How to Use the Water Storage Tank Sizing Calculator
Buildings like apartments, offices, and schools often keep a water storage tank so service can continue for a while even if municipal supply pressure drops or is briefly interrupted. Too small a tank means water runs out quickly during an outage; too large wastes space and money and can let water sit long enough to hurt water quality — so getting the size right matters.
This calculator multiplies occupancy by a per-capita daily water use rate for the building type (office ~15 gal, apartment/residential ~75 gal, school ~10 gal, hospital ~150 gal, factory ~20 gal per person per day — representative figures from common US plumbing design references), then multiplies by your chosen storage-days ratio (typically half a day). The formula is tank size = occupancy × daily use per person × storage-days ratio.
For an actual design, also consider whether fire-protection water is stored separately, more precise per-facility usage rates, and any local water-supply code requirements. Confirm final tank sizing with a mechanical engineer or plumbing designer.
Frequently Asked Questions
These are representative per-capita rates commonly referenced in US plumbing design guidance (office ~15 gal, apartment/residential ~75 gal, school ~10 gal, hospital ~150 gal, factory ~20 gal per person per day). Actual demand varies by facility, so check current code and your local water utility data.
A half-day (0.5) of storage is a common default for typical buildings. Facilities with higher outage risk or that want extra buffer often use a full day (1.0), while small facilities with very reliable supply sometimes size down to about 0.4.