Professional Heat Loss Calculator
Calculate the required heating power (Watts/BTUs) for your room
1. Room Dimensions
2. Temperatures
0.5 (Well Sealed/New)
1.0 (Average House)
1.5 (Drafty/Old)
2.0 (Very Drafty)
3. Insulation (U-Values)
Insulated: 0.3, Solid Brick: 2.0
Double Glaze: 1.6, Single: 4.8
4. Floor & Roof
Well Insulated: 0.15 – 0.25
Insulated: 0.25, Suspended Timber: 0.7
Estimated Heat Loss
0
Watts
0 BTU/hr
Understanding Room Heat Loss
Heat loss is the measure of total transfer of heat through the fabric of a building from the inside to the outside. Whether you are sizing a new radiator, installing underfloor heating, or upgrading a boiler, calculating heat loss is the most critical step to ensure comfort and energy efficiency.
The Heat Loss Formula
The core calculation for fabric heat loss is defined by the formula:
Q = U × A × ΔT
- Q: Heat loss in Watts (W)
- U: The U-value of the material (W/m²K)
- A: The surface area (m²)
- ΔT: The temperature difference between inside and outside (°C)
What are U-Values?
A U-value measures how effective a material is as an insulator. The lower the U-value, the better the material is at keeping heat inside your home. For example, a modern highly insulated wall might have a U-value of 0.18, whereas an old solid stone wall could be as high as 2.1.
| Building Element | Typical U-Value (Poorly Insulated) | Typical U-Value (Highly Insulated) |
|---|---|---|
| External Walls | 2.0 – 2.2 | 0.18 – 0.30 |
| Windows | 4.8 (Single) | 1.2 (Triple) |
| Roof / Loft | 2.5 | 0.13 – 0.20 |
| Ground Floor | 0.8 | 0.20 – 0.25 |
Ventilation and Air Infiltration
Not all heat is lost through walls. A significant portion escapes through gaps in windows, doors, and floorboards, or through intentional ventilation. Our calculator accounts for this using the "Air Changes per Hour" (ACH) method. Ventilation loss is calculated by multiplying the room volume by the air density and specific heat capacity (roughly 0.33 coefficient for standard conditions).
Example Calculation
Consider a room 5m long, 4m wide, and 2.5m high. If the outside temperature is -5°C and you want it to be 21°C inside (a 26°C difference):
- Wall Loss: 42m² (total wall minus windows) × 0.3 U-value × 26ΔT = 327.6 Watts
- Window Loss: 3m² × 1.6 U-value × 26ΔT = 124.8 Watts
- Ventilation Loss: (50m³ volume × 1 ACH) × 0.33 × 26ΔT = 429 Watts
Summing all elements gives the total wattage required to maintain the temperature during peak winter conditions.