Well Depth Is Not Pump Head: How to Calculate TDH for a Borehole Pump (With Worked Example)

First, Three Definitions

Static water level (SWL). The depth from ground level down to the water surface when the pump is not running. Measure it after the well has rested for several hours.

Dynamic water level (DWL). The depth from ground to the water surface while the pump is running at your required flow. When pumping starts, the water level drops. This is the level the pump actually works against.

Drawdown. The difference between the two: drawdown = DWL − SWL. Your driller’s pumping test gives you the dynamic water level at a given flow.

Key point: the pump lifts water from the dynamic water level, not from the bottom of the well, and not from the static level.

One practical warning: water levels change with the seasons. In the dry season, or after years of heavy pumping in the area, the dynamic level can drop lower than your driller’s first test. If you have dry-season measurements, use those. A pump that is short on head will deliver clearly less water than you need — and in the worst case the water may not reach the outlet at all.

The TDH Formula

TDH is the total height the pump must push water, plus all losses:

TDH = dynamic water level depth + outlet height above ground + pipe friction losses + required outlet pressure (required delivery head)

A note on terms: required outlet pressure (also called required delivery head) is a real demand of your system — for example the pressure a sprinkler line needs to work. It is not the same as a design margin. A design margin is extra head you add on purpose for safety or future changes. If you want such a margin, add it separately and tell our engineers — do not hide it inside the outlet pressure figure.

If you are not sure about friction losses, a rough field estimate for a straight rising main with a few fittings is acceptable for a first selection. Losses grow fast with small pipes, long pipe runs, and many bends and valves — using a larger rising-pipe diameter often cuts losses more than buyers expect. Our engineers refine the figure with your actual pipe diameter and length when we do the final selection — confirm the final figure with our engineers for large projects.

Required outlet pressure is zero when water falls freely into a tank. Add it only when you need pressure at the end of the line — for example for sprinklers or a long distribution network. When in doubt, tell us what the water must do at the outlet and we will help you set the right figure.

Worked Example: A Farm Well With an Elevated Tank

A farm project has these site conditions:

• Well depth: 80 m
• Static water level: 45 m below ground
• Dynamic water level: 60 m below ground (measured at the required flow)
• Elevated tank inlet: 12 m above ground
• Pipe friction losses: about 8 m
• Required outlet pressure: 0 m (water falls freely into the tank)

Calculation:

• Dynamic water level depth: 60 m
• Outlet height: + 12 m
• Friction losses: + 8 m
• Required outlet pressure: + 0 m

TDH = 60 + 12 + 8 + 0 = 80 m

Now compare with the wrong method. If the buyer had used the well depth, he would have asked for an 80 m pump for the wrong reason — here the two numbers match only by accident. If he had used the static level (45 m + 12 m + 8 m = 65 m), he would have bought a pump 15 m too weak: once drawdown appears, the flow would drop clearly, and in the worst case the water would not reach the tank at all.

A Candidate Model for TDH = 80 m

Assume the farm needs 80 m³/h and the well casing is 250 mm. Checking our selection manual for models that cover this duty point, one candidate is:

250QJ80-80/4 — design flow 80 m³/h, design head 80 m, 4 stages, matched motor 30 kW (YQS type; the manual also lists 28 kW JQS and 30 kW YQS·T options), maximum unit outer diameter 233 mm.

First checks:

• Well diameter 250 mm ≥ minimum well diameter 250 mm, and the unit’s 233 mm outer diameter passes the casing. ✔
• Design head 80 m covers TDH 80 m. ✔
• Design flow 80 m³/h matches the demand. ✔

Please note: this is a candidate model, not the final answer. The real duty point of a pump is where the pump curve meets your system curve, and the manual’s design point is only one point on that curve. If your duty point falls between two models, or the margin looks tight, the final choice must be confirmed with our engineers against the full pump curve. If the same farm had a 200 mm well instead, the manual offers 200QJ80 series models, and our engineers would shortlist the stage counts that cover 80 m for confirmation in the same way.

Installation Limits You Must Also Check

Selecting a candidate for the right head is not enough. The pump must be installed within the limits in our manual:

1. The first-stage impeller must be at least 3 m below the dynamic water level. In our example, DWL is 60 m, so the first impeller must sit at 63 m depth or deeper.
2. The pump unit must not be immersed more than 70 m below the static water level. SWL is 45 m, so the unit must not go deeper than 45 + 70 = 115 m — no problem in an 80 m well, but this limit matters in deeper wells.
3. The bottom of the motor must be at least 5 m above the well bottom. The well is 80 m deep, so the motor bottom must be at 75 m depth or shallower.

These three limits set the boundaries for the installation depth. Whether one single setting satisfies all three at once depends on the actual length of the pump-and-motor unit, which is not part of the example data — so do not fix the installation depth from the well data alone. Give these limits, together with your measured water levels and well depth, to our engineers. They will set the installation depth against the actual unit dimensions of the confirmed model. Also remember the motor is water-filled: fill the motor cavity with distilled or clean water before installation, and never run it dry.

Summary

• Well depth tells you how deep the hole is. It does not tell you the pump head.
• Measure the dynamic water level at your required flow — that is where the pump starts lifting.
• TDH = dynamic water level + outlet height + friction losses + required outlet pressure. A design margin, if you want one, is added separately.
• Any model you pick from a table is a candidate — the final duty point is confirmed on the pump curve.
• Check the installation limits with our engineers: impeller 3 m below dynamic level, immersion within 70 m below static level, motor bottom 5 m above the well bottom.