Well Pump Size Calculator: Find the Right GPM, TDH & Horsepower
Use this well pump sizing calculator to determine the correct pump capacity for your home. Enter your peak water demand, well depth, water level, pressure requirement, and piping details — the calculator will recommend a flow rate (GPM), total dynamic head (TDH), and pump horsepower class.
Best Well Pump for You:
Why Well Pump Sizing Matters: A Complete Guide to Flow Rate, Total Dynamic Head, and Horsepower
A well pump is the engine of your home's water system. It must deliver enough water to meet your household's peak demand while overcoming the physical forces that resist flow — lift, pressure, and friction. If the pump is undersized, you will experience low pressure, slow filling fixtures, and a pump that runs continuously without keeping up. If it is oversized, you pay more upfront, draw more electricity, and may cycle frequently, wearing out the motor and pressure switch. Correct sizing requires balancing three variables: flow rate (GPM), total dynamic head (TDH), and horsepower (HP). This calculator helps you estimate all three using your well characteristics, piping, and household demand.
The Three Pillars of Well Pump Sizing: GPM, TDH, and HP
Every well pump selection begins with three numbers. First, flow rate (GPM): the volume of water the pump must deliver per minute to meet peak household demand. This is determined by the fixtures you run simultaneously — showers, faucets, toilets, appliances. A typical home needs 8–12 GPM at peak. Second, total dynamic head (TDH): the total equivalent height the pump must push water, expressed in feet. TDH combines the vertical lift from the water level to the surface, the pressure requirement converted to feet, and friction losses in the piping. Third, horsepower (HP): the motor size needed to deliver the required GPM at the required TDH. Horsepower is determined by the pump curve — the relationship between flow and head for a specific pump model. Our calculator estimates TDH and then maps it to a practical horsepower class.
Flow Rate: How Much Water Does Your Home Really Need?
Peak demand is not the sum of every fixture in the house; it is the sum of fixtures that might run at the same time. A typical morning routine might include two showers (2.0 GPM each), a bathroom faucet (1.5 GPM), and a kitchen faucet (1.5 GPM), totaling 7 GPM. Add a clothes washer (2.0 GPM) and you reach 9 GPM. Add a dishwasher (2.5 GPM) and you reach 11.5 GPM. Most homes are well served by 8–12 GPM. If you have a large family, multiple bathrooms, or irrigation, you may need 15 GPM or more. Our calculator uses the GPM you enter as the design flow. If you are unsure, count your fixtures and add the ones likely to run together. A common rule of thumb is 1 GPM per fixture, but modern low-flow fixtures use less. When in doubt, size up slightly — a pump running below its capacity is more efficient than one running at its limit.
Total Dynamic Head: The Real Measure of Pump Work
Total dynamic head (TDH) is the most misunderstood part of well pump sizing. It is not simply the depth of the well. TDH is the sum of three components: (1) vertical lift — the distance from the pumping water level to the discharge point, usually the pressure tank; (2) pressure head — the system pressure converted to feet of water (1 psi = 2.31 feet); and (3) friction loss — the resistance to flow through pipes, fittings, and valves. For example, a well with a static water level of 80 feet, a drawdown of 20 feet, a system pressure of 50 psi, and 100 feet of 1-inch pipe might have a TDH of 100 (lift) + 115 (pressure) + 10 (friction) = 225 feet. The pump must be able to deliver the design GPM at 225 feet of head. If you undersize the pump for the TDH, it will not reach the cut-out pressure and will run continuously.
Step-by-Step: How the Calculator Estimates Well Pump Size
The calculator follows a transparent, repeatable formula that mirrors professional sizing methods:
| Factor | How It Is Applied | Typical Impact |
|---|---|---|
| Peak demand (GPM) | User-entered design flow | Sets the required pump capacity |
| Vertical lift | Static water level + drawdown | Dominant TDH component in deep wells |
| Pressure head | System psi × 2.31 | ~115 ft for 50 psi |
| Friction loss | Based on pipe size, length, and flow | 10–30 ft for typical residential systems |
| Total dynamic head | Lift + pressure + friction | Used to select pump horsepower |
| Horsepower class | Matched to GPM at TDH | 1/2 HP to 5 HP typical residential |
After calculating TDH, the calculator estimates the hydraulic horsepower required and applies a pump efficiency factor (typically 50–60% for submersible pumps) to determine the brake horsepower. It then rounds up to a standard motor size (1/2, 3/4, 1, 1.5, 2, 3, 5 HP) and provides a practical pump class recommendation. It also checks whether the well depth and water level are compatible with the selected pump type.
