Metric Unit Calculator
The formula, your substituted values, and the evaluated result are shown below.
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View offerAffiliate link — we may earn a commission at no extra cost to you.Calculate rainwater tank capacity for a site works construction scenario using clearly labelled project inputs.
dailyDemand * storageDays / 1000
Formula: dailyDemand * storageDays / 1000. Keep every input in the stated units. This result is a quantity-planning estimate; verify site measurements, waste factors, product yields, supplier data, applicable codes, and professional design requirements before construction.
Calculate rainwater tank capacity for a site works construction scenario using clearly labelled project inputs.
This calculator sizes the storage you need to keep supplying water through a dry spell:
Storage (m³) = daily demand (L/day) × days of storage ÷ 1,000
Multiply the answer by 1,000 to get litres, which is how tanks are sold.
A tank is only useful if your roof can fill it. The yearly harvest is:
Litres per year ≈ roof area (m²) × annual rainfall (mm) × 0.8
The 0.8 allows for first-flush diversion, evaporation and overflow. A 150 m² roof in an area with 600 mm of annual rain gives about 150 × 600 × 0.8 = 72,000 L a year, about 200 L a day on average. That supports the 300 L/day example only in wetter months, so the tank will draw down in summer. A local rainfall history helps balance the two.
| Use | Rough daily use |
|---|---|
| Toilet (dual flush, 4 people) | 80–120 L |
| Washing machine | 50–100 L |
| Garden watering | 100–500 L (seasonal) |
| Whole house (if tank is the only supply) | 150–200 L per person |
Multiply the water you'll draw each day by the longest dry spell you want to cover. For 300 L/day over 60 days, that's 18,000 L.
Roughly roof area × rainfall (mm) × 0.8. Each millimetre of rain on 100 m² of roof yields about 80 L.
Not if your roof can't fill it. Size the tank to both your demand and your roof's catchment.