Dewatering Pump Size Calculator

Enter dewatering design inputs

Excavation length (m)

Plan length of the excavation or sump zone.

Excavation width (m)

Plan width of the area being lowered.

Initial water depth to remove (m)

Average water depth above the target working level.

Desired lowering time (h)

Hours allowed for the initial drawdown stage.

Steady inflow (m³/h)

Expected continuing seepage, recharge, or nuisance flow.

Safety factor

Use to cover uncertainty, silting, and wet weather.

Static lift (m)

Vertical lift from water level to discharge point.

Drawdown allowance (m)

Allowance for extra suction margin and uneven water level.

Discharge pipe length (m)

Equivalent pipe length used in head-loss calculation.

Pipe inside diameter (mm)

Inside diameter for each duty pump discharge line.

Hazen-Williams C

Typical temporary hose values often range near 120 to 150.

Minor loss coefficient K

Combined fitting, valve, bend, and outlet losses.

Pump efficiency (%)

Overall pump efficiency used for brake power.

Duty pumps in service

Number of pumps sharing the required duty flow.

Standby pumps

Backup pumps kept ready for outages or maintenance.

Formula used

1) Initial water volume
V = L × W × d

2) Initial drawdown rate
Q_drawdown = V / t

3) Required total flow
Q_required = (Q_drawdown + Q_inflow) × Safety Factor

4) Flow per duty pump
Q_pump = Q_required / Number of Duty Pumps

5) Friction head
h_f = 10.67 × L × Q^1.852 / (C^1.852 × d^4.8704)

6) Minor head loss
h_m = K × v² / (2g)

7) Total dynamic head
TDH = Static Lift + Drawdown Allowance + h_f + h_m

8) Brake power
P = (ρ × g × Q × TDH) / (1000 × η)

This calculator uses SI units. Hazen-Williams flow uses m³/s internally, with pipe diameter in meters and equivalent discharge length per pump line.

How to use this calculator

Enter the excavation length, width, and average water depth above the required dry working level.
Set the target time allowed for the first lowering stage.
Enter the steady inflow expected from seepage, groundwater recharge, or nuisance water sources.
Choose a safety factor to reflect site uncertainty and temporary works risk.
Fill in static lift, discharge line length, diameter, and loss values.
Enter pump efficiency and the number of duty and standby pumps.
Press Calculate Pump Size to place the results above the form.
Use the export buttons to save a CSV summary or a PDF report.

Example data table

Scenario	Length (m)	Width (m)	Water Depth (m)	Inflow (m³/h)	Static Lift (m)	Duty Pumps	Indicative Flow per Pump (m³/h)
Basement excavation	20	12	1.8	45	8	2	67.50
Utility trench sump	15	6	1.2	18	6	1	41.25
Foundation pit with standby	28	16	2.2	75	10	3	56.11

Frequently asked questions

1) What is a dewatering pump size calculation?

It estimates the pump flow, head, and power needed to lower groundwater or remove seepage from an excavation. A reliable estimate helps keep the working area dry, reduces delays, and supports safer temporary works planning.

2) Why is safety factor included?

Field conditions often differ from desk assumptions. Soil variability, storm inflow, silting, hose wear, and uneven recharge can all raise actual demand. A safety factor provides practical allowance so the installed system still performs during tougher site conditions.

3) What does total dynamic head mean?

Total dynamic head is the total head the pump must overcome. It includes vertical lift, drawdown allowance, pipe friction, and minor losses from fittings, valves, bends, and discharge arrangements.

4) How many pumps should I install?

At minimum, install the number of duty pumps required to carry the design flow. On active construction sites, one standby pump is commonly added so work can continue during maintenance, blockage, or sudden inflow changes.

5) Why does pipe diameter change pump sizing?

Smaller discharge pipes increase velocity and friction losses, which raises the required head. Larger pipes usually lower head loss, reduce wear, and may allow a more efficient pump selection, especially on longer discharge runs.

6) Is this calculator enough for final engineered design?

No. It is a practical sizing tool for planning and screening. Final design may require soil permeability testing, wellpoint layout checks, drawdown modeling, discharge permits, electrical review, and manufacturer pump curves.

7) What pump efficiency should I use?

Use realistic field efficiency for the pump and operating point, not a best-case catalog number. Temporary dewatering systems often perform below peak efficiency because of wear, suction conditions, partial blockage, and variable operating levels.

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

Only with caution. This calculator assumes water-like fluid properties. Heavy sediment, slurry, or abrasive solids can change friction losses, reduce efficiency, increase wear, and require a solids-handling pump with different performance data.