Why Undersizing a Well Pump Is a Common Mistake
Many homeowners replace a failed pump with the same size, assuming the original was correct. But household demand changes over time — new bathrooms, irrigation systems, larger families. An undersized pump will not reach the pressure switch cut-out, so it runs continuously, trying to build pressure it cannot achieve. This wastes electricity and burns out the motor. It also delivers low pressure at fixtures, making showers weak and appliances slow to fill. In extreme cases, the pump may run dry if the well cannot supply the demand, damaging the motor. Sizing the pump correctly the first time avoids these problems and ensures reliable water pressure for years.
Why Oversizing a Well Pump Is Also a Problem
An oversized pump delivers more flow than the well can supply, which can draw the water level down below the pump intake, causing the pump to run dry. It also cycles more frequently if the pressure tank is not sized to match, wearing out the pressure switch and motor. Oversized pumps draw more amperage and may require larger electrical circuits, adding installation cost. They are also more expensive upfront. The goal is to match the pump's best efficiency point (BEP) to your household's design flow and TDH. A pump operating near its BEP uses less energy per gallon and lasts longer. Our calculator aims for that sweet spot.
Submersible vs. Jet Pumps: Sizing Differences
Submersible pumps are installed inside the well, below the water level. They are efficient, quiet, and can lift water from deep wells (100–400+ feet). Jet pumps are installed above ground and use a suction pipe to draw water. They are limited to shallow wells (typically less than 25 feet for single-drop jet pumps, up to 90 feet for deep-well jet pumps with two pipes). Convertible jet pumps can be configured for either shallow or deep well applications. Submersible pumps are generally more efficient and are the standard choice for wells deeper than 25 feet. Jet pumps are common for shallow wells, cisterns, and older systems. Our calculator supports all three types and adjusts the efficiency and depth limits accordingly.
Real-World Examples and Sanity Checks
Consider a home with a peak demand of 10 GPM, a static water level of 80 feet, a drawdown of 20 feet, a system pressure of 50 psi, and 100 feet of 1-inch pipe. Vertical lift = 80 + 20 = 100 feet. Pressure head = 50 × 2.31 = 115.5 feet. Friction loss for 10 GPM through 100 feet of 1-inch pipe is roughly 8–12 feet. Total dynamic head = 100 + 115.5 + 10 = 225.5 feet. A pump that delivers 10 GPM at 225 feet of head is typically a 1.5 HP submersible. Now consider a shallow well with a static water level of 15 feet, a drawdown of 5 feet, 40 psi pressure, and 50 feet of 1-inch pipe: lift = 20 feet, pressure = 92.4 feet, friction = 5 feet, TDH = 117 feet. A 3/4 HP jet pump could deliver 10 GPM at that head. These examples show how TDH varies with well depth and pressure.
Additional Factors the Calculator Cannot Capture
No online calculator can replace a professional well system assessment, but our tool covers the dominant variables for pump sizing. Still, be aware of factors that can affect performance. Well yield: if the well produces less water than the pump demands, the pump may run dry. The well's recovery rate (GPM the well can sustain) must exceed the pump's flow rate. Pump setting depth: the pump must be set below the drawdown water level but above the well bottom. Sand and sediment: sandy wells may require a sand-handling pump or a screen. Water quality: acidic or hard water affects pump materials. Electrical supply: voltage drop over long cable runs reduces motor performance; use the correct wire gauge. Freezing: pipes from the well to the house must be buried below frost line or heat-traced. If your situation is unusual in any of these dimensions, consult a licensed well contractor.
