Water Tank Size Calculator

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:

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

Enter 300 as the daily usage and 14 as the days of storage. That comes to 4,200 liters, and the calculator rounds up to the next size a manufacturer actually sells: a 5,000 liter tank, which buys you 16 days rather than 14.

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

The 0.85 is what is left after the first flush, evaporation off a warm roof, and what the gutters throw over the edge in a downpour. Enter your roof area above and the calculator works out how much rain it takes to fill the tank, which is the number that tells you whether the tank is sized for your climate or just for your garden.

Standard Tank Sizes

Polyethylene tanks are sold in fixed sizes, and these are the ones the calculator rounds up to:

• Metric: 500 L, 1,000 L, 2,500 L, 5,000 L, 10,000 L, 25,000 L
• Imperial: 50, 100, 250, 500, 1,000, 1,500, 2,500, 5,000 and 10,000 gallons

Practical Tips:

• Put the tank on a flat, solid base. Water weighs a kilogram a litre, so a full 1,000 L tank is a tonne sitting on four small feet, and a paving slab on soft ground will tilt within a season
• A first flush diverter throws away the first few litres off the roof, which carries most of the dirt, bird droppings and leaf debris
• Keep it covered. Sunlight through a translucent tank grows algae, and an uncovered inlet breeds mosquitoes within a fortnight
• Screen every inlet and the overflow with fine mesh, not just the top