Pump Sizing Calculator

Frequently Asked Questions

How is total dynamic head (TDH) calculated?

TDH = static lift (vertical rise) + friction loss (Hazen-Williams) + pressure head (required psi × 2.31). A pump lifting 20 ft, with 8 ft of friction, delivering 40 psi = 20 + 8 + 92 = 120 ft TDH.

How do I match a pump curve to my system?

Plot your system curve (TDH vs flow) and overlay the pump curve. The operating point is where they cross - design so it sits near the pump's best-efficiency point (BEP), usually 80–110% of BEP flow.

How is pump horsepower estimated?

HP = (GPM × TDH) ÷ (3,960 × η_pump × η_motor). At 30 GPM, 120 ft TDH, 60% pump efficiency, 90% motor: HP = 3,600 ÷ (3,960 × 0.54) ≈ 1.7 → 2 HP motor.

Why oversize the motor slightly?

Motors should run near 75–90% of nameplate for best efficiency and cooling. Designing the operating point well below nameplate gives margin for fouling, lift increases, and starting torque.

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Estimates for informational purposes only.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

A pump is sized to deliver a target flow rate against the total resistance of the system it serves. That resistance - called total dynamic head (TDH) - is the sum of three things: the vertical distance from the water source to the highest point of discharge (static lift), the friction loss in the suction and discharge piping at the design flow (friction head), and any pressure required at the discharge such as the city-side residual or a tank pre-charge (pressure head). Once you know GPM and TDH the brake horsepower follows from a simple energy balance, then you pick a real pump whose published curve passes above your design point.

Formula / Method

TDH (ft) = static lift + friction head + pressure head, where pressure head (ft) = psi × 2.31. Water horsepower (the energy actually added to the fluid) is WHP = Q × TDH ÷ 3960, with Q in gpm and TDH in feet. Brake horsepower - what the motor must deliver to the shaft - is BHP = WHP ÷ η_pump, where pump efficiency typically falls in the 50–75% range for residential centrifugals and 75–85% for well-matched commercial pumps. The motor must be sized at or above BHP, usually rounded up to the next nominal motor size (1/2, 3/4, 1, 1.5, 2 HP). For deep lifts above about 25 ft a single-stage centrifugal cannot prime; use a jet or submersible.

Worked Example

Pumping 15 gpm from a shallow well 40 ft below the discharge, through 80 ft of 1 in line losing 8 ft of friction at that flow, into a pressure tank set at 46 ft (20 psi). TDH = 40 + 8 + 46 = 94 ft. WHP = 15 × 94 ÷ 3960 ≈ 0.356 HP. At 60% efficiency BHP = 0.356 ÷ 0.60 ≈ 0.59 HP, so a 3/4 HP motor is the right call - round up to the next standard size, never down. Verify by laying your operating point (15 gpm at 94 ft) on the manufacturer's pump curve and confirm it sits below the curve in the recommended efficiency band.

Common Mistakes

Forgetting the pressure head - a 50 psi house system needs 116 ft of head just to maintain that pressure at the highest fixture.

Using free-flow (zero head) GPM from a brochure as the design point - that point delivers nothing usable.

Assuming a single-stage centrifugal can prime more than 25 ft of lift - physics caps it near 33 ft and real-world cavitation cuts that further.

Ignoring fitting losses - suction-side losses are especially punishing because they directly reduce NPSH available.

Sizing motor exactly to BHP with no margin - voltage sag, future fouling, and viscosity changes can overload it.

Tips & FAQ

Centrifugal or jet? Centrifugal pumps are quiet, efficient, and cheap but cannot prime themselves - use them for flooded suction or below-water installations. Jet pumps use a venturi to draw water up from shallow or deep wells (deep-well jets handle 100+ ft); submersible pumps push from the bottom of the well and are the standard for any well over 25 ft. What efficiency should I assume? 55–65% for residential, 70–80% for commercial well-matched pumps; if you have a curve, use the value at your operating point. How much GPM does a house need? A typical 3-bath home runs comfortably on 8–15 gpm; large irrigation or fire-suppression jumps to 30+ gpm. Why round up to nominal HP? Motors are sold in 1/4-HP steps; running an undersized motor at 100% load is a thermal failure waiting to happen. NPSH: always check NPSH available exceeds NPSH required by at least 2 ft to avoid cavitation. Variable speed: a VFD-controlled pump matches flow to demand and saves substantial energy in larger systems. Pressure tank sizing: the tank's drawdown volume sets how often the pump cycles - aim for at least 1 minute of run time per start. This is a planning estimate; for permitted or well work consult a licensed pump contractor.

Frequently Asked Questions

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