This heat loss calculator gives a guide to the approximate radiator size or wood burning stove output needed to keep a room warm on a cold day. It produces a breakdown and total of the surface and ventilation heat losses for a specific room in kWs (Kilowatts) and BTUs (British Thermal Units) so is hopefully one of the best btu calculators available.
The pull down menus for materials assume u-values for a typical home built before 1990. From the 1990’s onwards Building Regulations insulation standards have been gradually tightened so in these cases it is better to manually enter u-values prevailing when the house was built. See Building Regulations And U-values: How have they changed? for greater detail.
Scroll to the bottom of the page to find out more about using the calculator.
Heat Loss Calculator
How to use the heat loss calculator
Select the type of room and choose either metres of feet for dimensions. Enter room dimensions. Where the room is not a pure rectangle then just adjust dimensions to reflect the overall room area. Then enter information about the walls using the pull down menu or alternatively enter specific u-values if you know them.
To identify particular walls assume wall 1 and wall 3 are the longer walls (length) and wall 2 and 4 are the shorter walls (width). Entering a u-value overrides the information in the specific pull down menu. Enter the approximate window and door area in each of the four walls; if there are no windows/doors in a wall then just leave blank. For the windows, doors and ceiling select the material type in the same way. Then choose the ventilation adjustment. When entries are complete press the calculate button and the calculated heat loss and energy required in kWs and BTUs will appear below.
FAQ
Q: What does K stand for?
K is short for Kelvin and is the temperature scale used for building calculations. A temperature difference of 1K is the same as a difference of 1 degree Centigrade (C). The two units start at different points. A temperature of O C is the freezing point of water whilst this would be 273.15 K. Likewise 0 K would be -273.15 C.
Hello John,
There are many radiator calculators online, which take in a minimal amount of information and, as if by magic, produce an estimate of the output required to heat a room. Your calculator is quite different – and therefore most welcome.
Out of interest, I decided to develop a calculator myself based on the video at
https://www.deals4homes.co.uk/video/calculating-a-Uvalue.html
In calculating the U-value for an external cavity wall with insulation, based on the U-values of plaster (15), block (100), insulation (100) and brick (100), I came up with a U-value of 0.24, which translates into a heat loss of 90W. Your U-value for the same kind of wall is 0.6. How did you calculate your U-value?
Many thanks.
Jeff.
Hi Jeff,
Thanks for the feedback.
As you can imagine, there are insulated cavity walls in UK homes with many different U-values as over the years building regulations have gradually demanded increased insulation in building designs. This design value, when translated into a real building, may or may not be achieved by the builder. The latest Building regs “notional dwelling specification” requires a 0.18 value for walls but that is not representative of the existing housing stock.
BRE have done some helpful work on measuring typical U-values of real insulated cavity walls in the housing stock and that, along with the 1990 building regs figure, is where the average 0.6 figure comes from. Of course if you have a more accurate figure based on greater knowledge of the specific building and its construction then that can also be used in the calculator. Do beware of using purely theoretical calculations as the average space heating demand of a new building is often double or more the SAP design calculations, for a variety of reasons.
All the best in developing your own calculator.
Hope this helps.
Kind regards
Jon
Good radiator sizing tool.
Hello.
Firstly thank you for the great calculator.
I have a query regarding internal doors, should these be in the calculation if the rooms at either side of the doors are the same temperature?
Likewise with internal block walls?
Regards
Anthony
Hi Anthony,
Thanks for the comment. You only need to consider external doors. The heat losses between two adjacent heated rooms are pretty minimal, although arguably an ensuite bathroom at 22C may lose a little heat to a bedroom at 19C, especially as often the doors between the rooms may be open. I have amended the text in the article to hopefully make it a little clearer that it is external doors to the outside that should be included in the calculations..
Kind regards
Jon
Excellent thanks Jon
HI,
is the calculation output for T50 or T60?
thanks
The figure calculated is the heat demand required to maintain the specified room temperature i.e. balance heat losses. How the required heat demand is delivered will depend on the design of the specific heating system. The UK standard for specifying a radiator heating from a domestic gas boiler is Δt50 but for lower water temperature systems, such as air or ground source heat pumps, the radiator supplier will normally provide figures for Δt30 and Δt40.
Hi Jon,
Great website, one of the better btu calculators I have seen. Especially when you consider the range of insulation that is available to use in exterior walls and under floor.
One question though. What temperature do you recommend using for the winter design temperature?
Thanks
Lee
Thanks Lee,
We have tried to make this the best BTU calculator available for free use. On the winter design temperature we have chosen -1C but it will vary by area of the country. We do apply a 15% margin for error so even if it was a degree or two colder then you should still have a little more energy in hand.