Hot Water Cylinder Sizing Calculator — What Size Cylinder Do I Need?

Our hot water cylinder calculator estimates the correct capacity based on the number of occupants and their hot water usage. It follows the Hot Water Association (HWA) sizing method, consistent with BS 8558:2015 (the current UK guide to domestic water services, which superseded the withdrawn BS 6700).

1. Daily hot water demand

Daily demand = Number of occupants × Usage per person per day. The HWA allows 35–45 litres of stored hot water (at 60°C) per person per day. We bias this within the band by bath habit:

• Shower only: ~35 litres per person per day
• Bath & shower: ~45 litres per person per day
• Mostly baths: ~55 litres per person per day

2. Peak simultaneous draw-off

The cylinder must also buffer the busiest period (typically the morning), when several outlets run at once. Using HWA draw-off volumes — a bath is about 60 litres of stored hot water at 60°C (100 litres at 40°C) and a shower roughly 55 litres — we work out the peak simultaneous draw-off, limited by how many bathrooms you have. The cylinder is never sized below this peak.

3. Recommended size

The required stored capacity is the larger of the day's demand and the peak draw-off (plus an optional 10–20% spare-capacity margin for future-proofing, as the HWA suggests). We then round up to the nearest standard UK cylinder size: 90L, 120L, 150L, 180L, 210L, 250L, or 300L — never below 120 litres for an occupied home, the smallest size routinely specified for an unvented install. These align with commonly available products from manufacturers like Megaflo, Heatrae Sadia, and Telford. For a family of four this typically lands at 150–180 litres.