Calculator Inputs
Use basin working depth, not full pit depth. Enter flow in gallons per minute.
Formula Used
How the calculator works:
First, it finds the working water volume inside the sump basin. Next, it adds incoming water to the pump-down flow needed to clear that stored water within your selected drawdown time.
Then it estimates total head using vertical lift and pipe friction. Finally, it suggests a minimum pump flow at that head and gives a general motor class range.
Always compare your final target against the manufacturer pump curve, because actual delivered flow falls as head rises.
How to Use This Calculator
- Choose the basin shape that matches your pit.
- Enter the working water dimensions, not the full pit size.
- Add your expected inflow rate in gallons per minute.
- Set the drawdown time you want after the pump starts.
- Enter lift, discharge length, fittings, and pipe diameter.
- Add a safety factor for dirty water, wear, and uncertainty.
- Review recommended flow, total head, cycling, and pump class.
- Select a real pump whose curve meets that flow at that head.
Example Data Table
| Basin Shape | Diameter / L × W | Active Depth | Inflow | Lift | Pipe Length | Pipe Dia. | Recommended Flow | TDH | Suggested Class |
|---|---|---|---|---|---|---|---|---|---|
| Round | 24 in | 18 in | 12 GPM | 10 ft | 30 ft | 1.5 in | 31.30 GPM | 11.90 ft | Typical 1/2 HP class |
| Rectangular | 30 × 24 in | 18 in | 10 GPM | 12 ft | 35 ft | 1.5 in | 35.64 GPM | 14.98 ft | Typical 1/2 HP class |
| Round | 30 in | 20 in | 16 GPM | 14 ft | 40 ft | 2.0 in | 39.59 GPM | 15.09 ft | Typical 1/2 HP class |
These rows are examples for planning. Final selection should always be checked against the actual pump performance curve.
FAQs
1) What does sump pump capacity mean?
It is the flow a pump can deliver over time, usually in gallons per minute, at a specific total dynamic head. Capacity always depends on lift and pipe losses.
2) Why is total dynamic head important?
A pump delivers less water as head increases. Head includes vertical lift, friction losses, and fitting losses. A pump that looks strong at zero head may underperform in the field.
3) Why add a safety factor?
Real jobs have uncertainty. Debris, aging equipment, rough piping, changing groundwater, and dirty water can reduce performance. A reserve margin helps maintain dependable drainage during peak events.
4) Can I size only from basin volume?
No. Basin volume affects pump-down time and cycling, but inflow matters too. If water enters quickly, the pump must handle both incoming water and the stored basin volume.
5) What if my cycles per hour are too high?
High cycling can shorten motor life. Increase usable basin volume, widen float travel, reduce inflow where possible, or choose controls that reduce rapid on-off operation.
6) Does a larger pipe help capacity?
Usually yes. A larger discharge pipe lowers friction loss, which reduces total head. Lower head can improve delivered flow when the pump curve supports it.
7) Is the suggested horsepower exact?
No. It is a planning guide only. Always use the manufacturer curve, impeller details, voltage, solids handling limits, and motor data before purchasing or specifying equipment.
8) Can this calculator be used for construction pits?
Yes, for preliminary planning. It can help with temporary dewatering estimates for pits, basements, crawlspaces, and utility works. Field conditions should still be verified on site.