Great Home

Independent Advice On Improving Your Home

Radiator Sizing Guide And BTU Calculator

BTU calculator and radiator sizing calculator


The Great Home heat loss calculator gives a guide to the 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 kW (Kilowatts) and BTUs (British Thermal Units). We try to keep it as one of the best btu calculators available on the web. If using for radiator sizing rather than heat loss then the output assumes Δt50.

Existing Homes

The pull down menus for materials assume u-values for a typical home built before 1990. Homes built from the 1990’s onwards have higher insulation standards which have been gradually tightened. If your home is newer than 1990 it may be 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.

Extensions

Where the calculator is used for the heat losses in a new extension, then the pull down menus also offer u-values for current Building Regulations. For some elements such as windows and doors, this is from Part L1A (new dwellings) whilst for walls, floors and roofs this is from Part L1B (existing dwellings) which are not as demanding as for new build homes.

Scroll to the bottom of the page to find out more about using the calculator.

Heat Loss Calculator

Winter design temperature °C

Information On Room Ventilation: Some homes have greater air leakage with gaps in windows frames, floorboards and ventilation through chimneys and air bricks; other homes are very air tight and use an MVHR system to achieve the required air changes in a room without losing much heat. It’s not an exact science but if you have a room with a particular extreme then select an option to adjust for it in the calculations. The calculator measures ventilation heat losses in terms of air changes per hour in the room.

Ventilation Adjustment

Information About The Walls: Wall 1 and 3 use the room length dimension, wall 2 + 4 use the width. Is each an internal, external or party wall (a party wall is a dividing wall between two adjoining buildings)? Choose wall material or if known enter the u-value. Enter the external window/door area in the wall if any in m².

Wall
(length)
Wall type
Wall material
or U-value
Window
area m²
Door
area m²
Wall 1:
Wall 2:
Wall 3:
Wall 4:

Information About The Windows & External Doors/Floor/Ceiling: What type of glazing material is used for the windows and external doors in the room? Choose a glazing type or alternatively enter the u-value if known. What is below the floor (another room or the ground)? What is above the ceiling (another room or the roof)? Choose a material type for each or alternatively enter the u-value if known.

Material Type
or U-value
Windows/glazing
Doors
Floor: What’s below?
Ceiling: What’s above?

NDS – Notional Dwelling Specification, 2010 Building Regulations L1A (new dwellings), L1B (existing dwellings).

Note: this heat loss calculator is a guide only and you should consult your heating engineer to calculate a precise figure based on knowledge of the specific materials used for the walls, floors, ceiling, windows and doors as well as ventilation losses.

Entry error: check red highlighted input boxes above
 View detail in separate tab

TOTAL HEAT LOSS RESULTS

Fabric Heat Loss (Watts)
Ventilation Heat Loss (Watts)
15% capacity allowance (Watts)
Total Heat loss (Watts)
kW Output Radiator Needed at Δt50
BTU Output Radiator Needed at Δt50

This total heat loss is made up of the total of (A) surface heat loss and (B) ventilation heat loss with a 15% increase applied as a margin of error. The detailed calculations are below.

Note: When sizing radiators check what the Δt of the heating system is. Historically radiator manufacturers technical data specifies output at Δt50. Some provide data for Δt30 as used with a heat pump. Typically heat output is halved when a radiator at Δt50 is operated at Δt30. See FAQ below.

The calculated losses in Watts/K (and Watts/C), without the 15% margin of error are:

Fabric Heat Loss (Watts/K)
Ventilation Heat Loss (Watts/K)
Total Heat Loss (Watts/K)

A: SURFACE HEAT LOSS

Material

Area m²

U-value

Temp diff °C

Surface Heat loss (Watts)
Wall 1

Wall 2

Wall 3

Wall 4

Windows

Doors

Floor

Ceiling

Subtotal surface loss

B: VENTILATION HEAT LOSS

Air change rate

Volume m³

Specific heat

Temp diff °C

Ventilation Heat loss (Watts)

Subtotal ventilation loss

C: TOTAL HEAT LOSS DATA

TOTAL HEAT LOSS

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.

