Quick answer: the rule of thumb is 35–45 litres per person — about 150–180 L for a family of four. But that ignores the thing that decides it: an 80 L bath at 40 °C draws only 48 L of stored water from a 60 °C cylinder, and a cylinder only delivers about 75% of its nominal volume at full temperature. Size from your peak hour, not your headcount.
You never use hot water at cylinder temperature
This is the calculation that separates a properly sized cylinder from a guess, and it is the one most sizing tools skip entirely.
Nobody bathes at 60 °C — that would cause a serious burn. A bath is run at around 40 °C, blended from stored hot and cold mains. The share coming from the cylinder is:
hot fraction = (Tuse − Tcold) ÷ (Tstore − Tcold)
For a 40 °C bath from a 60 °C cylinder with 10 °C mains: (40 − 10) ÷ (60 − 10) = 0.6. So an 80-litre bath consumes just 48 litres of stored hot water.
| Draw-off | Volume | Temp | From cylinder |
|---|---|---|---|
| Bath | 80 L | 40 °C | 48.0 L |
| Shower — mixer, 8 L/min × 8 min | 64 L | 40 °C | 38.4 L |
| Shower — power, 12 L/min × 8 min | 96 L | 40 °C | 57.6 L |
| Kitchen sink | 15 L | 45 °C | 10.5 L |
| Washing-up bowl | 10 L | 45 °C | 7.0 L |
| Basin wash | 5 L | 40 °C | 3.0 L |
Why this matters so much. Size a cylinder assuming a bath needs 80 litres of stored water and you will buy one roughly 40% bigger than necessary — paying more upfront, waiting longer for reheat, and losing more heat every day of its life. The blending fraction is not a refinement; it is the main event.
A 210-litre cylinder does not give you 210 litres
As hot water leaves the top of a cylinder, cold mains enters the bottom. The two layers separate reasonably cleanly — that is stratification, and it is what makes storage work at all — but the boundary mixes, so the outlet temperature starts dropping before the cylinder is empty.
Plan on 70–80% of nominal volume being usable at full temperature:
| Cylinder | Usable at temp | Baths (48 L each) | Mixer showers (38.4 L) |
|---|---|---|---|
| 120 L | 90 L | 1 | 2 |
| 150 L | 113 L | 2 | 2 |
| 180 L | 135 L | 2 | 3 |
| 210 L | 158 L | 3 | 4 |
| 250 L | 188 L | 3 | 4 |
| 300 L | 225 L | 4 | 5 |
Those counts assume back-to-back use with no reheat in between — the genuine worst case a cylinder has to survive. If your household spreads its demand across the morning, the reheat rate starts doing some of the work for you.
Reheat rate can matter more than size
Heating water takes a fixed amount of energy: E = m × c × ΔT, with water's specific heat capacity c = 4.186 kJ/kg·K. What changes is how fast you can deliver it.
| Cylinder | Energy | 3 kW immersion | 24 kW boiler coil |
|---|---|---|---|
| 120 L | 6.279 kWh | 2.09 h | 16 min |
| 150 L | 7.849 kWh | 2.62 h | 20 min |
| 180 L | 9.419 kWh | 3.14 h | 24 min |
| 210 L | 10.988 kWh | 3.66 h | 27 min |
| 250 L | 13.081 kWh | 4.36 h | 33 min |
| 300 L | 15.698 kWh | 5.23 h | 39 min |
The gap is dramatic — 27 minutes against 3.7 hours for the same 210-litre cylinder. A 150 L cylinder on a fast boiler coil recovers between the morning's showers and effectively behaves much larger. A 250 L cylinder on a 3 kW immersion is a one-shot device: once it is gone, it is gone for four hours.
So the sizing question is really two questions. How much do you need at once? sets the volume. How fast does it come back? decides whether you can get away with less. If you are heating on an immersion or a heat pump, size generously. On a well-matched boiler coil, you can size lean.
60 °C to store, 43 °C to deliver
There is a real tension in cylinder temperature, and resolving it wrongly is dangerous in either direction.
| Temperature | Bacteria | Scald risk (adult skin) |
|---|---|---|
| 20–45 °C | Legionella multiplies | Safe |
| 50 °C | Growth slows | Full-thickness burn in ~5 minutes |
| 55 °C | Largely suppressed | ~30 seconds |
| 60 °C | Suppressed — correct storage | ~5 seconds |
Both columns matter, and they point in opposite directions. The resolution is to separate storage from delivery:
- Store at 60 °C. Do not turn it down to save energy — that moves the cylinder into the range where Legionella thrives. It is a genuine health risk, not a theoretical one.
- Deliver at 43–46 °C via thermostatic mixing valves at the outlets. Scalding happens at the tap, so that is where it should be controlled.
Children, older people and anyone with reduced sensation are burned considerably faster than the adult figures above. A TMV on the bath is the single most worthwhile safety fitting in the system.
Size for winter, not for average
Incoming mains temperature swings seasonally — around 15 °C in late summer, down to 5 °C or lower in winter. Colder mains means a larger share of each blended draw must come from the cylinder:
| Mains temp | Hot fraction | Stored water used | vs 10 °C |
|---|---|---|---|
| 15 °C (summer) | 0.5556 | 44.4 L | −7.4% |
| 10 °C (spring/autumn) | 0.6000 | 48.0 L | — |
| 5 °C (winter) | 0.6364 | 50.9 L | +6.1% |
Roughly 6% more stored water in winter — and that is exactly when demand for hot baths peaks. A cylinder sized on summer figures is the one that runs cold in January. Use your coldest realistic mains temperature.
