Water Tank Size Calculator
Calculate water tank size from daily usage and storage days.
Returns required volume in gallons and liters for rainwater harvesting and off-grid water systems.
Garden water tank size is calculated based on your garden’s total daily water requirement and how many days of storage you need between rainfall events or municipal water top-ups.
The Formula:
Tank volume (liters) = Daily water use (liters) × Storage days
Daily water use (liters) = Garden area (m²) × Daily water depth (mm)
Daily Water Depth Reference:
| Season / Climate | Daily Water Need |
|---|---|
| Cool / humid climate | 2–3 mm/day |
| Temperate climate | 3–5 mm/day |
| Hot / dry climate | 5–8 mm/day |
| Drip irrigation (efficient) | 2–4 mm/day |
Note: 1 mm of water over 1 m² = 1 liter.
Worked Example:
Garden area: 50 m². Hot dry summer, needing 6 mm/day. Want 14 days of storage:
Daily use = 50 × 6 = 300 liters/day Tank size = 300 × 14 = 4,200 liters
Choose a 5,000-liter tank (next standard size up) to allow headroom.
Rainwater Harvesting Calculation:
Roof catchment yield (liters) = Roof area (m²) × Rainfall (mm) × 0.85 (runoff efficiency)
For a 100 m² roof after 20 mm of rain: 100 × 20 × 0.85 = 1,700 liters
Standard Tank Sizes (polyethylene):
500L, 1,000L, 2,500L, 5,000L, 10,000L, 25,000L
Practical Tips:
- Place tank on a flat, solid base — a full 1,000L tank weighs 1,000 kg
- First flush diverters prevent contaminated roof runoff from entering the tank
- Cover the tank completely — sunlight promotes algae growth inside
- Screen all inlets with mosquito mesh to prevent mosquito breeding
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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