| Peak Demand (GPM) | Typical TDH (feet) | Recommended Pump | Typical Use Case |
|---|---|---|---|
| 5–8 | 50–150 | 1/2 HP submersible or jet | Small cabin, 1 bathroom, shallow well |
| 8–12 | 150–250 | 3/4 – 1 HP submersible | Typical single-family home, 2 bathrooms |
| 12–15 | 200–300 | 1.5 HP submersible | Large home, 3 bathrooms, irrigation |
| 15–20 | 250–400 | 2 HP submersible | Very large home, high demand, deep well |
| 20+ | 300–500+ | 3–5 HP submersible or multiple pumps | Commercial, farm, multi-family |
Pump Curves: Why Horsepower Alone Is Not Enough
A pump's horsepower rating tells you the motor size, but it does not tell you how much water the pump will deliver at a given head. That information is found on the pump curve — a graph that shows flow (GPM) on the horizontal axis and head (feet) on the vertical axis. A 1 HP pump may deliver 15 GPM at 100 feet of head, but only 8 GPM at 250 feet. When selecting a pump, you must find a model whose curve passes through your design point (design GPM at design TDH). A pump that is too far to the right of its curve will not reach the required pressure; one that is too far to the left will cycle frequently. Our calculator estimates the design point (GPM and TDH) and suggests a horsepower class, but you should always verify against the manufacturer's pump curve before purchasing.
Pressure Tanks and Well Pump Sizing: A Critical Partnership
The well pump and pressure tank work together. The pump fills the tank, and the tank provides drawdown to keep the pump from cycling too frequently. If the pump is sized correctly but the tank is too small, the pump will short-cycle and wear out. If the tank is sized correctly but the pump is too small, the pump will run continuously and never reach cut-out. The two must be sized as a system. A common rule is that the pressure tank should provide at least 1–2 minutes of runtime at the pump's flow rate. For a 10 GPM pump, that means 10–20 gallons of drawdown, which typically requires a 30–50 gallon diaphragm tank. If you are replacing a pump, consider whether the existing tank is still adequate. Our pressure tank calculator can help you check.
Energy Efficiency, VFDs, and Operating Cost
Well pumps are significant energy users, especially in rural homes where they may run several hours per day. A correctly sized pump operating near its best efficiency point uses less energy per gallon than an oversized or undersized pump. Variable frequency drives (VFDs) can further improve efficiency by modulating pump speed to match demand, maintaining constant pressure and reducing starts. A VFD system may cost more upfront but can cut energy use by 20–40% and extend pump life. If you are building a new system or replacing an old pump, consider a constant-pressure VFD system, especially if you have high or variable demand. Our calculator does not include VFD, but the GPM and TDH results are still the starting point for selecting a VFD-compatible pump.
Frequently Asked Questions
How many GPM do I need for a house?
A typical single-family home needs 8–12 GPM at peak demand. That covers two showers running simultaneously, a kitchen faucet, and a bathroom faucet. If you have three or more bathrooms, irrigation, or a large family, you may need 15 GPM or more. Count your fixtures and estimate which ones run together, then add their flow rates. Use the calculator above with your peak demand to get a pump recommendation.
What is total dynamic head (TDH)?
Total dynamic head is the total equivalent height the pump must push water, expressed in feet. It is the sum of vertical lift (from the pumping water level to the discharge point), pressure head (system pressure converted to feet, where 1 psi = 2.31 ft), and friction loss (resistance in pipes and fittings). TDH is the number you use to select a pump from its curve, along with the design flow rate.
How do I calculate friction loss in my piping?
Friction loss depends on pipe diameter, length, flow rate, and the number of fittings. You can use friction loss tables or online calculators. As a rough rule, 1-inch pipe at 10 GPM loses about 0.1 psi per 10 feet, or roughly 8–12 feet of head over 100 feet. Larger pipe (1-1/4" or 1-1/2") reduces friction loss significantly. Our calculator applies an approximate friction loss based on your pipe size, length, and flow rate.
Can I replace my well pump with a larger one?
You can, but the well must be able to supply the additional flow. If the well's recovery rate is lower than the pump's flow rate, the pump will draw the water level down and run dry, damaging the motor. Also check that the pressure tank, pressure switch, and electrical circuit can handle the larger pump. Before upsizing, have a well contractor measure your well's yield and recovery rate.
