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Calcimator

Pump Sizing Calculator

Size a centrifugal pump by calculating hydraulic horsepower, brake horsepower, motor size, NPSH required, and annual energy cost.

About this calculator

This calculator estimates centrifugal pump sizing from the standard hydraulic horsepower relationship, WHP = (Q x H x SG) / 3960, where Q is flow rate in gallons per minute, H is total dynamic head in feet, and SG is the fluid's specific gravity relative to water. This is the widely used real formula for the theoretical power needed to move a given flow against a given head -- the constant 3960 converts the units so the result comes out directly in horsepower. Dividing hydraulic horsepower by pump efficiency gives brake horsepower, the actual shaft power the motor must deliver once mechanical and hydraulic losses inside the pump are accounted for; this calculator then rounds up to the next standard NEMA motor frame size, since motors are manufactured in fixed steps rather than built to an exact horsepower.

Annual energy cost assumes continuous (8,760 hours/year) operation at an illustrative electricity rate -- adjust both assumptions for your actual duty cycle and local rate. The NPSH-required figure shown here is a rough placeholder, not derived from your specific pump's impeller design or suction specific speed: real NPSH-required is a curve published by the pump manufacturer for that exact model, and should always be checked against your system's actual available NPSH before finalizing a pump selection, since cavitation from insufficient NPSH margin can quickly damage an impeller.

Inputs

GPM
ft
lb/ft³
%

Results

Brake Horsepower

1.68 HP

≈ 21 laptops

Motor Size

2 HP

≈ 25 laptops

Hydraulic Horsepower1.26 HP
Motor Sizing NoteOK
NPSH Required (est.)7.5 ft
Annual Energy Cost$1,100.00
How to Use This Calculator
  1. Enter the Flow Rate in GPM required at the design operating point.
  2. Enter the Total Dynamic Head in ft — sum static lift, friction losses in piping, and any pressure differential required at the destination.
  3. Set the Fluid Density in lb/ft³: water at 60°F = 62.4 lb/ft³; adjust for other fluids.
  4. Set the Pump Efficiency %: small centrifugal pumps ≈ 50–70%, large centrifugal ≈ 75–90%.
  5. Review Brake Horsepower and the recommended Motor Size in HP — the motor should be the next standard size above BHP. If the Motor Sizing Note reports a warning, your required brake horsepower exceeds this calculator's standard motor catalog (300 HP) and needs custom/large industrial motor selection instead of the figure shown.
  6. Check the estimated NPSH Required in ft against the available NPSH at the pump suction to prevent cavitation.

How the result changes with Pump Efficiency

Pump EfficiencyBrake HorsepowerMotor Size
383.32 HP5 HP
562.25 HP3 HP
951.33 HP1.5 HP

What each input means

Flow Rate
Required volumetric flow rate in gallons per minute at the design operating point.
Total Dynamic Head
Total head the pump must deliver, including static lift, friction losses, and velocity head.
Fluid Density
Density of the fluid being pumped. Water at 60°F = 62.4 lb/ft³; light oil ≈ 55; brine ≈ 75.
Pump Efficiency
Expected pump efficiency at the design point. Small pumps ≈ 50-70%; large centrifugal ≈ 75-90%.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Flow Rate = 100, Total Dynamic Head = 50, Fluid Density = 62.4, Pump Efficiency = 75 = 4 input(s) provided
  2. Calculate Brake Horsepower
    Brake Horsepower
    1.68 = 1.68
  3. Calculate Motor Size
    Motor Size
    2 = 2
  4. Calculate Hydraulic Horsepower
    Hydraulic Horsepower
    1.26 = 1.26
  5. Calculate NPSH Required
    NPSH Required
    7.5 = 7.5

Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Is the NPSH-required value in this calculator accurate for my pump?

No -- treat it as a rough placeholder, not an engineering figure. Actual NPSH required depends on a specific pump's impeller eye design and suction specific speed, and is published by the manufacturer as a curve versus flow rate for that exact model. Always check your pump manufacturer's actual NPSHR curve against your system's available NPSH before finalizing a selection -- running with insufficient NPSH margin causes cavitation, which can damage an impeller quickly.

Why does brake horsepower divide by efficiency instead of multiply?

Hydraulic horsepower is the theoretical power delivered to the fluid; a real pump loses some of the power it draws to internal friction, recirculation, and other inefficiencies before that power reaches the fluid. Dividing by efficiency (a fraction less than 1) scales the required input power up, so a lower-efficiency pump needs a larger brake horsepower -- and therefore a larger motor -- to deliver the same hydraulic output.

Why does the recommended motor size jump to a round number above my brake horsepower?

Motor makers only offer a limited catalog of frame sizes (0.5, 0.75, 1, 1.5 HP and so on) rather than building a custom unit to an exact calculated horsepower. This calculator rounds your brake horsepower up to the next size in that catalog, since specifying a motor below your calculated requirement would leave insufficient power margin for real-world conditions.

Does the annual energy cost reflect my pump's actual operating pattern?

Only if your pump runs continuously at the entered flow and head. This calculator assumes 8,760 hours per year of operation (24/7, all year) at an illustrative electricity rate, both of which are simplifying assumptions -- a pump that cycles on and off, runs at variable speed, or operates at a different local electricity rate will have a different actual annual cost than this figure.

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