Total Dynamic Head Explained

Why "how deep is the well" isn't the whole answer

Total dynamic head (TDH) is the total resistance a pump has to work against, measured in equivalent feet of vertical lift. Well depth is only one piece of it — the well pump sizing calculator needs the full TDH, not just depth, because undersizing this number is one of the most common well pump sizing mistakes.

The three components of TDH

• Vertical lift — the distance from the water's pumping level (which drops below static level once the pump is running) up to the pressure tank or highest point of use.
• Friction loss — resistance from the water moving through the pipe itself, which increases with pipe length, decreases with larger pipe diameter, and depends on the pipe material's roughness.
• Discharge pressure — the pressure you want maintained at the tank, converted to equivalent feet of head (roughly 2.31 ft of head per 1 psi of pressure).

A rough field estimate

Many well contractors use roughly 1.5× the well's pumping-level depth as a fast estimate when a full calculation isn't available — it bakes in a reasonable allowance for friction loss and discharge pressure without calculating each separately. It's a starting point, not a substitute for an actual calculation or your well driller's report when precision matters.

Worked example

A well with a 100 ft pumping-level depth: the rough estimate gives 100 × 1.5 = 150 ft TDH — which is exactly the example used in the well pump sizing calculator's own worked example.

Frequently asked questions

Is TDH the same as static water level?

No — static level is the water's resting depth before pumping starts. Pumping level (also called dynamic level) is deeper, since the water level drops while the pump is actively drawing from the well. TDH uses the pumping level, not the static level, since that's the condition the pump actually has to overcome while running.

Does pipe size affect TDH?

Yes — smaller diameter pipe increases friction loss for the same flow rate, raising TDH. This is part of why undersized well piping can make an otherwise correctly-rated pump underperform.

Where do I get an exact TDH instead of the rough estimate?

A well driller's completion report typically includes pumping-level data, and a licensed well or pump contractor can calculate exact friction loss for your specific pipe run — worth getting for anything beyond a simple, shallow well replacement.