Hot water cylinder size calculator
The calculator needs JavaScript. The worked example, formula and figures below show exactly how it works, so you can do the sum yourself. A guide, not advice. Check your supplier's or maker's figures before ordering.
Short answer
Worked example
| You enter | Value |
|---|---|
| Start from | A cylinder size |
| Cylinder size | 210 L |
| Storage temperature | 60 °C |
| Cold feed temperature | 10 °C |
| Heated by | Immersion |
| Heat into the cylinder | 3 kW |
Cylinder: 210 litres, about 4.1 h to heat from cold (indicative)
| Result | Value |
|---|---|
| Cylinder size (indicative) | 210 L |
| Energy to heat it from cold | 12.21 kWh |
| Reheat time from cold | 4.07 h |
Show the working
- Cylinder: 210 L (your figure)
- Energy: 210 L × 4.186 × (60 − 10) °C ÷ 3600 = 12.21 kWh
- Reheat time: 12.21 kWh ÷ 3 kW = 4.07 h
How it works
- Storage = people × litres per person + extra per bathroom after the first, then the next stock size up
- Energy (kWh) = litres × 4.186 × (storage temperature − cold feed) ÷ 3600
- Reheat time (h) = energy ÷ heat input (kW): the coil output, not the boiler's or heat pump's full output
Unvented cylinders must be fitted by a competent person (Building Regulations G3) and need an expansion vessel or air bubble.
Any gas boiler work, including connecting a cylinder coil, is for a Gas Safe registered engineer.
Heat pump cylinders are usually larger with a bigger coil: follow the heat pump maker's and MCS guidance.
Our default (adjustable)
These are typical planning figures and our own allowances, not facts about your product. Change any of them in the calculator to match your bag, pack or tin.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Stored hot water per person: Showers | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB4: litres of stored hot water per person, mostly showers. Researcher to source to MCS MIS 3005-D or a cylinder maker. |
| Stored hot water per person: Baths | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB4: litres of stored hot water per person, mostly baths. Researcher to source to MCS MIS 3005-D or a cylinder maker. |
| Extra storage per bathroom after the first | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB4: extra storage per bathroom after the first. Researcher to source to a cylinder maker's sizing guide. |
| Storage temperature | 60 °C | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB4 (HW1 test value): storage temperature. Researcher to cite (HSE L8 / cylinder maker). |
| Cold feed temperature | 10 °C | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 PB4 (HW1 test value): cold feed temperature. Researcher to cite. |
Regulations and standards
- Building Regulations Part G3 (unvented hot water storage; England, with equivalents in Wales, Scotland and Northern Ireland)
- MCS MIS 3005-D (domestic hot water sizing for heat pumps)
Explained: Approved Document G (hot water safety), MCS MIS 3005-D (heat pump design).
Approved Document G covers England (Wales has its own version). Scotland uses the Technical Handbooks and Northern Ireland its Technical Booklets.
| Table | Clause | Nations | Status |
|---|---|---|---|
| Hot water cylinder stock sizes TODO: Researcher to add the maker or standard URL | Cylinder makers' ranges (Researcher to fill from at least two UK makers) | UK-wide | 0/8 values entered Not yet checked against the source |