TryBuildCalcSump Pump Sizing Calculator Dewatering pump capacity and count estimator
Calculate the pump capacity and count needed for excavation dewatering.
🕒 Last updated: July 30, 2026
Excavation Footprint
ℹ️Length of the excavation base.
ℹ️Width of the excavation base.
ℹ️Set to more than 1 if several identical pits share this same footprint and depth.
Depth & Water Table
ℹ️Total depth from existing ground level to the base of the excavation.
ℹ️Depth from ground level to the natural groundwater level — confirm on site, since it can be shallower or more seasonal than a regional assumption.
Seepage Estimate
ℹ️A rough planning-stage seepage estimate — confirm with a pumping test or geotechnical report for anything beyond a small, low-risk excavation.
Pump Sizing
ℹ️Rated discharge capacity of a single pump you plan to use, from its spec sheet.
ℹ️Common minimum is 1 standby pump equal to the largest duty pump — increase for higher-consequence sites.
ℹ️Groundwater seepage is continuous, so most sites pump 24 hours/day — reduce only if a sump buffer allows controlled intermittent pumping.
ℹ️How long the excavation needs to stay dewatered — typically tied to the foundation/structure work duration.
Cost
With 4.00 m of excavation below the water table, you need to remove approximately 2,400.00 m³ (84,755.3 cft) of standing water initially, then keep pumping 15.00 LPS of ongoing seepage using 4 pump(s) (3 duty + 1 standby).
Standing Water
Depth below water table: 4.00 m
Initial volume to clear: 2,400.00 m³ (84,755.3 cft)
Initial pump-down time: 44.4 hours
Seepage & Pumps
Method: Sandy Soil / High Permeability
Seepage rate: 15.00 LPS (54.0 m³/hr, 238 GPM)
Duty pumps: 3 + Standby: 1
Total pumps on site: 4
Ongoing Operation
Operating hours/day: 24
Duration: 30 days
Daily pumped volume: 1,296.00 m³
Total water over duration: 41,280.00 m³ (1,457,790.8 cft)
Assumptions Used
Initial volume = area × depth below water table | Seepage rate is a rough planning estimate by soil permeability class (or your measured/custom rate) — not a substitute for a pumping test or geotechnical assessment | Duty pumps sized with 1 unit of margin (round-up), standby pumps added on top for redundancy | Cost excludes mobilization, discharge permits, and site-specific power supply.
Approximate results for planning only. Verify with a professional.
Dewatering Cross-Section
Looking for the verification checklist, reference tables, or tips and mistakes?See the complete Dewatering Calculator.
Sump pump sizing estimate
This page is pump-sizing focused, using a larger 30 m x 20 m excavation (600 m²) with a shallower 1 m water table and the sandy soil/high permeability preset (2.5 LPS per 100 m²) — a combination that produces meaningfully higher seepage than the general default example. At 4 m below the water table, initial volume is 2400 m³, and estimated seepage is 15 LPS (600 x 2.5 ÷ 100).
With a 5 LPS pump capacity, that 15 LPS seepage rate needs 3 duty pumps (ceil(15 ÷ 5) = 3), plus 1 standby pump — 4 pumps total, with 15 LPS of duty capacity. Initial pump-down takes 44.44 hours, and ongoing seepage at full 24-hour operation over 30 days totals 38,880 m³ — illustrating how sizing scales once seepage genuinely exceeds a single pump's capacity.
- 600 m² footprint, sandy soil preset, 4 m below water table → 2400 m³ initial volume.
- Seepage: 15 LPS → needs 3 duty pumps (5 LPS each) + 1 standby = 4 pumps total.
- Pump-down: 44.44 hours. Ongoing: 38,880 m³ over 30 days.
How Is Dewatering Calculated?
The calculation happens in stages — excavation area, standing water volume, seepage rate, pump sizing — then optional cost on top.
Step 1 — Excavation Area
Area = Length × Width
Total Area = Area × Number of Identical Sections
If sections differ in size: Total Area = Sum of (Length × Width × Count) for every section
The base area is what every downstream volume and seepage figure is built on. Number of Identical Sections multiplies the whole result for repeated pits of the same size; for pits of different sizes, switch on "Multiple Different-Sized Sections" and the calculator sums every row's area into one combined total automatically.
Step 2 — Depth Below Water Table & Initial Volume
Depth Below Water Table = MAX(0, Excavation Depth − Water Table Depth)
Initial Volume = Total Area × Depth Below Water Table
If the excavation doesn't reach the water table, this is zero and no dewatering is needed. Otherwise, the initial volume is the standing groundwater that floods in and needs to be pumped out once before work can begin.
