Heat Loss Calculator
Use the Heat Loss Calculator to calculate heat loss in any room or building by entering dimensions, materials, and temperatures in metric or imperial units.
Calculate room or house heat loss in seconds
Account for walls, floors, doors, windows, ceilings, and ventilation
Use metric or imperial units
Download a professional PDF report with your results
| Parameter | Value |
|---|---|
| Ventilation Heat Loss: | 0.158 kW |
| Material Heat Loss: | 2.26 kW |
| Thermal Bridging: | 0.114 kW |
| Total Heat Loss: | 2.55 kW |
What Is Heat Loss & Why Does It Matter?
Heat loss refers to the transfer of heat from a warm space to a colder one, typically through walls, windows, doors, floors, ceilings, and ventilation. Understanding and calculating heat loss is critical for engineers, consultants, and installers when designing HVAC systems, selecting heating equipment, or meeting MCS and energy efficiency standards.
Accurate heat loss calculations help ensure the right boiler or heat pump is specified, avoiding underperformance or wasted energy. This is especially important in low-energy buildings or retrofit projects where energy conservation is a top priority.
Heat Loss Variables
Room Dimensions
The room's width, height, and length define its total volume and surface area. Larger spaces lose more heat through walls, floors, and ceilings.
Temperature Inputs
External, internal, and ground temperatures create the temperature difference that drives heat transfer. Accurate values are essential for reliable heat loss results.
Ventilation Rate
Air changes per hour account for heat lost through ventilation and infiltration. This factor is especially important in draughty or poorly sealed buildings.
Material Selection
The materials used for walls, floors, ceilings, windows, and doors each have different thermal properties. These affect how much heat is transferred through surfaces.
Surface Materials
Each layer, like brick, plasterboard, or timber, has a specific thermal conductivity. This impacts how quickly heat flows through the building envelope.
Window & Door Openings
Wider or taller windows and doors result in a larger total opening area. These often allow more heat to escape, especially if glazing or insulation is poor.
Thermal Bridging
Heat can bypass insulation at junctions, frames, and structural supports. These bridges increase total heat loss and are often underestimated.
External Wall Exposure
The greater the percentage of walls exposed to the outside, the more area is available for heat to escape. This increases heat transfer to the external environment.
Heat Loss Formula
Heat loss is calculated using a formula that accounts for surface area, temperature difference, U-value, room volume, and air changes per hour.
Imperial Equation
Following the formula below, you can calculate the heat loss:
Components:
Q = Heat loss (BTU/hr)
A = Surface area (ft²)
ΔT = Temperature difference (Indoor - Outdoor, °F)
U = U-value of material (BTU/hr·ft²·°F)
V = Room volume (ft³)
ACH = Air changes per hour
In imperial units, heat loss is calculated in BTU per hour (BTU/hr) using surface area, temperature difference, U-value, room volume, and air changes per hour. This formula helps you determine heat loss from the space.
Metric Equation
Following the formula below, you can calculate the heat loss:
Components:
Q = Heat loss (Watts)
A = Surface area (m²)
ΔT = Temperature difference (Indoor - Outdoor, °C)
U = U-value of material (W/m²K)
V = Room volume (m³)
ACH = Air changes per hour
In metric units, heat loss is calculated in Watts (W) using the same core variables. The result shows how much heat is lost from the space.
Heat Loss Calculations In h2x
h2x simplifies complex heat loss calculations into an easy-to-use tool that improves both speed and accuracy.
See why engineers and contractors use h2x to calculate heat loss:
Smart Layouts: Draw and adjust systems quickly with auto-connected components.
Verified Sizing: MCS-aligned calculations ensure confidence in your design.
Easy Exports: Download branded PDFs or AutoCAD and Revit files in one click.
Detailed Reports: Get heat loss data to system design and sizing in h2x.
