Dewatering Pump Calculator

Calculator Inputs

Enter site values. Use decimals for better accuracy.

Excavation Length (m)

Plan view length of area to drain.

Excavation Width (m)

Plan view width of area to drain.

Average Water Depth (m)

Water depth to remove from the excavation.

Target Dewatering Time (hr)

Shorter time increases required flow.

Estimated Inflow / Seepage (m³/hr)

Add groundwater seepage or rainfall inflow.

Safety Factor (%)

Common range: 10–30%.

Vertical Lift (m)

Elevation from water surface to discharge point.

Discharge Pipe Length (m)

Total straight pipe length (approx.).

Pipe Inner Diameter (mm)

Diameter strongly affects friction losses.

Hazen–Williams C

Typical: 140 (PVC), 120 (steel), 100 (older pipe).

Allowance on Head (%)

Covers aging, bends, uncertainty, and site changes.

Additional K (custom)

Extra minor-loss factor if you know it.

90° Elbows (qty)

Typical K ≈ 0.9 each.

45° Elbows (qty)

Typical K ≈ 0.4 each.

Gate Valves Open (qty)

Typical K ≈ 0.15 each.

Check Valves (qty)

Typical K ≈ 2.0 each.

Tees Through (qty)

Typical K ≈ 0.6 each.

Pump Efficiency (%)

Use 55–75% for many site pumps.

Motor Efficiency (%)

Typical: 85–92%.

Reset

Tip: If TDH rises, check diameter and fittings first.

Example Data

Sample inputs and typical outputs for a medium excavation.

Case	Length (m)	Width (m)	Water Depth (m)	Time (hr)	Inflow (m³/hr)	Lift (m)	Pipe (mm / m)	Flow (m³/hr)	TDH (m)
Baseline	20	10	1.2	6	8	12	100 / 50	~56	~16–22
Fast drain	20	10	1.2	3	8	12	100 / 50	~104	~20–32
Larger pipe	20	10	1.2	6	8	12	150 / 50	~56	~14–18

Ranges reflect fitting counts and allowance settings. Always check manufacturer curves.

Formula Used

This calculator estimates flow, head, and power for site dewatering.

1) Required Flow

Volume = L × W × Depth
Qdraw = Volume / Time
Qbase = Qdraw + Qinflow
Qrequired = Qbase × (1 + Safety%)

2) Total Dynamic Head

TDH = Lift + hf + hm + Allowance
Allowance = (Lift + hf + hm) × Allowance%
hm = Ktotal × v² / (2g)

3) Friction Head (Hazen–Williams)

SI form with Q in m³/s, d in m, L in m.

hf = 10.67 × L × Q^1.852 / (C^1.852 × d^4.87)

4) Power

Phyd (kW) = g × Q × TDH (for water)
Pin = Phyd / (ηpump × ηmotor)

Power is an estimate. Add margin for solids, suction conditions, and wear.

How to Use This Calculator

A quick workflow for pump sizing in the field.

Measure excavation length, width, and average water depth.
Set the target dewatering time based on work sequence.
Estimate inflow from seepage, rainfall, or nearby sources.
Enter vertical lift to the discharge location or tank.
Add discharge pipe length and diameter, then select a C-value.
Count major fittings and valves to capture minor losses.
Use realistic efficiencies and a practical head allowance.
Press Calculate, then compare flow and head on pump curves.

Field note: For deep excavations, confirm suction limits, priming method, and NPSH. Consider staged pumping or wellpoints if inflow is high.

Dewatering Pump Planning Guide

Eight practical notes to interpret the calculator outputs on site.

1) Scope and deliverables

This calculator estimates a temporary dewatering duty by combining drained volume, inflow, and total dynamic head (TDH). Outputs include required flow (m³/hr, L/s, gpm), TDH (m, ft), and input power (kW, HP). Shortlist pumps with these numbers, then confirm selection on the vendor curve.

2) Volume and target time

Drained volume uses Volume = L × W × Depth. A 20 m × 10 m excavation with 1.2 m water depth holds 240 m³. If the target time is 6 hours, the drawdown component is 40 m³/hr before seepage. Halving the time roughly doubles the flow requirement.

3) Inflow plus safety margin

Inflow covers seepage, run-on, and rainfall. Estimate it from sump rise rate converted to m³/hr. A safety factor of 10–30% is typical when conditions are uncertain. Example: 40 m³/hr drawdown + 8 m³/hr inflow, with 20% safety, gives about 57.6 m³/hr required flow.

4) Pipe diameter and velocity

Velocity is computed from v = Q/A. Many temporary lines run smoothly at roughly 1.0–3.0 m/s. Higher velocities increase friction and coupling stress; very low velocities can allow fines to settle in long flat runs. If velocity is high, increase diameter or split the flow.

5) Friction head with Hazen–Williams

Friction uses the SI Hazen–Williams equation: hf = 10.67 × L × Q^1.852 / (C^1.852 × d^4.87). Typical C-values: about 140 for clean PVC, 120 for steel, and 100 for older rough pipe. Confirm internal diameter for layflat hose and reducers.

6) Fitting losses and custom K

Minor losses are estimated by hm = Ktotal × v²/(2g). Typical K values include 90° elbow ≈0.9, 45° elbow ≈0.4, and check valve ≈2.0. If you have manifolds, couplers, or complex routing, add a custom K to reflect extra loss.

7) Allowance, efficiency, and power

A 5–15% head allowance helps cover kinks, throttling, and performance drift. Hydraulic power for water is Phyd = g × Q × TDH, then input power is Pin = Phyd/(ηpump×ηmotor). For electrical planning, include motor starting current and site derating.

8) Field checks and operations

Verify drawdown time and discharge rate in the first hour. Keep suction lines short, airtight, and well-primed to avoid cavitation. Use strainers where practical to protect the impeller. If TDH is high, reduce lift, shorten the run, or stage pumps for reliability.

FAQs

Quick answers for field use and procurement checks.

1) What does TDH represent in this calculator?

TDH combines vertical lift, pipe friction, fitting losses, and a head allowance. It’s the head the pump must overcome at the required flow. Use TDH and flow together when reading a pump curve.

2) How do I estimate inflow if I have no data?

Measure sump rise over time, then convert to m³/hr. If you cannot measure, assume conservative inflow and increase the safety factor. Adjust later once the excavation reaches steady conditions.

3) Why is pipe diameter so important?

Friction rises sharply as diameter decreases. In Hazen–Williams, head loss is proportional to approximately d^-4.87. A modest diameter increase can significantly reduce TDH, power demand, and wear.

4) Can I use this for muddy water or slurry?

The equations assume clean water. Muddy water increases losses and reduces efficiency. Use a larger allowance, select a solids-handling pump, and confirm with supplier curves for expected solids content.

5) What safety factor should I choose?

Common choices are 10–30%. Use higher values for variable groundwater, storm exposure, or critical schedules. Use lower values when inflow is measured and stable, and when you have standby capacity.

6) Does the power estimate include generator and start-up effects?

No. The tool estimates steady input power only. For generators, include motor starting current, derating for heat/altitude, and other loads. Suppliers can recommend kVA based on the selected motor.

7) When should I consider multiple pumps?

Use multiple pumps for redundancy, long discharge runs, or staged lifts. Parallel pumps increase flow capacity; series arrangements increase head. Multiple smaller units can also simplify transport and maintenance.