Radiator Sizing Guide And BTU Calculator

BTU calculator and radiator sizing calculator

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

Dimensions entered in:

Room Type

Winter design temperature °C

Room Length

Room Width

Room Height

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 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?

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

Ventilation Adjustment

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

TOTAL HEAT LOSS RESULTS

Total Heat loss (Watts)

kW Output Radiator Needed

BTU Output Radiator Needed

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.

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.

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Comments

Nicoletta says

May 15, 2019 at 7:55 pm

HI,

is the calculation output for T50 or T60?

thanks

- Jon Davies says
  
  May 16, 2019 at 10:58 am
  
  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.
  
Lee says

February 6, 2018 at 9:16 pm

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

- roofman says
  
  March 15, 2018 at 3:11 pm
  
  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.

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