Calculator Inputs
Example Data Table
| Item | Example Value | Notes |
|---|---|---|
| Pit size | 24 in × 24 in × 18 in | Working storage depth only, not full basin depth. |
| Refill time | 5 min | Represents how quickly seepage refills the active volume. |
| Desired drawdown time | 3 min | Shorter times need higher pump capacity. |
| Total dynamic head | 15.60 ft | Includes vertical lift, check valve loss, and pipe friction. |
| Recommended design flow | 13.99 gpm | Includes 25% safety factor on the governing flow. |
| Suggested motor size | 1/3 HP | Use actual manufacturer pump curves before final purchase. |
Formula Used
1) Pit storage volume
Pit Volume (cu ft) = (Length × Width × Working Water Depth) ÷ 1728 Pit Volume (gal) = Pit Volume (cu ft) × 7.48052This converts the active storage in the basin from cubic inches to cubic feet, then to gallons.
2) Refill-based inflow
Refill Inflow (gpm) = Pit Volume (gal) ÷ Refill Time (min)If you timed how fast the basin refills, this gives a field-based seepage estimate.
3) Required drawdown flow
Drawdown Flow (gpm) = Pit Volume (gal) ÷ Desired Drawdown Time (min)This is the flow needed to lower the water level over the chosen emptying period.
4) Equivalent discharge length and friction head
Equivalent Length (ft) = Horizontal Run + (Fittings × 5 ft) Friction Head (ft) = Equivalent Length ÷ 100 × Friction RateFittings add resistance. The friction rate is entered as feet of head lost per 100 feet of pipe.
5) Total dynamic head
TDH (ft) = Vertical Head + Check Valve Loss + Friction HeadThis combines lift and discharge losses into one sizing head value.
6) Final design flow
Base Required Flow = max(Effective Inflow, Drawdown Flow) Design Flow = Base Required Flow × (1 + Safety Factor ÷ 100)The larger governing flow is increased by the chosen reserve margin.
7) Horsepower and discharge velocity
Water HP = (Flow × TDH) ÷ 3960 Brake HP = Water HP ÷ Pump Efficiency Velocity (ft/s) = 0.4085 × Flow ÷ Diameter²These help estimate motor size and whether the chosen discharge pipe may be undersized.
How to Use This Calculator
- Measure the sump pit’s working length, width, and active water depth.
- Enter either a timed refill period, a manual inflow estimate, or both.
- Set the desired drawdown time that matches your acceptable pump run duration.
- Enter the discharge path details: vertical lift, horizontal run, fittings, friction rate, and check valve loss.
- Add a practical safety factor to cover uncertainty, seasonal peaks, and aging equipment.
- Review design flow, total head, brake horsepower, and motor recommendation together.
- Use the chart and notes to judge cycling frequency, velocity, and discharge layout quality.
- Confirm the final selection with actual manufacturer pump curves before buying equipment.
Frequently Asked Questions
1) What is a sump pump sizing calculator used for?
It estimates the pump flow, head, and motor range needed to remove water from a sump basin. It helps compare drainage demand with discharge losses and supports better planning before selecting a specific pump model.
2) Why does basin size matter?
Basin size controls how much water is stored between pump cycles. Larger active volume can reduce frequent starts, while smaller pits may cycle faster and increase wear if inflow remains high.
3) Should I use refill time or manual inflow?
Use both when possible. This calculator takes the higher inflow so the result stays conservative. Refill timing gives a field estimate, while manual inflow can reflect monitoring data or engineering assumptions.
4) What is total dynamic head?
Total dynamic head combines the vertical lift with pipe friction and check valve losses. A pump may move plenty of water at low head but much less at higher head, so this value matters greatly.
5) Why include a safety factor?
Actual conditions vary with season, sediment, aging components, and storm events. A safety factor adds reserve capacity so the chosen pump is less likely to struggle when inflow or resistance rises.
6) Is the motor recommendation the final pump choice?
No. It is a planning recommendation based on calculated horsepower. Final selection should always be verified against the manufacturer’s pump curve, shutoff head, solids handling, duty rating, and electrical compatibility.
7) What discharge velocity is reasonable?
Moderate velocity is usually preferred because very high velocity can increase noise, losses, and wear. If velocity is high, consider a larger discharge line or fewer fittings to improve system performance.
8) Can this calculator size backup pumps too?
Yes, as a planning tool. You can estimate the duty point for a backup unit, then compare run time, power source, and emergency capacity. For critical spaces, verify storage and outage-duration assumptions separately.