Heat Loss Calculator
Enter your room dimensions, insulation levels, temperature settings, and ventilation rate to find out exactly how much heat the room loses and what size heater you need. Results are shown in watts, kilowatts, and BTU per hour with a full breakdown by component: walls, windows, doors, ceiling, floor, and infiltration.
What is heat loss and why does it matter?
Every building continuously loses heat to the outside through its walls, roof, floor, windows, doors, and gaps where air leaks in. The rate of loss is measured in watts (W) or kilowatts (kW). To maintain a comfortable indoor temperature, a heating system must replace heat at the same rate it is lost. Calculating heat loss tells you the minimum heating capacity you need, prevents oversizing (which wastes money and reduces comfort), and shows exactly where insulation improvements will have the biggest effect.
How heat loss is calculated: the formula
Heat conduction through any flat surface follows a simple formula: Q = A x U x dT, where Q is the heat loss in watts, A is the surface area in square metres, U is the thermal transmittance in W/(m²K) (lower is better), and dT is the temperature difference between inside and outside in degrees Celsius or Kelvin. The calculator applies this formula independently to walls, windows, doors, ceiling, and floor, then adds infiltration loss - the heat carried out by cold outside air leaking through gaps. Infiltration loss is calculated as: Q_inf = 0.33 x ACH x V x dT, where ACH is air changes per hour and V is the room volume in cubic metres. Finally a safety factor (typically 10-20%) is added so your heating system has some headroom on the coldest days.
Choosing your outdoor design temperature
The outdoor design temperature is the coldest temperature your system must cope with, not the average winter temperature. In the UK a common design temperature is -3 to -5 °C; in northern Europe or Canada it may be -15 to -25 °C. Most building regulations and ASHRAE guidelines specify a "1% design temperature" - the temperature exceeded 99% of heating-season hours - for your location. Using an outdoor temperature that is too mild results in an undersized heating system that cannot keep up on the coldest days.
Understanding the component breakdown
The bar chart and individual output figures show how much of the total heat loss comes from each building element. In a poorly insulated older house, windows often account for 20-30% of total loss despite making up a small fraction of the wall area - because single glazing has a U-value nearly 15 times worse than a well-insulated wall. Infiltration losses can easily exceed 30% of total heat loss in an older house with poor draught-proofing. The breakdown helps you prioritise: the element with the highest loss percentage is usually the best target for improvement. For most older UK homes the order of cost-effectiveness is: loft insulation, draught-proofing, cavity wall insulation, double glazing, and finally solid-wall insulation.
Typical U-values for building elements
| Building element | Construction type | U-value (W/m²K) |
|---|---|---|
| External wall | Solid brick, 215 mm | 2.2 |
| External wall | Uninsulated cavity wall | 1 |
| External wall | Cavity wall + 100 mm mineral wool | 0.28 |
| External wall | Passive-House grade | 0.15 |
| Glazing | Single pane | 5.8 |
| Glazing | Double glazing | 2.8 |
| Glazing | Double + low-e | 1.6 |
| Glazing | Triple glazing | 0.9 |
| Roof / ceiling | Uninsulated | 2.3 |
| Roof / ceiling | 200 mm mineral wool | 0.18 |
| Roof / ceiling | Passive-House grade | 0.1 |
| Ground floor | Uninsulated solid slab | 0.7 |
| Ground floor | Insulated slab | 0.22 |
Lower U-values mean better insulation. These are representative figures; actual values vary by construction details.
Frequently asked questions
What is a U-value?
A U-value (thermal transmittance) measures how much heat flows through 1 square metre of a material for every 1 degree of temperature difference between the two sides. It is expressed in W/(m²K). A lower U-value means better insulation: a solid brick wall has a U-value around 2.2, while a triple-glazed window is around 0.9, and a Passive-House-grade wall is around 0.15. Multiplying the area by the U-value and by the temperature difference gives heat loss in watts.
What is ACH and how does it affect heat loss?
Air changes per hour (ACH) is the number of times the entire room volume is replaced by outside air in one hour. Even without opening windows, gaps around windows, under doors, through floorboards, and around pipes cause infiltration. A draughty older house may have 1.5-2.0 ACH, while a modern well-sealed house should be below 0.5. The heat lost through infiltration is: Q = 0.33 x ACH x volume (m³) x temperature difference. Halving the ACH by draught-proofing cuts infiltration loss by 50%.
How do I convert watts to BTU/hr?
Multiply watts by 3.412 to get BTU/hr. For example, 5,000 W x 3.412 = 17,060 BTU/hr. In the other direction, divide BTU/hr by 3.412. This calculator shows both units automatically so you can use whichever is more familiar or required by your heating equipment specifications.
What size heater do I need for my room?
Choose a heater whose maximum output in kW or BTU/hr equals or exceeds the recommended heater size this calculator shows. For radiators, the manufacturer rating is usually given at a specific flow temperature (e.g. 70 °C flow, 50 °C return, 20 °C room). If you use a heat pump or lower-temperature system, derate accordingly. Adding a 10-20% safety factor (which this calculator already includes when you select one) ensures the system keeps up on the coldest days.
Why does room position (ground, middle, top) matter?
Ground-floor rooms lose heat downward through the floor to the cooler ground beneath. Top-floor rooms lose heat upward through the ceiling or roof to the cold outside. A middle-floor room with heated rooms above and below loses very little heat through the floor or ceiling, so only walls, windows, and infiltration matter. Selecting the correct position ensures the calculator includes only the relevant surfaces.
What is a reasonable heat loss per square metre?
For a well-insulated modern room the total heat loss typically falls between 30-60 W per square metre of floor area at typical UK design conditions. An older poorly insulated room can easily exceed 100-150 W/m². Passive-House-certified buildings aim for below 10 W/m². The "heat loss per floor area" output lets you benchmark your room against these ranges.