Why bigger is not automatically better
Oversizing feels like the safe error, but it is not free:
| Penalty | Effect |
|---|---|
| Standing heat loss | Runs 24/7 whether you use the water or not — a modern well-insulated cylinder still loses roughly 1.5–2.5 kWh/day, and loss scales with surface area |
| Reheat time | 300 L on a 3 kW immersion is over 5 hours from cold |
| Space | Cylinders need clearance for the immersion element to be withdrawn |
| Upfront cost | Plus heavier structural loading — 300 L of water is 300 kg |
| Stagnation | Under-used stored water sits longer between full turnovers |
Go one standard size above your calculated peak demand, not two. The margin should cover a guest or an unusually heavy morning — not a household you do not have.
Common mistakes & pro tips
| Mistake | What happens | Fix |
|---|---|---|
| Sizing a bath at its full volume | Cylinder ~40% oversized | An 80 L bath draws 48 L of stored water |
| Assuming full capacity is usable | Undersized by ~25% | Plan on 70–80% usable |
| Using daily total instead of peak hour | Wildly oversized | Size for the busiest hour only |
| Ignoring reheat rate | Wrong size in both directions | Fast coil sizes lean; immersion sizes generous |
| Turning storage below 60 °C | Legionella risk | Store at 60 °C; limit at the tap with a TMV |
| Sizing on summer mains | Runs cold in winter | Use your coldest mains temperature |
| Forgetting power-shower flow | Undersized | 12 L/min uses 50% more than an 8 L/min mixer |
Pro tip on measuring your real usage. Do not estimate shower flow — measure it. Put a bucket under the running shower for 10 seconds at your normal setting, weigh or measure what you collect, and multiply by six for litres per minute. Real showers vary from 6 to 15 L/min, and that range alone changes the cylinder size by more than any other single input.
How to use this calculator
- Use peak demand mode and enter what happens in your busiest hour — baths, showers and sink use that genuinely overlap.
- Set your storage temperature (60 °C unless you have a reason), mains temperature (use your winter figure) and usable fraction (75% is a sound default).
- Pick your heat source from the dropdown to see reheat time for the recommended size.
- Read the recommended volume and the next standard cylinder size above it — that is what to buy.
- Or use quick mode for the litres-per-person estimate if you just want a sanity check.
Frequently asked questions
What size hot water cylinder do I need?
Common guidance is 35–45 litres per person — about 140–180 L for a family of four. That is a starting point, not an answer, because it ignores how the household actually uses water. Four people with one bathroom and no baths may be fine on 150 L; the same household with two power showers back-to-back needs 250 L or more. Sizing from peak-hour demand gives a far better result.
How much hot water does a bath use?
Far less than the bath volume, because you never bathe at storage temperature. A typical 80 L bath at 40 °C, from a 60 °C cylinder with 10 °C mains, uses only 48 L of stored hot water. The blending fraction is (40−10)/(60−10) = 0.6. Sizing as though a bath needs 80 L of stored water oversizes the cylinder substantially.
Can I use the full capacity of a hot water cylinder?
No. As hot water leaves the top, cold mains enters the bottom, and although the layers separate cleanly, mixing at the boundary means outlet temperature falls before the cylinder empties. Plan on 70–80% of nominal volume being usable at full temperature — a 210 L cylinder realistically delivers around 158 L.
How long does a hot water cylinder take to reheat?
It depends on the heat source. Heating 210 L from 15 °C to 60 °C needs 10.99 kWh (water takes 4.186 kJ/kg·K). On a 3 kW immersion that is about 3.7 hours; through a 24 kW boiler coil, roughly 27 minutes. This is why reheat rate matters as much as capacity — a 150 L cylinder on a fast coil can serve a household better than 250 L on an immersion.
What temperature should a hot water cylinder be set to?
60 °C. Hot enough to suppress Legionella, which multiplies between roughly 20 and 45 °C, while avoiding the accelerated scale and standing losses of higher settings. Do not lower storage temperature to save energy — that creates a genuine health risk. Fit thermostatic mixing valves instead to control temperature at the tap, where scalding actually happens.
Is 60 degree water dangerous at the tap?
Yes — which is why storage and delivery temperature must be treated separately. Water at 60 °C can cause a full-thickness burn on adult skin in about five seconds, and far faster on a child or older person. At 50 °C the same injury takes around five minutes. Store at 60 °C for bacterial safety, and fit a TMV to limit bath outlets to about 43–46 °C.
Does cold mains temperature affect cylinder sizing?
Yes — it is why a cylinder that copes in summer struggles in winter. Colder incoming water means more of each blended draw comes from the cylinder. With 10 °C mains an 80 L bath at 40 °C uses 48 L stored; with 5 °C winter mains the same bath needs 50.9 L, about 6% more. Size for winter, not annual average.
Should I choose a bigger cylinder to be safe?
Only up to a point. An oversized cylinder costs more, takes longer to reheat, occupies more space, and loses heat continuously whether or not you use the water — standing loss scales with surface area, and even a modern well-insulated cylinder loses roughly 1.5–2.5 kWh/day. Choose the next standard size above your calculated peak demand, not two sizes up.