| Scenario | Area | Depth / Rain | Removal time | Lift | Hose | Typical outcome |
|---|---|---|---|---|---|---|
| Small patio cover | 10 m² | 12 mm depth | 1.5 h | 1 m | 6 m, 25 mm | ~14–18 L/min, low head |
| Raised bed tarp | 22 m² | Rain 8 mm/hr for 3 h | 2 h | 2 m | 10 m, 19 mm | ~50–70 L/min, moderate friction |
| Large garden canopy | 45 m² | 20 mm depth | 2 h | 3 m | 15 m, 25 mm | ~170–220 L/min, check power |
- Volume: area × depth. Metric shortcut: 1 m² × 1 mm = 1 liter.
- Required flow: Q = Volume ÷ Removal time, then add safety margin.
- Hose friction (Hazen‑Williams): computes head loss from flow, hose length, diameter, and C value.
- Total Dynamic Head: TDH = static lift + friction head + minor-loss allowance.
- Power: P = ρgQH (metric) or WaterHP = Q×H/3960 (imperial), adjusted by efficiency.
- Pick your unit system, then enter cover area.
- Enter measured water depth, or switch to rainfall mode.
- Set a realistic removal time based on your needs.
- Add static lift and hose details from your discharge route.
- Choose a hose C preset, then set efficiency and margins.
- Press Calculate, then match a pump curve at the shown TDH.
A cover pump is selected on two numbers: flow rate and total dynamic head (TDH). If flow is too low, water remains on the cover longer, increasing sag risk and debris load. If head is underestimated, the pump may run continuously but still fail to move water. This tool combines volume, time, and head components so the selected pump performs under real garden conditions.
Volume is derived from cover area and water depth. In metric sizing, 1 m² at 1 mm equals 1 liter, making quick checks easy. For example, 22 m² with 15 mm represents 330 liters. If you want removal within 2 hours, the base requirement is 165 L/h, or 2.75 L/min, before margins. Shorter target times raise flow proportionally and should be matched to the pump’s published curve.
TDH includes the vertical lift plus hose friction and a minor-loss allowance for bends, fittings, and outlet restrictions. Friction rises sharply as diameter decreases and as flow increases. A 19 mm hose can create noticeably more loss than a 25 mm hose at the same flow. By selecting hose material roughness (C value) and adding a minor-loss percentage, the calculator approximates field routing more closely than lift-only rules.
Hydraulic power depends on water density, gravity, flow, and head. The motor power shown adjusts that hydraulic demand by pump efficiency, which varies widely for small units. Typical compact pumps often operate around 35–60% efficient. If efficiency is set too high, motor power will be underestimated. Use the result to choose a pump with a comfortable electrical rating and to anticipate running cost and heat buildup during long cycles.
After calculating, confirm the pump can deliver the required flow at the stated TDH, not just at “zero head.” Prefer a larger hose diameter when practical, keep the discharge path straight, and place the outlet to avoid backflow. A 10–20% safety margin helps cover partially blocked screens, aging hoses, and small leaks. If your calculated TDH exceeds common cover pump limits, a transfer-style pump may be more reliable.
1) Should I size to the pump’s maximum GPH rating?
No. Maximum ratings are usually at near-zero head. Always match the pump curve at your TDH, because lift and hose friction can reduce real flow substantially.
2) How do I estimate depth if I only know rainfall?
Use rainfall mode: depth equals intensity multiplied by storm duration. This gives a practical estimate for sizing when you cannot measure water depth directly on the cover.
3) What hose diameter is best for faster draining?
Larger diameters reduce friction dramatically, especially at higher flows. If your pump supports it, moving from 19 mm to 25 mm can lower TDH and improve delivered flow.
4) Why add minor losses and a safety margin?
Bends, fittings, check valves, and debris create extra resistance. Minor losses and a safety margin account for these realities and keep performance stable as conditions change.
5) What efficiency value should I enter?
If you do not know, use 50–55% for compact electric pumps. Use a lower value for small, inexpensive units or when pumping dirty water through restrictive hoses.
6) My calculated head is high. What should I do?
Reduce head by shortening hose, increasing diameter, removing sharp bends, or lowering the discharge point. If TDH remains high, consider a higher-head transfer pump.