Hydraulic Institute Equations

Total Dynamic Head (TDH) Calculator for Water Wells

Total Dynamic Head (TDH) is the true measurement of physical work a submersible well pump must produce. Sizing a pump solely by well depth without calculating TDH leads to weak pressure fixtures, pump overheating, and catastrophic motor burnout. Calculate your exact head requirements below.

Avg Home: 8–12 GPM | Homestead: 15–20 GPM
ft
Static level + drawdown under continuous pumping
ft
Vertical climb from wellhead to pressure tank
ft
Trench length from wellhead to house
RECOMMENDED PUMP MOTOR 3.0 HP (3 HP) 2.58 HP (Shaft Load)
TOTAL DYNAMIC HEAD (TDH) 493 ft Head 9.4 ft Friction Loss
CABLE WIRE GAUGE 10 AWG Copper
BREAKER SIZE 20A 2-Pole
MIN PRESSURE TANK 90+ Gal Tank
💧 Pressure Tank Drawdown Capacity:

Min 24 Gal Drawdown (provides safe 60s motor cycle)

Deep Well Submersible Pumping System Blueprint

Cross-sectional engineering diagram: Well casing, dynamic water level, submersible pump, pitless adapter, and residential pressure tank assembly.

TDH: 493 ft Head
Drilled Well & Submersible Pump Cross-Section Hydraulic blueprint illustrating deep well casing, submersible motor, drop pipe, pitless adapter, buried waterline, and indoor pressure tank. GRADE LEVEL (SURFACE) WATER LEVEL (320′) WELL SEAL 1.25″ Poly Water Pipe PUMP MOTOR UTILITY ROOM / BASEMENT PRESSURE TANK 60 PSI TO HOME CALCULATED OPERATING POINT 12 GPM @ 3.0 HP (3 HP) Total Dynamic Head: 493 ft
💡 Pumping Point: 12 GPM flow against 493 ft Total Dynamic Head requires a 3.0 HP (3 HP) motor.

The 4 Pillars of Total Dynamic Head (TDH)

TDH is the sum of four distinct components of fluid resistance expressed in equivalent vertical feet of water column:

The Master TDH Equation:

TDH = Pumping Water Level (ft) + Elevation Head (ft) + Pipe Friction Loss (ft) + Pressure Head (ft)

  • 1. Pumping Water Level: The vertical depth in feet from the ground surface down to the water level while the pump is actively running at its target flow rate. This includes the static water table depth plus well drawdown.
  • 2. Elevation Head: The vertical height difference between the wellhead casing cap and the highest plumbing fixture (or inlet of your pressure tank).
  • 3. Pipe Friction Loss: The resistance created by water dragging against the internal walls of the polyethylene, PVC, or steel pipe, calculated via the Hazen-Williams formula plus a standard 10% allowance for check valves, pitless adapters, and 90° elbows.
  • 4. Pressure Head: The energy required to overcome the air bladder pressure inside your home's pressure tank. Because 1 PSI = 2.31 feet of water column, a 40/60 PSI switch (cut-out at 60 PSI) demands 60 × 2.307 = 138.4 feet of head.

Hazen-Williams Pipe Friction Loss Table (Feet per 100 ft of Pipe)

Friction losses through smooth polyethylene (Poly / PE, C=150) and Schedule 40 PVC pipe based on continuous flow rate (GPM):

Flow Rate (GPM) 1.00″ Poly Pipe (ID: 1.049″) 1.25″ Poly Pipe (ID: 1.380″) 1.50″ Poly Pipe (ID: 1.610″) 2.00″ Poly Pipe (ID: 2.067″)
5 GPM 1.6 ft / 100′ 0.4 ft / 100′ 0.2 ft / 100′ 0.1 ft / 100′
8 GPM 3.8 ft / 100′ 1.0 ft / 100′ 0.5 ft / 100′ 0.1 ft / 100′
10 GPM 5.7 ft / 100′ 1.5 ft / 100′ 0.7 ft / 100′ 0.2 ft / 100′
12 GPM 8.0 ft / 100′ 2.1 ft / 100′ 1.0 ft / 100′ 0.3 ft / 100′
15 GPM 12.1 ft / 100′ 3.2 ft / 100′ 1.5 ft / 100′ 0.4 ft / 100′
20 GPM 20.6 ft / 100′ (High Velocity) 5.5 ft / 100′ 2.6 ft / 100′ 0.8 ft / 100′
* Velocities exceeding 5 feet/second (such as 20 GPM through a 1-inch pipe) lead to severe pipe erosion and water hammer damage. Always size to 1.25″ or 1.50″ for flows above 10 GPM.

Worked Calculation Example: Standard Rural 250-Foot Well

Site Data:

• Pumping Water Level: 190 feet
• Elevation Climb to House: 20 feet
• Piping: 240 ft of 1.25″ Poly Pipe discharging 10 GPM
• System Pressure: 40/60 PSI (Cut-out = 60 PSI)

Calculating the 4 Head Components:

  • Vertical Lift: 190 ft + 20 ft = 210 ft
  • Friction Loss: 240 ft of 1.25″ pipe @ 10 GPM produces 1.5 ft per 100′. (2.4 × 1.5 = 3.6 ft) + 10% fittings = 4.0 ft
  • Pressure Head: 60 PSI × 2.307 ft/PSI = 138.4 ft
Total Dynamic Head (TDH) = 210 + 4.0 + 138.4 = 352.4 Feet of Head

Sizing Result: To deliver 10 GPM at 352.4 feet of TDH:

Water HP = (10 × 352.4) / 3960 = 0.89 WHP

Required Shaft HP (at 58% efficiency) = 0.89 / 0.58 = 1.53 BHP

This installation requires a 1.5 HP (1-1/2 HP) Submersible Pump Motor. Installing a standard 0.75 HP or 1.0 HP pump would fail to reach the 60 PSI shutoff pressure, running continuously until the motor burned out.

Frequently Asked Questions About Total Dynamic Head

What is the difference between static water level and pumping water level?

Static water level is the resting depth of water in the well casing when the pump has been idle for several hours. Pumping water level (or dynamic water level) is the depth to which the water drops after the pump runs continuously for an hour or more. TDH must always be calculated from the pumping water level, never the static level.

Why do we multiply pressure switch PSI by 2.31 to calculate head?

A column of clean water exactly 2.31 feet high exerts 1.0 pound per square inch (PSI) of downward hydrostatic pressure at its base. To convert a pressure tank's cut-out pressure into equivalent vertical feet of lift that the pump must overcome, multiply operating PSI by 2.31. A 60 PSI switch represents 138.6 feet of equivalent vertical head.

How does pipe diameter impact Total Dynamic Head?

Under the Hazen-Williams fluid equation, friction loss increases exponentially as pipe diameter decreases (inversely proportional to the internal diameter to the 4.87 power). Pumping 15 GPM through a 1-inch pipe creates over 12 feet of friction loss per 100 feet of pipe, whereas sizing up to a 1.25-inch pipe drops that friction loss to just 3.4 feet per 100 feet.

System Cross-References:

Now that you know your TDH, verify wire gauge requirements in our Submersible Wire Size Calculator or determine pressure tank drawdown in the Pressure Tank Calculator.