Step 3 — Seepage Rate
Preset: Seepage Rate = Total Area × Permeability Rate (per 100 m²) ÷ 100
Custom: Seepage Rate = your measured/estimated rate, entered directly
The soil-permeability presets are rough, order-of-magnitude planning figures — real seepage depends on hydraulic gradient and soil layering that a simple area-based rule can't fully capture. Switch to "Custom / Measured Rate" once you have an actual pumping test or geotechnical report figure.
Step 4 — Pump Sizing
Duty Pumps = ROUND UP(Seepage Rate ÷ Pump Capacity)
Total Pumps = Duty Pumps + Standby Pumps
Duty pumps are sized to handle the full seepage rate with rounding up for margin. Standby pumps are added on top for redundancy — a duty pump failure during active dewatering can re-flood the excavation, so standby capacity is standard practice, not optional.
Step 5 — Pump-Down Time & Ongoing Operation
Initial Pump-Down Time = Initial Volume ÷ (Duty Pumps × Pump Capacity)
Daily Volume = Seepage Rate × Operating Hours per Day
Total Water Over Duration = Initial Volume + (Daily Volume × Duration)
Initial pump-down time is how long it takes to clear the standing water already in the excavation using only the duty pumps. After that, the same duty pumps keep running to handle ongoing seepage for the full dewatering duration.
Step 6 — Cost & Contingency (Optional)
Rental Cost = Total Pumps × Rental Rate per Pump per Day × Duration
Fuel Cost = Duty Pumps × Fuel Rate per Pump per Day × Duration
Total Cost = (Rental Cost + Fuel Cost) × (1 + Contingency % ÷ 100)
Rental cost applies to every pump on site (duty and standby both need to be rented and ready), while fuel/power cost applies only to the duty pumps actually running. Contingency is an optional buffer on top for unknowns — a higher-than-estimated seepage rate or schedule delays.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section. Each table shows the calculation, the values substituted in, and the result it produces.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Excavation footprint | 30 m × 20 m, 1 section(s) | Sets the base area every volume/seepage figure is built on |
| Depth / water table | 5 m excavation, 1 m water table | Sets depth below water table and initial volume |
| Seepage estimate | Sandy Soil / High Permeability | Sets the ongoing inflow rate to be pumped |
| Pumps & operation | 5 LPS/pump, 1 standby, 24 hrs/day, 30 days | Sets duty/standby pump count, run-time, and duration |
Step 1 — Excavation Area
| Calculation | Substitution | Result |
|---|---|---|
| Area | 30 m × 20 m | 600.000 m² |
| Total area | 600.00 × 1 | 600.000 m² (6,458.34 sqft) |
Step 2 — Depth Below Water Table & Initial Volume
| Calculation | Substitution | Result |
|---|---|---|
| Depth below water table | 5.00 − 1.00 | 4.00 m |
| Initial volume | 600.00 m² × 4.00 m | 2,400.00 m³ (84,755.3 cft) |
Step 3 — Seepage Rate
| Calculation | Substitution | Result |
|---|---|---|
| Seepage rate | 600.00 m² × rate ÷ 100 | 15.00 LPS (54.0 m³/hr, 238 GPM) |
Step 4 — Pump Sizing
| Calculation | Substitution | Result |
|---|---|---|
| Duty pumps | ROUND UP(15.00 ÷ 5.00) | 3 |
| Total pumps | 3 duty + 1 standby | 4 |
Step 5 — Pump-Down Time & Ongoing Operation
| Calculation | Substitution | Result |
|---|---|---|
| Initial pump-down time | 2,400.00 m³ ÷ (3 × 5.00 LPS) | 44.4 hours |
| Daily volume | 15.00 LPS × 24 hrs | 1,296.00 m³/day |
| Total water over duration | 2,400.00 + (1,296.00 × 30) | 41,280.00 m³ (1,457,790.8 cft) |
Step 6 — Cost & Contingency
| Calculation | Substitution | Result |
|---|---|---|
| Enable cost estimation above to see rental and fuel cost | — | |
Therefore, this excavation needs approximately 2,400.00 m³ pumped out initially, then 15.00 LPS of ongoing seepage handled by 4 pump(s).
This page keeps things focused on the calculation above. For the full construction guide — verification checklist, reference tables, usage steps, tips, common mistakes, and limitations.See the complete Dewatering Calculator.