Heat Loss Calculator
Estimate room heat loss and required heater power from dimensions, insulation, floor level, and temperature difference.
What Is a Heat Loss Calculator?
A heat loss calculator estimates how much heating power a room needs to stay warm. Enter room dimensions, insulation level, floor position, number of external walls, windows, doors, and the temperature difference to get required power in watts, kilowatts, and BTU per hour.
How Room Heat Loss Is Calculated
Heat flows through each surface proportional to its area, U-value, and temperature difference:
$$ Q = U \times A \times \Delta T $$
Wall U-values range from 2.2 W/(m²·K) for uninsulated solid brick to 0.6 W/(m²·K) for well-insulated cavity walls. Windows transfer more heat at 2.5 W/(m²·K), and ground floors add loss at 1.0 W/(m²·K).
For boiler sizing based on property type, see the Boiler Size Calculator and Furnace Size Calculator.
Frequently Asked Questions
What is a U-value?
A U-value measures how quickly heat passes through a material in watts per square meter per degree Kelvin (W/(m²·K)). Lower U-values mean better insulation and less heat loss.
How do I convert watts to BTU per hour?
Multiply watts by 3.412 to get BTU per hour. For example, 1,000 W equals 3,412 BTU/h. This calculator shows both units automatically.
Does floor level affect heat loss?
Yes. Ground floor rooms lose heat through the floor (U = 1.0). Top floor rooms lose heat through the ceiling (U = 0.7). Intermediate floors have no floor or ceiling loss to the outside.
How many external walls should I count?
Count walls that face the outside, not interior partition walls. A corner room typically has two external walls. An interior room may have only one or none.
Is this calculator accurate enough for professional sizing?
This calculator provides a useful estimate for room heaters and preliminary boiler sizing. Professional installations should include a full heat-loss survey that accounts for ventilation, thermal bridges, and local climate data.
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