groundwater systems. Agricultural and commercial jobs — irrigation, livestock watering — change the demand calculation completely, so size those separately.
Most sizing failures trace to five mistakes: using the static water level instead of the pumping level, ignoring friction losses, choosing by horsepower, never confirming well yield, and ignoring pipe and casing fit. Work the numbers in the right order and sizing becomes arithmetic: flow plus TDH gives you the duty point, and the curve picks the pump.
When your duty point lands on a 1 HP model and you’re ready to buy, our tested 1 HP submersible pump roundup puts the leading candidates side by side.
