Formula
Every well pump is chosen at a duty point: a flow, and the total dynamic head at that flow. Flow here is either a number you type or about 1 gpm per fixture, which is a pump-industry rule of thumb and not a plumbing-code fixture unit. Static level is the water surface with the pump off, in feet below the wellhead. Drawdown is how much farther it falls at the pumping rate. Pumping level is the sum. Leave drawdown at 0 only if you have not opened the well log yet. The page says so.
Pressure head = psi × 2.31
Friction = Hazen–Williams per 100 ft × L / 100
TDH = pumping level + elevation + pressure head + friction
Water hp = Q × TDH / 3,960
Brake hp = water hp / efficiency
Hazen–Williams is the same function as the friction-loss page: 0.2083 × (100/C)^1.852 × Q^1.852 / d^4.8655 feet of water per 100 feet. Q is gallons per minute and d is the inside diameter in inches. PE uses C = 140, which is the value the worked examples lock. PVC Schedule 40 uses C = 150, the smooth-pipe value on the friction page. Galvanized uses C = 120, the wet black-steel value. Inside diameters for 3/4, 1, 1-1/4, 1-1/2, and 2 inch are 0.824, 1.049, 1.380, 1.610, and 2.067. Those are ASME B36.10 Schedule 40, and ASTM D1785 PVC Schedule 40 agrees at these sizes. PE drop pipe is often a different dimension ratio. Use the ID printed on the pipe if it disagrees. Fitting equivalent length is added to the pipe before the formula runs, and a percent add-on multiplies the friction after that. Both default to zero.
3,960 is the water-horsepower constant: a gallon of water weighs about 8.34 pounds, and one horsepower is 33,000 foot-pounds per minute, so gpm × feet / (33,000 / 8.34) is gpm × feet / 3,960 for a specific gravity of 1. Efficiency is a placeholder at 0.55 until the curve gives you a better one. The page will not accept an efficiency outside 0.2 to 0.9. The next common motor size at or above the brake horsepower is only a starting point: 1/2, 3/4, 1, 1.5, 2, or 3 hp. Above 3 hp the page says the estimate is above 3 hp and the curve decides. Velocity above about 5 ft/s is flagged. A set depth that is not below the pumping level is flagged. Jet pumps are for shallow wells, roughly under 25 feet of lift. This page does not switch formulas for them.
Nothing here is a wire size. The motor’s amps are on its nameplate. Branch-circuit rules for motors are NFPA 70 (NEC) 2023, Article 430, cited by name only. The voltage drop and wire size calculators are context. Friction details stay on the friction loss calculator. The tank is the tank volume calculator.
Worked examples
Ten fixtures, 10 gpm. Static 80 ft and drawdown 20 ft is a 100 ft pumping level. The tank is 10 ft above the wellhead. Cut-out 60 psi is 138.6 ft. Two hundred feet of 1-1/4 inch PE, ID 1.380 in, C = 140, is 3.32 ft of friction. TDH is 251.9 ft. Water horsepower is 0.636. Brake horsepower at 0.55 is 1.16, so the page says to check 1.5 hp curves and notes that a 1 hp high-efficiency curve may also fit.
Smaller system. 7 gpm, static 60 ft, drawdown 15 ft, tank level with the wellhead, cut-out 50 psi (115.5 ft), 150 ft of 1 inch PE (ID 1.049 in). Friction is about 4.88 ft and TDH is about 195.4 ft. Water horsepower is about 0.345. Brake horsepower at 0.50 efficiency is about 0.69, so the page says to check 3/4 hp curves.
Pipe size. The first example in 1 inch PE is about 12.6 ft of friction. In 1-1/2 inch PE (ID 1.610 in) it is about 1.57 ft. The duty point moves with that friction. The same example with a well yield of 6 gpm shows the yield warning.
Reference
Nominal IDs used for every material on this page. PE often differs. Read the stamp. C is 140 for PE, 150 for PVC, 120 for galvanized.
| Nominal | Inside diameter, in |
|---|---|
| 3/4 in | 0.824 |
| 1 in | 1.049 |
| 1-1/4 in | 1.380 |
| 1-1/2 in | 1.610 |
| 2 in | 2.067 |
Pressure-switch presets use the cut-out, not the cut-in. 60 psi is 138.6 ft. 50 psi is 115.5 ft. 70 psi is 161.7 ft. 80 psi is 184.8 ft.
