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).
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
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.
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.
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
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.
TOTAL HEAT LOSS RESULTS
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
B: VENTILATION HEAT LOSS
C: TOTAL HEAT LOSS DATA