Home & Construction⏱ 4 min read
How to Calculate the Right Water Tank or Butt Size
Whether harvesting rainwater or storing emergency supply, here is how to calculate the volume you need and match it to standard tank sizes.
Water storage calculations arise in three scenarios: rainwater harvesting, emergency water storage, and off-grid supply. Each has different sizing logic.
Rainwater Harvesting
Annual yield = Catchment area x Annual rainfall x 0.85 (runoff coefficient)
Catchment 50m2, rainfall 900mm/year:
Yield = 50 x 0.9 x 0.85 = 38,250 litres/year
Garden demand (summer, 10m2 veg plot): 6,000 litres May-Sep
Recommended tank: 1,000-2,000 litres (captures winter rain for summer use)
Emergency Water Storage
NHS/FEMA guidance: 4.5 litres/person/day (drinking only)
Comfortable use (cooking, washing): 10-15 litres/person/day
Family of 4, 3-day emergency: 4 x 12 x 3 = 144 litres
2-week supply: 4 x 12 x 14 = 672 litres
Standard 1,000-litre IBC tank: covers family of 4 for 3+ weeks comfortably
Tank Volume Formulas
Rectangular: L x W x H (metres) x 1,000 = litres
Cylindrical: pi x r^2 x H x 1,000
Example: 1.2m diameter, 1.5m tall:
3.14159 x 0.36 x 1.5 x 1000 = 1,696 litres
Standard sizes: butts 100-250L, slimline 350-700L, IBC 1,000L, underground 1,500-10,000L