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Rule of thumb: 35-45 litres per person per day, plus headroom for simultaneous showers if bathrooms outnumber... behave. Round UP to the next stock size (120/150/180/210/250/300L): an undersized cylinder is a daily cold-shower argument, an oversized one just costs a little more standing heat loss (modern cylinders are well insulated: ~1-2 kWh/day). Heat pump households size UP again (~50L/person): lower stored temperatures mean more stored litres for the same baths. Cylinder stored at 60C+ (legionella): blend down at the taps.
What The Hot Water Cylinder Size Calculator Works Out
- Recommended cylinder: 40 litres of stored hot water per person per day, plus 30 litres of buffer for a second bathroom and another 30 for a third, rounded up to the next stock size on the 120/150/180/210/250/300 litre ladder. Four people and two bathrooms land at 210 litres.
- The sizing logic: cylinders fail on the Sunday-morning rush hour, not the Tuesday average. A bath draws about 80 litres of hot water in one go and a shower 40, so round up rather than down: an undersized cylinder is a daily argument while an oversized one costs only a little extra standing loss.
- Direct, indirect, unvented: indirect (coil heated by the boiler or heat pump) is standard. Unvented mains-pressure cylinders give power-shower performance everywhere but need G3-certified installation and annual servicing. Direct immersion-only cylinders suit off-gas homes paired with cheap-rate electricity.
- Litres per person (advanced): the 35 to 45 litre rule covers showers, basins and kitchen. Long-shower teenagers and bath households push towards 50 or more; a flat with a single short-shower occupant can work on 35.
- Heat source (advanced): heat pumps store at around 50C rather than 60C, so the same bath needs more stored litres; the calculator sizes up 25% and uses a heat pump coefficient of performance of 2.7 for the running costs. Immersion-only heating prices every kWh at the full electricity rate.
- Peak morning draw (advanced): baths x 80 litres plus showers x 40 litres run back to back. If that beats the daily allowance it sets the cylinder size, which is the honest test for a family of four with two en-suites.
- Energy to heat a full cylinder and reheat time (advanced): litres x 4.186 kJ per degree from 10C mains to the storage temperature, divided by 3,600 for kWh, then divided by the coil or immersion power. A 210 litre cylinder from cold is about 12 kWh, which an 18kW boiler coil reheats in 40 minutes and a 3kW immersion in four hours.
- Hot water cost per year (advanced): the daily hot water energy divided by the heat source efficiency (88% for a condensing gas boiler) at the unit rate, over 365 days. Default rates are the current Ofgem price cap and are editable for fixed deals or Economy 7 night rates.
- Standing heat loss per year (advanced): a modern foam-insulated cylinder loses 1 to 2 kWh a day; an old bare copper one with a red jacket can lose 4 or more. Priced through the same heat source, this is what keeping the cylinder hot costs before anyone turns a tap, and why a seven-day timer and a retrofit jacket are the cheapest energy measures going.