BTU calculator used for radiator sizing-
Identify Wall 1 and 3 as the longest length walls, Wall 2 and 4 as the shorter length walls.

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.

Q: What are the U-values required by the current Building Regulations for a new home or extension?
See post Building Regulations: Current U-values for Insulation in Homes

Q: What Δt does the calculator assume for the heating system and radiators.

The base 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. Historically radiator technical data supplied by manufacturers specified output at Δt50 for gas boilers. Some now provide data for Δt30 as typically used with a heat pump. As a simple rule of thumb, to calculate a radiators output on a heat pump operating at Δt30 then halve the Δt50 figure provided by the radiator supplier.
Example: A radiator’s output rated 1000W at Δt50 would produce circa 500W output on a heat pump at Δt30. Check individual manufacturers technical information for detail.

Comments

  1. Mike Wright says

    I’m trying to calculate the heating requirement of a bathroom which has no external windows. Your calculator doesn’t allow this option. I got round it (I hope) by user selecting a U value of 0.1 – it wouldn’t accept a 0 U value. Also as this room has no external windows it has an extractor fan on whenever the lights are on, which whenever the room is in use. To reflect this in your calculator I selected a ‘Very draughty room’. Would this be a reasonable approximation?

    Reply

  2. Mark Emerson says

    Hi Jon

    I don’t understand why you use a U-value of 1.6 for internal walls to calculate BTU requirements. If internal walls are dividing two rooms heated at the same temperature, then surely their U-values are irrelevant – there is no net heat transfer across them. It seems odd that the required BTU value of a room with three insulated external walls is much lower than a room of equal dimensions with only one insulated external wall.

    Please could you explain and help me adjust my BTU requirements if necessary.

    Reply

    • roofman says

      Hi Mark,
      Happy to clarify. Different types of room (bedroom, bathroom, kitchen) will often have different target design temperatures. So a bathroom with a target temperature of 22C next to a bedroom at 19C will lose some heat. Not a huge amount but likewise not zero and the calculator allows for this 3C difference in temperature in the calculations. It assumes that any room with an internal wall has a room on the other side at 19C. The 1.6 U-value is a standard assumption for internal walls which tend not to have much insulation value. You can see from the detailed calculations what the assumed temperature difference is for each wall and the impact it has on heating requirements.
      Hope that helps.
      Kind regards
      Jon

      Reply

  3. Bart Weenink says

    Hi
    I have an air source heat pump. The calculator suggests I need 5200 BTU output of the radiator. Can you advice as the delta factor is only 30 rather than 50. What does this mean for purchasing the right size of radiator

    Reply

    • roofman says

      Hi Bart,
      The figure calculated is the heat demand required to maintain the specified room temperature i.e. balance heat losses. It is independent of heating system. How the required heat demand is delivered will depend on the design of the specific heating system. Historically 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 sometimes provide figures for Δt30 and Δt40. You will need to refer to the radiator supplier to see what their 5200 BTU radiator (assuming this is their Δt50 figure) will output at Δt30 but typically it is half so you will likely get 2600 BTU / hour.
      Hope this helps
      Kind regards
      Jon

      Reply

  4. Guy says

    Hi,
    I found this very interesting; just one thought – how sensitive are the final values to the assumed temperatures of the rooms ? Those numbers must be just parameters in your program (and they don’t coincide with our way of life), so could you not allow user entered values without much change of the user interface ?

    Or would the result be within your 15% figure ?

    If so, just ignore this

    Reply

    • roofman says

      Hi Guy,
      Thanks for the feedback. The assumed temperatures are pretty standard for UK calculations. Generally an installer / designer would design a system for a typical user – that way it can accomodate a wide variety of requirements in the future rather than say just the current residents who happen to like cooler room temperatures and have a particular lifestyle. The heat demand is for sizing the radiator to cope with the lowest winter temperatures. Most of the year the radiators / boiler will not be anywhere close to that demand as the outside temperatures wont be below zero.

