Well Pump Horsepower Calculator
ANA›Life Services Authority›National Calculator Authority›Well Pump Horsepower Calculator
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Well Pump Horsepower Calculator
Calculate the required brake horsepower (BHP) and water horsepower (WHP) for a well pump based on flow rate, total dynamic head, and pump efficiency.
Flow Rate (GPM)
gallons per minute
Total Dynamic Head (ft)
feet
Pump Efficiency (%)
percent (1–100)
Motor Efficiency (%)
percent (1–100)
Specific Gravity of Fluid
dimensionless (water = 1.0)
Calculate
function welCalc() { const resultDiv = document.getElementById('wel-result');
const gpm = parseFloat(document.getElementById('wel-flow-rate').value); const head = parseFloat(document.getElementById('wel-total-head').value); const pumpEff = parseFloat(document.getElementById('wel-pump-efficiency').value); const motorEff = parseFloat(document.getElementById('wel-motor-efficiency').value); const sg = parseFloat(document.getElementById('wel-specific-gravity').value);
// --- Validation --- const errors = []; if (isNaN(gpm) || gpm 100) errors.push("Pump Efficiency must be between 1 and 100."); if (isNaN(motorEff) || motorEff 100) errors.push("Motor Efficiency must be between 1 and 100."); if (isNaN(sg) || sg 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = '⚠ ' + errors.join('⚠ ') + ''; return; }
const pumpEffDec = pumpEff / 100.0; const motorEffDec = motorEff / 100.0;
// Water Horsepower (WHP): the theoretical power needed to move the fluid // WHP = (GPM × TDH × SG) / 3960 // 3960 = 33,000 ft·lbf/min ÷ (8.34 lb/gal) = unit conversion constant const whp = (gpm * head * sg) / 3960.0;
// Brake Horsepower (BHP): power required at the pump shaft (accounts for pump efficiency) // BHP = WHP / pump_efficiency const bhp = whp / pumpEffDec;
// Motor Horsepower (MHP): power drawn from the electrical supply (accounts for motor efficiency) // MHP = BHP / motor_efficiency const mhp = bhp / motorEffDec;
// Overall system efficiency const overallEff = pumpEffDec * motorEffDec * 100.0;
// Approximate electrical power draw (kW) // 1 HP = 0.7457 kW const kw = mhp * 0.7457;
resultDiv.style.display = 'block'; resultDiv.innerHTML = '### Results ' + '' + 'Water Horsepower (WHP)' + '' + whp.toFixed(4) + ' HP' + 'Brake Horsepower (BHP)' + '' + bhp.toFixed(4) + ' HP' + 'Motor Input Horsepower (MHP)' + '' + mhp.toFixed(4) + ' HP' + 'Electrical Power Draw' + '' + kw.toFixed(4) + ' kW' + 'Overall System Efficiency' + '' + overallEff.toFixed(2) + ' %' + '' + 'ⓘ Select a motor rated at or above ' + mhp.toFixed(2) + ' HP for safe continuous operation.
'; }
#### Formulas Used
Water Horsepower (WHP) — theoretical hydraulic power:
WHP = (GPM × TDH × SG) / 3960 Where 3960 is derived from: 33,000 ft·lbf/min ÷ 8.34 lb/gal (weight of one gallon of water).
Brake Horsepower (BHP) — power required at the pump shaft:
BHP = WHP / η_pump
Motor Input Horsepower (MHP) — power drawn from the power supply:
MHP = BHP / η_motor
Electrical Power (kW):
kW = MHP × 0.7457
Overall System Efficiency:
η_overall = η_pump × η_motor × 100
#### Assumptions & References
- Total Dynamic Head (TDH) includes static lift (depth to water + surface elevation), friction losses in pipes, fittings, and any pressure head required at the discharge point.
- The constant 3960 is the standard US unit conversion factor for GPM, feet, and horsepower (33,000 ÷ 8.34 ≈ 3960).
- Water specific gravity is 1.0 at ~60 °F (15.6 °C). Adjust for other fluids or temperatures.
- Typical centrifugal well pump efficiencies range from 50 % to 80 %; motor efficiencies typically range from 85 % to 95 %.
- Always select a motor with a nameplate rating at or above the calculated MHP to avoid overloading.
- Formula source: Groundwater and Wells, 3rd ed., Johnson Screens / Roscoe Moss Company; AWWA Manual M21.
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