How to Calculate Room Heat Loss: Watts, U-Values and Examples
A room loses heat through its walls, windows, roof and floor, and with the air that leaks out. A room 4 × 3.5 × 2.4 m with one external wall, 2 m² of window and the new-build U-values loses about 390 W (0.39 kW) at 21 °C inside and −3 °C outside. The room heat loss calculator does the sum for your room.
The formula
- Fabric. For each wall, window, door, roof and floor: U-value × area × temperature difference, in watts.
- Ventilation. 0.33 × air changes in an hour × volume of the room in m³ × temperature difference, in watts.
- Total. The sum of the two. Multiply by 3.412 for BTU an hour.
How the U-values change the heat loss
The room above at 21 °C inside and −3 °C outside:
| Wall U-value | Window U-value | Air changes | Heat loss | In BTU an hour |
|---|---|---|---|---|
| 0.26 | 1.6 | 1 | 390 W | 1,332 BTU/h |
| 0.5 | 1.6 | 1 | 434 W | 1,481 BTU/h |
| 2 | 1.6 | 1 | 708 W | 2,415 BTU/h |
| 2 | 5 | 1.5 | 1,004 W | 3,426 BTU/h |
| 0.26 | 1.2 | 0.5 | 238 W | 812 BTU/h |
How the temperatures change it
| Inside | Outside | Difference | Heat loss |
|---|---|---|---|
| 18 °C | 0 °C | 18 | 293 W |
| 21 °C | 0 °C | 21 | 342 W |
| 21 °C | -3 °C | 24 | 390 W |
| 21 °C | -5 °C | 26 | 423 W |
Where to find the U-values
- Your EPC and any survey give the U-values of the walls and the roof.
- For a new home, the limits in Approved Document L are 0.26 for the wall, 0.16 for the roof, 0.18 for the floor and 1.6 for windows and doors.
- An old solid brick wall is about 2, a cavity wall filled with insulation about 0.5, and single glazing about 5.
Using the answer
- Pick a radiator that gives out the heat loss, with 10 to 20% to spare, at the flow temperature of your system.
- Seal draughts first: halving the air changes can cut the loss by a lot in a small room.
- A top floor room under the roof and a room over an unheated space lose more.
See also the radiator size calculator and the insulation thickness calculator. This is a room by room estimate: a heat loss survey covers the whole house.
Frequently asked questions
How do I calculate the heat loss of a room?
Add U × area × temperature difference for the walls, windows, roof and floor, and 0.33 × air changes × volume × temperature difference for the air. This example is 390 W.
What is a U-value?
The watts lost for each square metre of a wall, window or roof for each degree of difference between inside and outside. A lower number means better insulation.
What outside temperature should I use?
A design temperature of about −3 °C is usual in much of the UK, and your installer has the figure for your area.
How do I turn watts into BTU?
Multiply by 3.412. 390 W is 1,332 BTU an hour.
Can I size a boiler with this?
No. A boiler or a heat pump needs a heat loss of the whole house and the hot water, from an installer. This page is for one room.
More guides
Tools & materials for this project
Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.
Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.