Boiler 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 |
|---|---|
| Boiler type | Combi |
| Hot water flow you want | 12 L/min |
| Temperature rise | 35 K |
| Heat loss of the house | 8,000 W |
Boiler output: at least 29.3 kW
| Result | Value |
|---|---|
| Boiler output needed | 29.3 kW (99,983 BTU/h) |
| For hot water | 29.3 kW |
| For heating | 8 kW |
Show the working
- Heating: 8,000 W ÷ 1,000 = 8 kW
- Hot water: 12 L/min × 4.186 × 35 K ÷ 60 = 29.3 kW
- Combi: the larger of 29.3 kW (hot water) and 8 kW (heating) = 29.3 kW
How it works
- Combi hot water (kW) = flow (L/min) × 4.186 kJ/kg·K × temperature rise (K) ÷ 60 (1 L of water ≈ 1 kg)
- Combi size = the larger of the hot-water figure and the heating figure (a combi does one at a time)
- Heating (kW) = house heat loss (W) ÷ 1000
- System or regular: cylinder reheat (kW) = cylinder energy (kWh) ÷ reheat time (hours); boiler = heating + reheat
Don't oversize: a boiler much bigger than the heat loss cycles on and off and runs less efficiently. Combis are sized for hot water, so check the incoming mains flow can supply it.
A Gas Safe registered engineer must fit and commission the boiler and will confirm the size.
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.
This calculator has no built-in defaults: every figure comes from you or your product's data.
Regulations and standards
- BS EN 12831-1 (heat loss, for the heating figure)
Explained: BS EN 12831 and CIBSE DHDG (heat loss).
Our checks use the guidance for England unless a badge says otherwise. Wales, Scotland and Northern Ireland have their own rules: check the source for where you live.