Enter drainage area or measured inflow. Add lift and pipe details. Get recommended capacity and horsepower. See common pump class suggestions instantly for homes.
This tool sizes a pump by matching a required flow rate at a calculated Total Dynamic Head (TDH). You can enter a measured inflow directly, or estimate inflow from a contributing area and rainfall.
Always confirm your chosen model’s pump curve delivers the target flow at the computed TDH.
| Scenario | Design inflow (gpm) | Safety factor | Required flow (gpm) | Lift (ft) | TDH (ft) | Suggested motor |
|---|---|---|---|---|---|---|
| Seepage test result | 28 | 1.30 | 36.4 | 10 | ~16 | 1/3–1/2 HP |
| Area-based estimate | 45 | 1.40 | 63.0 | 14 | ~28 | 1/2–3/4 HP |
| High head discharge | 60 | 1.30 | 78.0 | 25 | ~45 | 3/4–1 HP |
Measure carefully, choose safely, and keep basements reliably dry.
A sump pump should keep up with peak inflow while operating within a realistic head range. An undersized unit cycles constantly, overheats, and allows water to rise. An oversized unit can short-cycle, wasting energy and stressing switches. For many basements, target flows commonly fall in the 20–80 gpm range, but site conditions vary widely.
If you can measure inflow, it is the most defensible input. A quick method is a timed container test from a drain or a flow meter on a temporary discharge line. Convert to gpm, then apply a safety factor (often 1.2–1.5) to cover uncertainty, seasonal changes, and partial blockages.
When measurements are unavailable, inflow can be approximated from contributing area and rainfall intensity. Example: 2,000 ft² at 1.5 in/hr with C=0.90 and η=0.90 yields about 25 gpm before safety factor. Smooth roof and slab surfaces generally have higher runoff than landscaped soil.
Use a local design storm intensity if you have it, or choose a conservative short-duration burst for critical spaces. Intensities near 1–3 in/hr are common in many regions for brief events; higher values can be used when your risk tolerance is low or flooding consequences are high.
Total Dynamic Head (TDH) is not just vertical lift. It also includes friction losses along the discharge pipe, losses through elbows, check valves, and reducers, plus any extra margin. A typical residential lift might be 8–15 ft, while TDH can rise above 30 ft when long runs and fittings are added.
Small discharge lines increase velocity, which increases both friction and minor losses. If your required flow is above ~60 gpm, a larger diameter often reduces TDH noticeably and can shift the recommendation to a smaller motor. This calculator uses Hazen–Williams and a K-factor approach to capture those effects.
After computing required flow and TDH, select a pump that delivers at least that flow at the TDH point on its published curve. Avoid choosing based on “maximum gph” alone, because that value is often at near-zero head. A realistic operating point is what protects you during peak events.
Consider a battery backup or secondary pump for high-risk basements. Keep the pit clear, verify the check valve, and test float switches. A small maintenance routine can prevent failures more effectively than additional horsepower.
A common range is 1.2–1.5. Use the higher end if inflow varies seasonally, debris is likely, or the space is critical. Avoid extreme factors unless you have strong evidence.
Smaller pipes increase water velocity, which increases friction and fitting losses. Higher losses increase TDH, so the pump must work harder to deliver the same flow.
New smooth plastic piping is often modeled around 130–150. Older or rougher piping can be lower. If you are unsure, choose a conservative value to avoid underestimating head loss.
Add K values for major fittings: elbows, tees, check valve, and outlet. A practical quick estimate for several bends plus a check valve is K≈4–10, then refine if you know your fitting list.
No. Capacity is flow at a given head. Horsepower relates to the energy required. Two pumps with the same horsepower can deliver very different flow depending on design and head.
Maximum flow is usually measured at very low head. Your installation has lift and losses, so the real operating flow is lower. Use the pump curve at your TDH instead.
If flooding risk is high, power outages are common, or you store valuables in the basement, a backup system is strongly recommended. A battery backup or secondary pump adds resilience during peak events.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.