      One way of seeing the difference a 1C change in room temperature would have is to increase the winter design temperature by 1C ( from -1 to 0c). It will make about a 5% difference. It would also slow the speed with which the room would heat up from cold.

      Hope this helps.
      Kind regards
      Jon

      Reply

  5. Obaid Malik says

    Hi Jon,
    Great website!!! helped me alot. I had one question, Where do you get the room wise ACH values? ie.
    Living room 1.5
    Kitchen 2.0
    Bedroom 1.0
    Bathroom 2.0.

    Is there an official document like SAP 2012 version 9.92 (October 2013) that lists room wise ACH values? or is this just common practice

    Many Thanks
    Malik

    Reply

    • Jon Davies says

      Hi Malik,
      These figures come from BS EN 12831:2003 Heating systems in buildings — Method for calculation of the design heat load. The document specifies the air changes and also the target temperature for the room type. This standard has been updated as BS EN 12831-1:2017.

      Hope this helps.
      Kind regards
      Jon

      Reply

  6. Andrew Bayliss says

    Hi I am trying to calculate the BTU for a old loft bedroom probably done when the house was built in the 1800’s. It has no insulation with mo carpet. As the room is not square more a prism shape, how do I calculate its BTU?

    Reply

  7. huan1003 says

    Hi , ~confused ..for a kitche / diner 8 x 4.5 x 2.1 the calulcator recommends 12236 BTU.
    Would a single centrally placed Rad suffice or would you recommend multiple?
    I ask as one half will have seating the other half will be generally kitchen

    Reply

    • Jon Davies says

      Hi,
      Thanks for the comment. My personal preference would be to have two/three smaller radiators, on outside walls not covered by curtains/furniture. Two/three rads will give you a more even heat. With one larger radiator, whilst you get the same heat output, you can get cold spots in the room due to poor circulation of air. If cooking, the kitchen area is likely to be warmer so you may also wish to have slightly more heat output in the dining area.
      Hope this helps
      Kind regards
      Jon

      Reply

  8. Jeffrey WILLIAM Terrell says

    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.

    Reply

    • Jon Davies says

      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

      Reply

      • Jeffrey WILLIAM Terrell says

        Hello John,

        Many thanks for your quick reply.

        Just to clarify – I’m not developing a calculator for general use. I’m simply calculating the heat loss for a particular room – an extension that I’ve built recently, which I’m in the process of finishing. There are, in fact, 7 walls of various kinds in this L-shaped room: 4 internal, 3 external and 1 party wall.

        Of the external walls, 2 are new cavity walls with 100 insulation, and the other is an old cavity wall with 50 air gap. The party wall is solid 9″ brickwork.

        In calculating the heat loss in watts for the whole room, I’ve discarded the internal walls (on the basis that the temperatures either side of the walls will be the same) and broken down each of the remaining surfaces (walls, ceiling – part of which is a warm roof, floor, windows, door) into their constituents parts, and assuming a temperature delta of 22’C, I’ve calculated the U-values of each surface.

        My principle concern is that the heat loss I’ve calculated is about one third of the heat loss generated by simple online calculators.

        I’ve performed the calculations in an Excel spreadsheet. Would you be prepared to look at the figures – for a fee, of course – to see if I’ve calculated them correctly?

        Thanks.

        Regards,
        Jeff.

        Reply

  10. Anthony says

    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

    Reply

    • Jon Davies says

      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

      Reply

    • Jon Davies says

      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.

      Reply

  12. Lee says

    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

    Reply

    • roofman says

      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.

      Reply

Leave a Reply

Your email address will not be published. Required fields are marked *

Show you are a real person * Time limit is exhausted. Please reload CAPTCHA.