Well Pump Sizing Calculator
Size a submersible well pump by working out peak flow demand and total dynamic head, including the pressure and friction terms that most rules of thumb leave out.
How to use this calculator
- 1Tick the fixtures that could genuinely run at the same time. Sizing for every fixture in the house simultaneously buys a pump the well may not be able to feed.
- 2Get the static level and drawdown from the well log rather than estimating. Drawdown in particular is the figure that separates a working system from one that pumps the well dry.
- 3Enter the cut-out pressure from the switch, not the pressure you would like. It contributes more head than most people expect.
- 4Include the whole pipe run — down the well and buried out to the house — plus an allowance for fittings.
- 5Take the resulting flow and head to a pump curve. Do not buy on horsepower.
How the calculation works
Peak flow = Σ gpm of fixtures that may run together
Pumping level = static water level + drawdown
Pressure head (ft) = cut-out pressure in psi x 2.31
Friction (ft per 100 ft) = 0.2083 x (100 ÷ C)^1.852 x gpm^1.852 ÷ diameter^4.8655
Total dynamic head = pumping level + elevation rise + pressure head + friction head
Velocity (ft/s) = 0.4085 x gpm ÷ diameter²- Static level
- Depth to the water surface with the pump off. Not the depth the pump lifts from
- Drawdown
- How much further the level falls while pumping at the design flow. Measured during the driller’s yield test and stated on the well log
- 2.31
- Feet of water head per psi. A 50 psi cut-out is 115 feet of head before any water has moved
- C
- Hazen-Williams roughness coefficient: 150 for PVC, 140 for polyethylene and copper, 100 for older galvanised steel
- TDH
- Total dynamic head — everything the pump works against, in feet of water. Paired with flow, it is the point the pump curve must reach
A pump is specified by a curve, not a number. The same submersible delivers perhaps 15 gpm at 100 feet of head and 7 gpm at 250 feet, so quoting horsepower alone says almost nothing about whether it will serve a house. The output of this page is a duty point to look up on that curve.
The pressure term is the one most commonly omitted, and it is large. At 2.31 feet per psi, a 40/60 switch demands 138 feet of head purely to reach cut-out — frequently more than the lift from the well.
Hazen-Williams is the standard empirical relation for water in full pipes at ordinary temperatures. Its exponents mean friction rises with roughly the 1.85th power of flow and falls with the 4.87th power of diameter, which is why doubling the flow through an undersized line is so punishing.
Worked example
A house drawing 7 gpm from a 200 ft well on 1-inch poly pipe
- 1.Simultaneous demand: shower 2.5 + bathroom sink 1.5 + toilet refilling 3.0 = 7.0 gpm.
- 2.Pumping level: 60 ft static + 25 ft drawdown = 85 ft of lift.
- 3.Pressure head: 50 psi x 2.31 = 115.5 ft — larger than the lift.
- 4.Elevation to the tank: 10 ft.
- 5.Friction: 7 gpm through 1-inch poly (1.049 in ID, C = 140) loses 3.25 ft per 100 ft; over 286 equivalent feet that is 9.3 ft.
- 6.Total dynamic head: 85 + 10 + 115.5 + 9.3 = 219.8 ft.
- 7.So the pump must deliver 7 gpm at about 220 feet — and velocity is 2.6 ft/s, comfortably safe.
Result: 7 gpm at 220 ft of head
Why head, not horsepower, chooses a pump
Ask what size well pump a house needs and the answer usually comes back in horsepower — a half, three quarters, a full horse. That is close to meaningless on its own, because a pump does not have a capacity. It has a curve.
Every centrifugal pump trades flow against pressure. Push it to deliver more gallons per minute and the pressure it can generate falls; ask it for more head and the flow drops away. The manufacturer publishes this as a curve, and selecting a pump means finding one whose curve passes through the point your system actually demands. Two pumps of identical horsepower can have completely different curves — one built with many small impeller stages for a deep well at modest flow, another with fewer stages for a shallow well at high flow.
This is why the output of a sizing calculation is a pair of numbers, not one. Seven gallons per minute at 219 feet of head is a point on a graph. A pump whose curve passes above it will work; one whose curve passes below it will not, whatever the badge says.
The four components of total dynamic head
Total dynamic head is the sum of everything the pump works against, all converted into the same unit — feet of water. Four things contribute and it is unusual for someone to remember all four.
- 1Lift from the pumping level — not from the static water level. When the pump runs it draws the level down, sometimes by tens of feet, and the pump must lift from wherever the level settles at the design flow. The well log records this from the driller’s yield test.
- 2Elevation rise above the wellhead — any further vertical climb to the pressure tank. Only vertical distance counts here — a long horizontal run adds friction, not lift.
- 3Pressure head — the pump does not just move water, it pressurises the system. At 2.31 feet per psi, a 40/60 switch requires 138 feet of head simply to reach cut-out. On a shallow well this term can exceed the lift.
- 4Friction head — resistance in the pipe, rising steeply with flow and falling steeply with diameter. Modest on a short run in generous pipe, dominant on a long buried line in undersized pipe.
Drawdown, well yield and the failure that follows from ignoring them
The static water level is what you measure with the pump off, and it is not the depth the pump lifts from. As soon as water is drawn, the level in the casing falls, because water flows in from the surrounding rock at a finite rate. The distance it falls is the drawdown, and it grows with the flow being pumped.
That relationship is what defines a well’s yield. A well that gives 5 gallons per minute with 20 feet of drawdown may give 10 gallons per minute only with 80 feet, or may simply not sustain 10 at all. The driller measures this during the yield test and records it on the well log, which is why that document is worth finding before sizing anything.
