Updated Home Energy Tool

Heat Loss Calculator – Room & Whole-House BTU/hr Estimates

Estimate heat loss for a single room or your entire house. Combine walls, windows, doors, roof, floor and air infiltration, switch between Imperial and Metric units, and size your heating system in BTU/hr or kW.

Room & Whole-House Modes BTU/hr & Watts Windows, Doors & Infiltration Heating System Sizing

Interactive Heat Loss Calculator

Use the tabs to estimate simple room heat loss, full-house envelope losses, window and door losses, air infiltration and approximate heating system size. Choose Imperial or Metric units and adjust U-values, areas and temperature difference to match your building and climate.

All tabs use this unit choice. Internally, the calculator keeps results physically consistent.
Imperial: ft², Metric: m².
Imperial: °F, Metric: °C.
Imperial: BTU/hr·ft²·°F, Metric: W/m²·K. You can approximate by averaging wall, window and roof U-values.

This mode assumes a single representative U-value for the combined room envelope (walls, windows, ceiling, floor). For more detail, use the full-house and openings tabs.

Enter approximate exposed areas and U-values for each part of the building envelope. If some areas are unknown, you can leave them at zero and focus on the components you know best.

Imperial: ft², Metric: m².

This tab focuses on windows and doors only. Use it to see the impact of upgrading windows or doors on total envelope heat loss.

Imperial: ft³, Metric: m³.
Higher ACH means more infiltration and ventilation loss.

Infiltration and ventilation can be a major part of total heat loss, especially in older or leaky buildings. This tab uses a common approximation based on air changes per hour and room or house volume.

Imperial: BTU/hr; Metric: Watts. You can bring this from other tabs.
Adds margin for extreme conditions and warm-up. Common values: 10–25%.

This tab treats your total design heat loss as the required output to the building. It then applies a safety factor and accounts for system efficiency to estimate input capacity and approximate annual energy use and cost.

Heat Loss Calculator – Plan Insulation, Windows and Heating Capacity

Heat loss is a key design number for any home or building that needs heating. It tells you how much heat you must deliver to maintain a comfortable indoor temperature when it is cold outside. This Heat Loss Calculator helps you estimate losses through surfaces like walls, windows, roof and floor, as well as losses due to air infiltration.

Instead of relying on a single rule-of-thumb BTU/hr per square foot, this tool breaks heat loss down into separate components. That makes it easier to see which upgrades—such as better windows, more attic insulation or reduced air leakage—will have the biggest impact on comfort and energy bills.

How the Heat Loss Calculator is Organized

The calculator is divided into five modes that match common planning questions:

  • Simple Heat Loss: A quick room-level estimate using area, an overall U-value and temperature difference.
  • Full-House Heat Loss: Component-based calculation for walls, windows, doors, roof and floor.
  • Window & Door Loss: Focused look at openings, including typical U-values by window and door type.
  • Air Infiltration Loss: Estimate losses from air changes per hour (ACH) and building volume.
  • Heating System Sizing: Convert total design heat loss to required system capacity and energy use.

A single unit selector lets you choose between Imperial (ft², °F, BTU/hr) and Metric (m², °C, Watts). The calculator converts internally so you can compare results in both BTU/hr and Watts where it is useful.

Mode 1: Simple Heat Loss for a Room or Zone

In the simple mode you enter the room area, indoor and outdoor design temperatures and an overall U-value. The calculator multiplies U × area × ΔT to estimate heat loss and then converts between Watts and BTU/hr. If you supply heating hours per day, heating days per year and an energy price per kWh, it also estimates seasonal energy use and cost.

Mode 2: Full-House Heat Loss by Component

The full-house mode separates the building envelope into five parts: walls, windows, doors, roof/ceiling and floor. For each component you provide an exposed area and U-value. The tool calculates heat loss for each part and the total, in your chosen unit system and in the alternate unit for easy comparison.

This mode is useful when you have building plans, surface areas or U-values from energy reports or manufacturer data sheets. It also helps you see which components dominate heat loss so you can target upgrades efficiently.

Mode 3: Window & Door Heat Loss

Windows and doors often represent a large share of envelope heat loss. In this tab you enter total window and door area and choose typical window and door types. The calculator applies representative U-values for single-, double- and triple-glazed windows and for different door constructions, or you can enter your own custom U-values.

The results show window heat loss, door heat loss and the combined total. You can experiment with different window types to see how upgrading glazing or frames affects the overall result.

Mode 4: Air Infiltration Heat Loss

Even with well-insulated walls and windows, unwanted air leakage can carry away substantial heat. The infiltration tab uses a standard approximation based on air changes per hour (ACH), the volume of the house or room and the temperature difference between indoors and outdoors.

You can also include heating hours per day, heating days per year and an energy price per kWh. This allows the calculator to estimate how much infiltration alone might contribute to annual heating costs, highlighting the value of air sealing and controlled ventilation.

Mode 5: Heating System Sizing

After you have estimated total heat loss from the other tabs, the sizing mode helps translate that into the capacity of a furnace, boiler or heat pump. You enter:

  • Total design heat loss (BTU/hr or Watts, depending on unit system)
  • A safety or oversizing factor, typically 10–25%
  • Estimated system efficiency
  • Heating hours per year and energy price

The calculator reports the required output and input capacity, output in kW, and estimates annual energy use and cost. Final equipment selection should still be done by a qualified heating professional who can account for local codes, equipment options and detailed building characteristics.

Tips for Using the Heat Loss Calculator Effectively

  • Start with the Simple Heat Loss tab to get a quick sense of required BTU/hr for a room or zone.
  • Use the Full-House and Openings tabs to refine estimates and explore upgrade scenarios.
  • Check Air Infiltration if you suspect drafts or know your building has high ACH values.
  • Combine the results in the System Sizing tab to plan approximate heating capacity and energy costs.
  • Remember that real buildings are dynamic; this tool assumes steady-state conditions for simplicity.

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Heat Loss & Heating FAQs

Frequently Asked Questions About Heat Loss and Heating Capacity

Use these answers as a quick reference while you experiment with different scenarios in the calculator.

You’ll get the best results if you know approximate areas for walls, windows, doors, roof and floor, plus reasonable U-values for each component. If those are not available, you can start with typical values for your building type or energy report data and refine over time.

Usually, systems are sized with some safety margin above the calculated design heat loss to handle colder-than-expected conditions and warm-up after setbacks. The sizing tab lets you apply a safety factor, but final decisions should be reviewed by a heating professional familiar with your climate and building.

In some buildings, large roof or wall areas with poor insulation can contribute more to heat loss than windows. Component-based calculations show which surfaces dominate losses, so you can target upgrades that deliver the biggest reduction per dollar spent.

The underlying formulas are general, so you can use them for many building types. However, complex commercial or industrial projects often need more detailed modeling and ventilation analysis, which goes beyond the scope of this tool.

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