How deep should a submersible pump be set?
The pump should be set below the drawdown water level — the lowest level the water reaches when the pump runs — but at least 10–20 feet above the well bottom to avoid pulling in sand and sediment. The pump setting depth must also be below the well's casing perforations or screen. A licensed well contractor can determine the correct setting depth based on your well's construction and yield.
What size pressure tank do I need for my well pump?
The pressure tank should provide enough drawdown to keep the pump from short-cycling. A common rule is 1–2 minutes of runtime at the pump's flow rate. For a 10 GPM pump, that means 10–20 gallons of drawdown, which typically requires a 30–50 gallon diaphragm tank at a 30/50 psi setting. Use our pressure tank size calculator to get a precise recommendation based on your pump flow and pressure switch settings.
How long does a well pump last?
A properly sized submersible pump typically lasts 10–15 years. Jet pumps may last 10–20 years. Lifespan depends on water quality, sand content, cycling frequency, and electrical conditions. A pump that short-cycles due to an undersized pressure tank or an oversized pump will fail sooner. Regular maintenance, including checking the pressure tank air charge and monitoring for sand, can extend pump life.
Why does my well pump run continuously?
A pump that runs continuously usually indicates one of four problems: the pump is undersized for the TDH and cannot reach cut-out pressure; the pressure tank is waterlogged and provides no drawdown; the pressure switch is faulty or set too high; or there is a leak in the system. Check the pressure gauge — if it stays below the cut-out setting, the pump cannot build pressure. If it fluctuates rapidly, the tank may be waterlogged. Diagnose the cause before replacing the pump.
What is the difference between a submersible and a jet pump?
A submersible pump is installed inside the well, below the water level. It pushes water up to the surface and is efficient for deep wells (100–400+ feet). A jet pump is installed above ground and uses suction to draw water up. Jet pumps are limited to shallow wells (typically less than 25 feet for single-drop, up to 90 feet for deep-well two-pipe configurations). Submersible pumps are the standard choice for wells deeper than 25 feet.
Do I need a variable frequency drive (VFD) for my well pump?
A VFD is not required, but it offers benefits: constant pressure at fixtures, fewer pump starts, lower energy use, and longer pump life. VFD systems cost more upfront but can pay back over time, especially in homes with high or variable water demand. If you are replacing a pump and want constant pressure, a VFD system is worth considering. The pump must be compatible with the drive; consult the manufacturer.
Well Pump Sizing Checklist
- Calculate peak household demand in GPM (sum of simultaneous fixtures)
- Measure well depth, static water level, and drawdown
- Determine system pressure (cut-out psi) and convert to feet (psi × 2.31)
- Estimate friction loss from pipe size, length, and flow
- Sum the three components to get total dynamic head (TDH)
- Use the calculator to get recommended GPM, TDH, and horsepower class
- Verify the pump curve passes through your design point
- Confirm the well's yield exceeds the pump's flow rate
- Match the pressure tank to the pump flow and pressure switch
- Check electrical circuit capacity and wire gauge for the pump
Professional Sizing vs. Rule-of-Thumb
Well contractors use detailed calculations that consider pump curves, well yield, pipe friction, electrical voltage drop, and local code requirements. Those calculations are more precise than any online tool, but they are also time-consuming and often overkill for a single-family home. Our calculator sits between the crude "one size fits all" approach and a full manual calculation. It captures the variables that matter most for well pump sizing — flow demand, vertical lift, pressure, and friction — while keeping the interface simple. If you are sizing for a commercial system, a farm, or a well with unusual characteristics, consider hiring a licensed well contractor to verify the design. For a typical residence, the calculator's result will serve you well.
Final Thoughts: Matching the Pump to the Well and the Home
The right well pump size is the one that delivers your peak demand at your system pressure without running continuously or cycling excessively. Use this calculator as your first step, then check the pump curve on any model you consider. Remember that the pump and pressure tank work as a team — sizing one without the other leads to problems. If you are replacing a pump, take the opportunity to verify your well's yield, inspect the pressure tank, and check the electrical circuit. Bookmark this page, run your numbers before you buy, and enjoy reliable, efficient water pressure for years to come.