The failure mode when this is skipped is specific and expensive. A pump sized for more flow than the well can sustain draws the level down to the pump intake, at which point the pump runs dry. Submersible pumps are cooled by the water flowing past them, so running dry overheats the motor within minutes and destroys it. Modern installations use a low-water cut-off or a pump-saver device precisely because this happens so often.
Where the demand genuinely exceeds the well’s sustainable yield, the answer is storage rather than a bigger pump: a cistern filled slowly by a modest pump, with a second pump pressurising the house from it.
Pipe size is not the place to save money
Friction loss falls with roughly the fifth power of pipe diameter, which is a relationship steep enough to be worth restating: going from 1-inch to 1¼-inch pipe cuts friction by around two thirds at the same flow.
On a short drop pipe this rarely matters. On a long buried run from a well to a house set back from the road, it matters enormously. Three hundred feet of 1-inch pipe at 10 gallons per minute loses over 16 feet of head; the same run in 1¼-inch loses about 5. That difference has to be paid for by the pump, every minute it runs, for the life of the installation.
There is a second constraint alongside friction: velocity. Water moving faster than about 5 feet per second erodes fittings over time, produces audible noise, and makes water hammer more severe when a valve closes quickly. Checking velocity is a useful cross-check on pipe size, because a line that passes on friction can still be uncomfortably fast.
Since the pipe is buried and the labour to dig the trench dwarfs the cost of the pipe itself, undersizing it is a decision that is very expensive to revisit.
The pressure tank, and why pumps die young
A pressure tank is not storage in any meaningful sense. It is a cushion of compressed air separated from the water by a bladder, and its purpose is to let the pump deliver a batch of water and then stop, rather than switching on for every glass poured.
The volume that actually matters is the drawdown — the water delivered between the switch cutting in and cutting out — which is a fraction of the tank’s nominal size. A 20-gallon tank might have a usable drawdown of 6 or 7 gallons depending on the switch settings.
The rule that follows is that the drawdown should hold at least a minute of the pump’s flow. A pump that starts and stops every twenty seconds is short-cycling, and it is the single most common cause of premature submersible failure — motor start current is many times running current, and the heat it generates is what wears the motor out. Cycles, not hours, are what kill these pumps.
The usual cause of short-cycling in an existing system is a tank that has lost its air charge, either through a failed bladder or simply through slow leakage over years. Checking the pre-charge is a two-minute job with a tyre gauge: drain the system, and the pressure at the schrader valve should read about 2 psi below the cut-in setting. A great many pumps replaced as worn out were killed by a tank nobody checked.
What this assumes, and where it stops
Assumptions
- Peak demand is the simple sum of the fixtures ticked, with no diversity factor applied.
- Friction follows Hazen-Williams with the coefficient for the material selected, applied to the whole run at the peak flow.
- Fittings are covered by a flat percentage added to the pipe length rather than counted individually.
- Pressure head uses the cut-out setting, since that is the highest pressure the pump must reach.
- The pump is assumed to be a submersible set below the pumping level, so no suction lift limit applies.
Limitations
- This produces a duty point, not a pump selection. The final choice has to be made against a manufacturer’s curve, and the curve is where horsepower, stage count and efficiency actually live.
- Well yield is not verified. If the drawdown entered is optimistic, or the well cannot sustain the design flow, the pump will draw the level to its intake and burn out.
- Fitting losses are approximated by a percentage. A run with many elbows, a long horizontal section and several valves may need considerably more.
- Water temperature, dissolved gas and sediment all affect real pump performance and are not modelled.
- Electrical supply, wire sizing down the well, control box selection and local well code are outside this calculation and all matter for the installation.
Common questions
What size well pump do I need?
Size it against two numbers, not horsepower: the peak flow the house may draw, typically 6 to 12 gallons per minute for a family home, and the total dynamic head, which for a 200-foot well on a 50 psi system is commonly 200 to 250 feet. Take that pair to a manufacturer’s pump curve and choose a pump whose curve passes above the point.
What is total dynamic head?
Everything the pump works against, expressed in feet of water: the lift from the pumping level, any further rise to the tank, the pressure the system must hold, and friction in the pipe. The pressure term is the one most often forgotten — at 2.31 feet per psi, a 50 psi cut-out is 115 feet of head before any water moves.
Why does drawdown matter more than static water level?
Because the pump lifts from wherever the water sits while it is running, not while it is resting. Drawing water pulls the level down, and the pump has to overcome that greater depth. Sizing from the static level alone understates the head, and worse, it hides the possibility that the well cannot sustain the flow at all — which ends with the pump running dry and burning out.
Does pipe size really change the pump I need?
On a long run, substantially. Friction falls with roughly the fifth power of diameter, so 300 feet of 1-inch pipe at 10 gallons per minute loses over 16 feet of head where 1¼-inch loses about 5. Since the pipe is buried and the trench costs more than the pipe, undersizing it is an expensive decision to revisit.
Why do well pumps fail early?
Usually short-cycling. Motor starting current is many times running current, and it is cycles rather than hours that wear a submersible out. The common cause is a pressure tank that has lost its air charge, so the pump switches on and off every few seconds. Check the pre-charge with a tyre gauge on a drained system — it should read about 2 psi below the cut-in pressure.
Sources
- Private drinking water wells — US Environmental Protection Agency
- Water well basics and well construction — US Geological Survey
- Private ground water wells — safety and maintenance — Centers for Disease Control and Prevention
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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