Sizes minimum cold water storage capacity by building/occupancy type (BS 8558 / CIPHE Design
Guide), with a stagnation warning against HSE HSG274's 24-hour (12-hour for healthcare premises) guideline.
Tender-stage only. Verify final design to project spec and a suitably qualified engineer.
For design guidance only. Always verify with a qualified engineer.
About this cold water storage calculator
This free cold water storage calculator sizes the minimum cold water storage capacity a building needs by occupancy type, using the BS 8558 / CIPHE Design Guide reproduction tables, and flags whether a proposed tank is oversized against the HSE stagnation guidance. It is aimed at UK plumbing and public-health engineers, M&E designers and contractors sizing storage cisterns and break tanks for dwellings, offices, schools, hotels and healthcare premises at tender stage. Everything runs in your browser. Nothing is uploaded.
How cold water storage is calculated
Storage is a straightforward table sum: for each occupancy type you enter a count, and the calculator multiplies it by that type's litres-per-unit figure, 135 L per dwelling occupant, 200 L per hotel bed space, 45 L per office employee with a canteen, 7 L per restaurant meal and so on: from the BS 8558 / CIPHE Design Guide and CIBSE Guide G tables, then totals every row. It compares your proposed (or the table) storage against a stagnation limit: the HSE HSG274 Part 2 §2.36 guideline that stored water should not normally exceed one day's use, 24 hours, or a nominal 12 hours (half the table total) for healthcare premises, and warns when the tank exceeds it. A break-tank/booster advisory is included for information; booster hydraulic sizing is a separate calculation.
Frequently asked questions
Why does oversizing a cold water tank matter?
Oversized storage means water sits longer, raising the risk of stagnation and Legionella growth. HSG274 Part 2 §2.36 advises storage should not normally exceed one day's use, so the calculator warns when a proposed volume goes beyond the 24-hour (or 12-hour healthcare) guideline.
What counts as a healthcare premises?
Children's homes, residential nurseries, nurses' homes, nursing/convalescent homes and hospitals. For these the stagnation ceiling drops to a nominal 12 hours, half the table storage total, reflecting the tighter water-safety regime.
Does it size the booster set or break tank?
No. The break-tank/booster note is advisory only, direct mains-fed storage is not always acceptable to the water undertaker. Sizing a break tank (minimum ~15 minutes at the booster's peak flow) and the booster set's hydraulics is a separate calculation. Verify the final design with a qualified engineer.