Cistern / Storage Reserve Days and Required Volume
The reserve a storage tank carries and the tank a target reserve needs, a straight mass balance: days = usable storage / daily demand, and required volume = daily demand x target days. A 2,500-gal usable cistern at 150 gpd carries 16.7 days; banking a 30-day dry spell needs 4,500 gal. Use USABLE storage (above the pump intake and below overflow, past any fire or first-flush reserve) and a realistic peak demand, not the annual average. For a rain-fed cistern the reserve must bridge the longest dry spell between refills; for a hauled or well-fed tank it sets the refill interval. A planning estimate; local rainfall or the source yield, the storage detail, and the AHJ (for potable use) govern.
Formula and source
reserve_days = usable_storage_gal / daily_demand_gpd; required_gal_for_target = daily_demand_gpd x target_days.
Rainwater-harvesting / off-grid storage mass balance (ARCSA / TWDB rainwater manuals; water-storage practice), by name; local rainfall or the source yield and the AHJ govern.
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