What This Solves
Calculates sump pump capacity requirements, pit sizing, cycle frequency, and total dynamic head for basement and crawlspace dewatering.
Best Used When
- You need to select a sump pump capacity based on groundwater inflow rate
- You want to size the sump pit to achieve an appropriate cycle frequency
- You are calculating total dynamic head (TDH) to select a pump from manufacturer curves
Do NOT Use When
- You need to design the foundation drain system that feeds the sump — Use Foundation Drain Calculator
- You need to design a French drain for yard drainage rather than foundation dewatering — Use French Drain Calculator
Key Assumptions
- Groundwater inflow rate is constant (steady-state design condition)
- Total dynamic head includes static lift, pipe friction losses, and fitting losses
- Pump capacity is selected at the operating point on the pump curve (not maximum capacity)
- Cycle frequency is calculated from pit volume and inflow rate to avoid excessive motor cycling
- Check valve prevents backflow when the pump shuts off
Input Quality Notes
Inflow rate is the most uncertain input. Use foundation drain calculations or field measurements during wet season. Size the pump with a safety factor of at least 1.5 above calculated inflow.
Size a sump pump for basement or foundation drainage. Enter your inflow rate, discharge lift and pit dimensions to get the required pump capacity in GPM, the total dynamic head the pump must overcome, and a check that the cycle time protects the motor.
Calculate Sump Pump Requirements
For educational purposes only. Not a substitute for professional engineering judgment.
Sump Pump Design Overview
Sump pump design balances pump capacity against inflow rate while ensuring adequate cycle time to protect the pump motor. Storage volume in the pit allows the pump to cycle on and off rather than run continuously.
- Pump Capacity - Must exceed inflow rate with safety factor
- Total Dynamic Head - Static lift plus friction losses
- Cycle Time - Minimum 3 minutes to protect motor
- Duty Cycle - Target less than 50% for longevity
Typical Inflow Rates (GPM)
| Source | Min | Typical | Max |
|---|---|---|---|
| Foundation Drain | 2 | 10 | 30 |
| Groundwater Infiltration | 5 | 20 | 50 |
| Floor Drain | 1 | 5 | 15 |
| Combined Sources | 5 | 15 | 40 |
| Laundry Discharge | 5 | 15 | 25 |
Source: Engineering practice, IPC guidelines
Pump Type Characteristics
| Pump Type | Typical GPM | Max Head (ft) | Description |
|---|---|---|---|
| Pedestal Pump | 40 | 20 | Motor above pit, pump below |
| Submersible Pump | 50 | 25 | Motor and pump submerged in pit |
| Battery Backup Pump | 25 | 15 | DC pump with battery power |
| Water-Powered Backup | 15 | 10 | Uses water pressure, no electricity |
| Combination (Primary + Backup) | 50 | 25 | Primary AC with battery backup |
Source: Manufacturer specifications, engineering practice
Minimum Recommended Pit Sizes
| Pump Type | Min Diameter (in) | Min Depth (in) |
|---|---|---|
| Pedestal Pump | 18 | 24 |
| Submersible Pump | 18 | 24 |
| Battery Backup Pump | 14 | 18 |
| Water-Powered Backup | 14 | 18 |
| Combination (Primary + Backup) | 24 | 30 |
Source: IPC 2021, manufacturer recommendations
How sump pump sizing works
Sump pump design balances capacity against inflow while keeping the pump cycle long enough to protect the motor. The calculator works through five relationships, all referenced to the International Plumbing Code (IPC 2021) and Mays, Water Resources Engineering (2011).
- Friction head loss (Hazen-Williams): losses in the discharge pipe are
estimated as
h_f = 10.67 · L · Q^1.852 / (C^1.852 · D^4.87), with flow Q in GPM, equivalent length L in feet, internal diameter D in inches and roughness coefficient C from the material. Elbows, check valves and other fittings are added to L as equivalent lengths of straight pipe. - Total dynamic head:
TDH = h_static + h_f— the vertical lift from the pit to the discharge outlet plus the friction loss above. This is the head at which you must read the pump's published flow. - Required capacity:
Q_pump = Q_inflow × SF, where SF is a safety factor (default 1.25) covering pump wear and peak flows. - Active storage & cycle: the storage between the pump-on and pump-off
levels is
V = π · D² · h / (4 · 231)gallons (231 in³ per gallon). The cycle time is the fill time plus the pump-down time,t_cycle = V / Q_in + V / (Q_pump − Q_in). - Duty cycle:
Duty = t_pump / t_cycle × 100%. Targets are a cycle time of at least the minimum (default 3 minutes) and a duty cycle below 50%.
Variables: Q = flow (GPM / L·min⁻¹), L = equivalent pipe length, D = diameter, C = Hazen-Williams coefficient, h_static = static lift, h_f = friction loss, SF = safety factor, V = active storage volume.
Discharge friction reference
The Hazen-Williams roughness coefficient C depends on pipe material, and each
fitting adds an equivalent length of straight pipe to the friction calculation. These are the
exact values this calculator uses for residential discharge lines (roughly 1.5–2 in pipe).
| Pipe material | Hazen-Williams C |
|---|---|
| PVC | 150 |
| ABS | 150 |
| Galvanized steel | 120 |
| Cast iron | 100 |
A higher C means a smoother pipe and less friction loss.
| Fitting | Equivalent length |
|---|---|
| 90° elbow | 5.0 ft |
| 45° elbow | 2.5 ft |
| Check valve | 10.0 ft |
| Gate valve | 1.0 ft |
| Ball valve | 3.0 ft |
Source: IPC 2021 Appendix E; Mays (2011).
Frequently asked questions
How do I size a sump pump?
Size a sump pump on two axes: flow and head. First, the pump capacity must exceed the peak inflow rate into the pit, typically with a safety factor of about 1.25 to 1.5 to allow for pump wear and peak storm flows. Second, the pump must deliver that flow against the total dynamic head (TDH) — the vertical lift from the pit to the discharge point plus friction losses in the discharge pipe and fittings. Pick a pump whose performance curve meets or exceeds the required GPM at the calculated TDH.
What is total dynamic head (TDH) on a sump pump?
Total dynamic head is the total resistance the pump must overcome, expressed as a height of water. It equals the static head (the vertical rise from the pump to the discharge outlet) plus the friction head loss in the discharge pipe and fittings. Pump curves are published as flow vs. head, so you must read the pump's capacity at the actual TDH — not its free-flow rating. Friction loss here is estimated with the Hazen-Williams equation, and fittings such as elbows and check valves are added as equivalent lengths of pipe.
Why does sump pump cycle time and duty cycle matter?
A pump that switches on and off too rapidly (short cycling) can overheat and burn out its motor. A minimum cycle time of roughly 3 minutes is a common rule of thumb for motor protection, and it is governed by the active storage volume between the pump-on and pump-off float levels relative to the inflow rate. Duty cycle — the share of time the pump runs — should generally stay below about 50% so the motor has time to cool and so there is reserve capacity for heavier inflow. If the cycle is too short, increase the pit diameter or widen the on/off level spread rather than just buying a bigger pump.
What size sump pit do I need?
A larger pit holds more active storage, which lengthens the pump cycle and protects the motor. As a baseline, an 18 in (450 mm) diameter by 24 in (600 mm) deep pit suits most residential pedestal and submersible pumps; battery-backup units can use 14 in, while combination primary-plus-backup systems typically want 24 in diameter by 30 in deep so two pumps fit. The calculator solves for the minimum diameter needed to meet your target cycle time and compares it against these manufacturer minimums.
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Last verified: February 2026