Questions
What size well pump do I need?
You need a pump whose curve passes through your duty point: the gallons per minute the house asks for, at the total dynamic head of the well. Head is pumping level, plus elevation from the wellhead to the tank, plus pressure-switch cut-out turned into feet, plus pipe friction. This page estimates that point and a horsepower to start looking. The manufacturer curve is the selection. A 1 hp high-efficiency curve can cover a duty point whose rough horsepower looks like 1.5.
What is total dynamic head?
Total dynamic head is every foot of lift and pressure the pump has to produce at the same time. Pumping level is the static water level plus drawdown, both measured down from the wellhead. Elevation is how much higher the pressure tank is than the wellhead. A downhill run is a negative elevation. Pressure head is the cut-out psi times 2.31, because one psi is 2.31 feet of water. Friction is the Hazen–Williams loss in the drop pipe and the horizontal run, including any fitting allowance you add.
How many GPM does a house need?
A common pump-industry rule of thumb is about 1 gpm per fixture. Ten fixtures, about 10 gpm. It is a rule of thumb, and this page lets you type a flow instead. It is not a fixture-unit calculation from the plumbing code. If the well’s tested yield is lower than that flow, the page warns you. A pump that outruns the well pulls the water level down.
How deep should the pump be set?
Below the pumping level, so the intake is still under water while the pump is running. Pumping level is static plus drawdown, not the static level alone. If you enter a set depth and it is not deeper than that pumping level, the page warns you. Deeper means a larger number of feet below the wellhead. The well log and the installer set the real depth. Jet pumps are a shallow-well tool, roughly under 25 feet of lift, and this page does not have a separate jet mode.
What horsepower well pump for a 200 ft well?
There is no single horsepower for a 200 ft well. A 200 ft pumping level with no drawdown left is a different pump from 80 ft static plus 20 ft of drawdown. Water horsepower is gpm times total dynamic head, divided by 3,960. Brake horsepower divides by the pump efficiency. At 0.55, the 10 gpm example on this page is about 1.16 brake horsepower, so the page says to check 1.5 hp curves and notes that a 1 hp high-efficiency curve may also fit. The curve decides.
Is 1 hp enough, or do I need 1.5 hp?
Check both curves at the duty point, not the horsepower badge. The 3,960 formula with a 0.55 efficiency is a placeholder until you read the curve. A more efficient pump makes the same water horsepower with a smaller motor. A less efficient pump needs more. This page rounds up to the next common motor size only as a place to start looking: 1/2, 3/4, 1, 1.5, 2, or 3 hp. Above 3 hp it says so and stops.
Can a bigger pump fix low pressure?
Only if the well can supply the flow and the pipe can carry it. A larger pump at the same small pipe adds velocity and friction, and it can pull the well down if the yield is the real limit. Low pressure at the house is often the pressure switch, the tank, or the pipe size. Motor amps are on the pump nameplate. This page does not size the wire. Motor branch circuits are covered by NEC 2023 Article 430, and the voltage-drop and wire-size calculators are context only.
Sources
- Hazen–Williams in the form used on the friction loss calculator: 0.2083 (100/C)^1.852 Q^1.852 / d^4.8655, feet of water per 100 ft. Spears and PPI TR-14 use that US-unit form. C = 140 is the PE value locked by the examples on this page.
- Hydraulic Institute pump basics. Water horsepower is flow in gpm times head in feet times specific gravity, divided by 3,960. The manufacturer’s pump curve is what selects the pump at the duty point. This page only estimates the point.
- Flow rate calculator for the GPM a bucket test or a pipe velocity gives you, before this duty point.
- Generator size calculator once you have the pump’s nameplate watts. This page does not list a well-pump wattage.
- U.S. Geological Survey, Water Science School, “Groundwater Wells.” Pumping lowers the water level around a well. Static level is the non-pumping level. Drawdown is the extra drop while pumping. The well log is the number to type.
- NFPA 70, National Electrical Code, 2023 edition, Article 430, motors. Named here only. No conductor or breaker is calculated. The voltage drop calculator and the wire size calculator are context, and the amps come from the pump nameplate.