Inputs
Results
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Required Flow at Inflow
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Total Dynamic Head
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Recommended Pump Flow (with Safety Factor)
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Estimated Runtime Per Cycle
Detailed Breakdown
| Item | Value | Units | Notes |
|---|
Formula Used
Runoff inflow Qr = C × i × A.
- Imperial: Qr (gpm) = C × Aft² × iin/hr × 7.48052 / 720.
- Metric: Qr (L/min) = C × Am² × imm/hr / 60.
Total dynamic head TDH = Hs + hf.
- Hazen–Williams friction: hf (ft) = 4.52 × L × Q1.85 / (C1.85 × d4.87), using L in ft, Q in gpm, diameter d in inches.
- Fitting equivalent length approximation: Leq = 30D per 90°, 20D per 45°, 75D per check valve; D is inside diameter.
How to Use This Calculator
- Select units and enter catchment area, runoff coefficient, and storm intensity.
- Add extra inflows for drains, seepage, or appliances if applicable.
- Enter discharge height, pipe details, and expected fittings.
- Provide basin geometry and on/off difference for runtime estimation.
- Click Calculate to see required flow, TDH, and recommendations.
- Export a CSV record or print to PDF for documentation.
Example Data
| Scenario | Area | Coefficient | Intensity | Extra Inflow | Static Lift | Pipe | Result: Flow / TDH |
|---|---|---|---|---|---|---|---|
| Detached garage roof | 600 ft² | 0.95 | 1.5 in/hr | 3 gpm | 10 ft | 1.5 in, 45 ft PVC | ~11 gpm / ~15 ft |
| Basement perimeter drains | 80 m² | 0.60 | 30 mm/hr | 15 L/min | 3 m | 40 mm, 18 m PVC | ~44 L/min / ~6 m |
| Heavy storm combined | 1500 ft² | 0.90 | 2.0 in/hr | 6 gpm | 12 ft | 2.0 in, 55 ft PVC | ~30 gpm / ~20 ft |
Use these as references. Your site conditions may differ significantly.
FAQs
Use your local design storm intensity from rainfall maps or codes, typically a short-duration event (e.g., 15–60 minutes) for the chosen return period.
It is an accepted approximation for water in pressurized pipes. Use an appropriate C-factor for your material and age, or consider detailed hydraulic analysis if needed.
Common practice ranges from 1.15 to 1.50 depending on uncertainty, clogging potential, and consequences of overflow. The default 1.25 balances efficiency and resilience.
Basin volume between on/off levels should store inflow during pump lag. Larger volumes reduce short-cycling and wear. This tool estimates runtime per cycle based on your geometry.
Select a pump whose performance curve exceeds the recommended flow at the calculated TDH. Verify with manufacturer curves at your specific head.
Yes, consider battery backups or a secondary pump for redundancy, especially where flooding consequences are severe or power outages are common.
Reference: Typical Runoff Coefficients (C-values)
| Surface | C Range | Notes |
|---|---|---|
| Roofs (impervious) | 0.90 – 1.00 | Metal, tile, shingles; clean drainage. |
| Concrete / Asphalt | 0.85 – 0.95 | Driveways, patios, parking areas. |
| Pavers (tight joints) | 0.70 – 0.85 | Limited infiltration through joints. |
| Gravel surfaces | 0.25 – 0.70 | Varies with compaction and fines. |
| Lawn, sandy, flat | 0.05 – 0.20 | Good infiltration; low runoff. |
| Lawn, clay or steep | 0.30 – 0.50 | Poor infiltration or slope increases runoff. |
Choose values reflecting your site’s soil, slope, and surface condition.
Reference: Pipe Diameter vs Practical Flow (2–8 ft/s)
| Nominal Diameter | ~Flow @2 ft/s | ~Flow @8 ft/s |
|---|---|---|
| 1.25 in | 7.6 gpm (29.0 L/min) | 30.6 gpm (115.8 L/min) |
| 1.50 in | 11.0 gpm (41.7 L/min) | 44.1 gpm (166.8 L/min) |
| 2.00 in | 19.6 gpm (74.1 L/min) | 78.3 gpm (296.5 L/min) |
| 2.50 in | 30.6 gpm (115.8 L/min) | 122.4 gpm (463.3 L/min) |
| 3.00 in | 44.1 gpm (166.8 L/min) | 176.3 gpm (667.2 L/min) |
Select velocities that balance friction losses, noise, and scouring.