Calculator
Example data table
| Scenario | Daily demand | Reserve days | Rain harvest | Suggested capacity |
|---|---|---|---|---|
| Small raised beds | 80 L/day | 5 | 0 L/day | 500–750 L |
| Medium lawn and shrubs | 220 L/day | 7 | 20 L/day | 1,500–2,000 L |
| Large kitchen garden | 500 L/day | 10 | 60 L/day | 5,000–7,500 L |
Formula used
- Garden demand (area method): Demand(L/day) = Area(m²) × Depth(mm/day) ÷ Efficiency.
- Rain capture (optional): Harvest(L/day) = Catchment(m²) × Rain(mm/day) × Runoff coefficient.
- Net deficit: Net(L/day) = max(0, Demand − Harvest).
- Usable storage: Usable(L) = Net × Reserve days.
- Add safety: Usable+Safety = Usable × (1 + Safety%).
- Allow dead storage: Liquid total = (Usable+Safety) ÷ (1 − Dead%).
- Allow freeboard: Nominal tank = Liquid total ÷ (1 − Freeboard%).
- Cylinder volume: V = π × (D²/4) × H.
- Rectangular volume: V = L × W × H.
How to use this calculator
- Choose a demand method: direct liters/day or area-based watering depth.
- Set reserve days to match refill frequency and dry spells.
- Optionally add rain capture to reduce the net deficit.
- Adjust safety, dead storage, and freeboard for realism.
- Select a tank shape to get practical dimension suggestions.
- Press Submit and download CSV or PDF if needed.
Demand-driven sizing for irrigation cycles
Size tanks from peak-day irrigation, not weekly averages. For area-based input, multiply garden area by watering depth in millimeters; one millimeter over one square meter equals one liter. Divide by efficiency to reflect leaks, wind drift, and uneven emitters. Multiply net daily deficit by reserve days to cover pump failures or delivery delays, then add safety margin for hotter spells and crop changes during midseason growth.
Rainwater capture contribution and runoff losses
If you harvest rainwater, subtract expected capture from demand before sizing. Capture per day equals catchment area times rainfall depth times a runoff coefficient. Smooth metal roofs often achieve 0.85–0.95, while rough surfaces and first-flush diverters reduce yield. Use local average rainfall or conservative dry-season values. Even small roofs can offset hand-watering for containers and seed beds. For planning, treat gutters, filters, and overflow as constraints.
Reliability factors: safety, dead storage, freeboard
Operational allowances protect usable water. Safety factor covers forecasting error and occasional extra watering; many gardens use 10–25%. Dead storage accounts for sludge, pump pickup height, and outlet placement; 3–8% is common for small tanks. Freeboard leaves headspace for inlet surges and wave action; 5–15% helps prevent overflow. The calculator applies these as divisors, not simple additions. Higher percentages improve resilience, but increase footprint and budget.
Shape selection and dimension checks
Dimension suggestions help you check space and access. A vertical cylinder suits tight footprints and stable platforms, while a horizontal cylinder spreads weight and can fit under sheds. Rectangular tanks are easy to build but may need bracing. For cylinders, volume equals pi times diameter squared over four times height. Compare the suggested diameter or length with doorways, maintenance clearances, and base pad size before purchase.
Cost planning and staged expansion
Cost varies by material, fittings, and installation. If you know a typical price per liter, the calculator multiplies it by the nominal capacity so you can compare options quickly. Use the standard-capacity recommendation for shopping, because catalog tanks come in steps. When budgets are tight, stage expansion: start with a smaller tank, add a second tank in parallel, and share filtration as planting areas increase seasonally.
FAQs
What reserve days should I choose?
Choose reserve days based on refill access and risk. Five to seven days suits many gardens with regular supply. Ten to fourteen days is safer for remote plots, heat waves, or unreliable pumps.
Should I size for summer peak or yearly average?
Always size for the highest expected daily demand. Peak summer watering prevents crop stress and avoids emergency refills. If you irrigate seasonally, you can use separate peak and off-season scenarios.
How do I estimate irrigation efficiency?
Efficiency reflects how much applied water reaches roots. Drip systems may be 80–95% when maintained. Sprinklers can be 60–80% depending on wind and overlap. Use a conservative value if you are unsure.
What runoff coefficient should I use?
For smooth roofs, 0.85–0.95 is typical. For rough surfaces or systems with first-flush diversion, use 0.70–0.85. Keep it lower when gutters leak, screens clog, or debris causes losses.
Why is the recommended standard capacity larger?
Nominal capacity includes safety factor, then adjusts for dead storage and freeboard. These allowances reduce usable water, so the tank must be larger than the purely usable volume to deliver the reserve days you selected.
Can I connect two tanks together?
Yes. Two tanks in parallel increase capacity and improve flexibility. Use balanced plumbing, isolation valves, and overflow routing. Ensure the foundation can support full weight, and keep a common filter and screen to simplify maintenance.