A submersible well pump is sized by flow rate and total dynamic head, and 1/2 HP covers many 4-inch residential wells.
Well depth alone tells you almost nothing about pump size, which is why two neighbors on the same street can get very different results from the same 1/2 HP pump. Working out how to size a submersible well pump comes down to two numbers: the peak flow rate the house needs, in gallons per minute, and the total dynamic head the pump has to push against. For a large share of 4-inch residential wells, those numbers land on 1/2 HP.
The formula, the pressure math, and the wiring details all matter before you order.
What Two Numbers Decide The Pump Size?
Flow rate and total dynamic head set the pump size, and horsepower is only the result of those two figures. The working formula is HP = (GPM × TDH) ÷ (3,960 × pump efficiency).
Flow rate is the peak draw the house can put on at once — a couple of showers, a toilet refilling, a washing machine. Total dynamic head is everything working against the water’s trip from the aquifer to the tap: how far the water sits below ground, how far the level drops while the pump runs, friction in the drop pipe and fittings, and the pressure the system must deliver. Set the intake below the anticipated drawdown, safely above the bottom of the casing.
Pressure is the piece people skip. Every 10 PSI of system pressure is another 23 feet of head the pump must produce.
| Pressure Setting | Head It Adds |
|---|---|
| 10 PSI | 23 feet |
| 20 PSI | 46 feet |
| 30 PSI | 70 feet |
| 40 PSI | 90 feet |
| 50 PSI | 115 feet |
| 60 PSI | 140 feet |
The last check is the manufacturer’s pump curve: your GPM and TDH should intersect a point that sits away from both ends, in the range where the pump runs efficiently. The Water Systems Council’s well pump sizing sheet walks through the full worksheet if you’d rather fill one in by hand.
Why 1/2 HP Fits So Many Homes
A 1/2 HP pump sits in the sweet spot for homes with moderate demand and wells that don’t ask for more than roughly 150 to 200 feet of total head. General residential sizing guidance places 1/2 HP and 3/4 HP in the shallow-well, lower-demand range, with 1/2 HP commonly tied to about 5 to 15 GPM.
The model range is deeper than most buyers expect. Franklin Electric’s J-Class 4-inch 1/2 HP pumps include 10 GPM models with a shutoff height of 244 feet, offered in both 115 VAC and 230 VAC. Little Giant’s 4-inch 1/2 HP pump uses 8 stages and lists a 312-foot shutoff height at either voltage. Shutoff height is the ceiling — the head at zero flow — so a working installation stays well under it, which is exactly why one horsepower rating covers so many different wells.
Head climbs when the static level sits deeper, when irrigation is added, or when the pressure switch is set high; at that point the same 1/2 HP needs more stages, or the job moves to 3/4 HP. Comparing stage counts against your own head figure is easier side by side than across separate spec sheets, and our tested 1/2 HP submersible well pump roundup puts the common 10 GPM and 8-stage models next to each other
