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Pump Sizing Calculator

Calculate pump HP from flow rate, head, and friction losses

Required HP

1.57 HP

TDH

80.9 ft

Recommended

2 HP

Velocity

5.1 ft/s

Units
Required HP

1.57

Recommended HP

2 HP

Total Dynamic Head

80.9 ft

NPSH Available

29.0 ft

TDH Breakdown

Static Head20.0 ft
Friction Head14.7 ft
Pressure Head46.2 ft

Operating Point

Flow Rate50.0 GPM
Pipe Velocity5.1 ft/s
Equiv. Pipe Length140 ft
Efficiency65%
Velocity StatusMarginal

Example Calculations

1Residential Well Pump (50 GPM, 20 ft lift, PVC)

Inputs

Flow Rate50 GPM
Static Head20 ft
Pipe Diameter2 inches
Pipe Length100 ft
Pipe MaterialPVC (C=150)
Elbows/Fittings4
Discharge Pressure20 PSI
Pump Efficiency65%

Result

Required HP1.57 HP
Recommended Pump2 HP
Total Dynamic Head81.0 ft
Friction Head14.8 ft
Pipe Velocity5.1 ft/s

Equivalent length = 100 + 4×10 = 140 ft. Friction head = 14.4 ft via Hazen-Williams. Pressure head = 20×2.31 = 46.2 ft. TDH = 20+14.4+46.2 = 80.6 ft. HP = (50×80.6)/(3960×0.65) = 1.57. Next standard size: 2 HP.

2Commercial System (100 GPM, 30 ft lift, Steel)

Inputs

Flow Rate100 GPM
Static Head30 ft
Pipe Diameter3 inches
Pipe Length200 ft
Pipe MaterialSteel (C=120)
Elbows/Fittings6
Discharge Pressure30 PSI
Pump Efficiency70%

Result

Required HP4.34 HP
Recommended Pump5 HP
Total Dynamic Head120.3 ft
Friction Head21.0 ft
Pipe Velocity4.5 ft/s

Equivalent length = 200 + 6×10 = 260 ft. Friction head = 21.0 ft (steel C=120, higher friction). Pressure head = 30×2.31 = 69.3 ft. TDH = 30+21.0+69.3 = 120.3 ft. HP = (100×120.3)/(3960×0.70) = 4.34. Next standard size: 5 HP.

3Irrigation Booster (25 GPM, 10 ft lift, PVC)

Inputs

Flow Rate25 GPM
Static Head10 ft
Pipe Diameter1.5 inches
Pipe Length150 ft
Pipe MaterialPVC (C=150)
Elbows/Fittings3
Discharge Pressure15 PSI
Pump Efficiency60%

Result

Required HP0.74 HP
Recommended Pump3/4 HP
Total Dynamic Head70.0 ft
Friction Head25.4 ft
Pipe Velocity4.5 ft/s

Equivalent length = 150 + 3×10 = 180 ft. Friction head = 25.4 ft (small 1.5" pipe increases friction). Pressure head = 15×2.31 = 34.65 ft. TDH = 10+25.4+34.65 = 70.0 ft. HP = (25×70.0)/(3960×0.60) = 0.74. Next standard size: 3/4 HP.

Frequently Asked Questions

Q

How do I calculate what size pump I need?

Pump size depends on flow rate (GPM) and total dynamic head (TDH). TDH is the sum of static head (elevation), friction head (pipe losses), and pressure head (desired discharge PSI). Use HP = (GPM × TDH) / (3960 × efficiency) to find required horsepower.

  • Always round UP to the next standard pump size — undersizing causes motor burnout
  • Typical pump efficiency ranges from 50% to 80%; use 60–65% for conservative sizing
  • Add 10–15% safety factor for aging pipes and future capacity needs
  • Centrifugal pumps lose efficiency at very low or very high flow rates
  • A pump curve from the manufacturer shows exact HP at your operating point
Flow RateTDHRequired HP (65% eff)Standard Size
10 GPM50 ft0.19 HP1/4 HP
25 GPM60 ft0.58 HP3/4 HP
50 GPM80 ft1.55 HP2 HP
100 GPM100 ft3.89 HP5 HP
200 GPM120 ft9.32 HP10 HP
Q

What is Total Dynamic Head (TDH)?

TDH is the total equivalent height the pump must push water, measured in feet. It combines three components: static head (elevation difference), friction head (energy lost to pipe friction), and pressure head (desired outlet pressure converted to feet of head at 2.31 ft per PSI).

  • Static head is simply the vertical distance from the water source to the highest discharge point
  • Friction head increases exponentially with flow rate — doubling flow roughly triples friction loss
  • Each 1 PSI of desired discharge pressure adds 2.31 ft of head
  • Pipe fittings (elbows, tees, valves) add equivalent pipe length — each 90° elbow adds about 10 ft
  • Longer pipe runs and smaller diameters dramatically increase friction head
TDH ComponentHow to MeasureTypical Range
Static HeadElevation difference (ft)5–100 ft
Friction HeadHazen-Williams formula5–50 ft
Pressure HeadPSI × 2.3123–116 ft (10–50 PSI)
Q

What pipe material has the lowest friction loss?

PVC has the lowest friction loss with a Hazen-Williams C value of 150. Smoother pipes have higher C values and less resistance. Copper (C=140) is close, while steel (C=120) and cast iron (C=100) have significantly more friction due to rougher interior surfaces.

  • PVC and CPVC are the smoothest common pipe materials with C=150
  • Steel and cast iron C values drop significantly with age due to corrosion and scale buildup
  • Replacing old cast iron (C=70) with PVC (C=150) can cut friction losses by 75%
  • Higher friction means more HP required — switching from steel to PVC can reduce pump size by 20–30%
Pipe MaterialC ValueRelative Friction
PVC / CPVC150Lowest (baseline)
Copper (new)140~15% more
Steel (new)120~50% more
Cast Iron (new)100~2× more
Cast Iron (aged)60–803–5× more
Q

What is NPSH and why does it matter for pump sizing?

NPSH (Net Positive Suction Head) is the absolute pressure available at the pump inlet minus the vapor pressure of the liquid. If NPSH available is too low, the pump cavitates — forming vapor bubbles that collapse violently, destroying the impeller and reducing flow.

  • NPSH Available must always exceed NPSH Required (from the pump curve) by at least 2–3 ft
  • Atmospheric pressure provides about 33.9 ft of head at sea level — this drops at higher elevations
  • Suction lift (pump above water source) reduces NPSH Available — keep suction lines short and large
  • Hot water has higher vapor pressure, reducing NPSH Available — critical for boiler feed pumps
  • Signs of cavitation: loud rattling noise, vibration, reduced flow, and pitting on the impeller
Q

How do pipe fittings affect pump sizing?

Each pipe fitting (elbow, tee, valve) creates turbulence that acts like additional pipe length. A standard 90° elbow adds roughly 10 ft of equivalent pipe length. A system with many fittings can have 50–100% more effective pipe length than the straight-run distance.

  • Count all fittings in the system and add their equivalent lengths to the total pipe run
  • A typical residential well system with 6 elbows adds ~60 ft of equivalent pipe length
  • Reducing the number of elbows is often cheaper than increasing pump size
  • Use sweep (long-radius) elbows instead of standard 90° to cut fitting losses by 40%
Fitting TypeEquivalent Length (2" pipe)Notes
90° Elbow~10 ftMost common fitting
45° Elbow~5 ftAbout half of 90°
Gate Valve (open)~2 ftMinimal resistance
Check Valve~15 ftHigher than elbows
Globe Valve~35 ftHighest common fitting

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Last Updated: Mar 9, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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