Heating BTU Calculator: Estimate Your Home's Heat Loss

How heat loss is calculated

Heat leaves a house in two ways: conduction through walls, roof, floor and windows, and air leakage, where cold outside air replaces warm inside air. Both are proportional to the temperature difference between inside and outside. A professional ACCA Manual J calculation adds up each surface (area ÷ R-value, or area × U-value) and the infiltration rate. This calculator keeps the two weakest surfaces, windows and doors, separate and collapses everything else into a single loss factor for the opaque envelope.

ΔT = Indoor − Outdoor design
UA envelope = Area × (Ceiling ÷ 8) × Loss factor × Floor type
UA windows = Window area × Window U-value
UA doors = Doors × 20 sq ft × 0.4
Heat loss (BTU/h) = (UA envelope + UA windows + UA doors) × ΔT

U-value is the inverse of R-value: the BTU/h that pass through one square foot for each °F of difference. Typical whole-window values are about 1.1 for single pane, 0.5 for clear double pane, 0.35 for double pane with a low-E coating and 0.25 for triple pane. An insulated steel or fiberglass entry door is around 0.4. The floor-type multiplier nudges the envelope figure: a vented crawlspace or a room over an unheated garage loses more through the floor than a slab, and a heated basement loses a little less because the soil around it stays well above outdoor air temperature.

UA is the house's heat loss per degree of temperature difference. It is useful well beyond sizing: multiply it by any outdoor temperature to see the load on that day, which is how the heat pump sizing calculator finds the balance point.

Worked example

The calculator's defaults: a 2,000 sq ft slab-on-grade house with 8-foot ceilings, average insulation (0.27), 300 sq ft of clear double-pane windows, two exterior doors, kept at 70 °F in a location with a 5 °F design temperature:

1. ΔT = 70 − 5 = 65 °F
2. UA envelope = 2,000 × (8 ÷ 8) × 0.27 × 1.0 = 540 BTU/h·°F
3. UA windows = 300 × 0.5 = 150 BTU/h·°F
4. UA doors = 2 × 20 × 0.4 = 16 BTU/h·°F
5. UA total = 706 BTU/h·°F
6. Heat loss = 706 × 65 = 45,890, shown as 45,900 BTU/h

That is 13.4 kW, or about 23 BTU/h per square foot, and 10,800 BTU/h of it (about 24%) leaves through windows and doors, which make up a small fraction of the surface area. Swap to double-pane low-E glass and the window UA drops to 105, cutting the total to about 43,000 BTU/h; with single-pane windows it would be about 57,600. Upgrading the walls and attic to the "good" level (0.20) with the original windows brings it to about 36,800 BTU/h. A heater sized for the old figure would then be roughly 25% too big.

Outdoor design temperatures

The design temperature is not the record low. It is the temperature that the location stays above for about 99% of the hours in a typical year (the ASHRAE 99% heating design value). Approximate values:

Use your local building department's value if you can get it; codes often publish a design temperature per county.

Choosing the loss factor

The loss factor covers walls, roof, floor and air leakage, but not windows or doors, which you enter separately.

• 0.40 – poor: uninsulated or lightly insulated walls, a thin attic, a leaky attic hatch and drafts you can feel.
• 0.27 – average: most 1970s–1990s houses, perhaps with an attic top-up.
• 0.20 – good: code-built in the last 15–20 years, or an older house with dense-pack walls, R-38 or more in the attic and air sealing done.
• 0.14 – very good: blower-door-tested tight construction and continuous exterior insulation.

A cathedral ceiling or a slab with no edge insulation pushes a house toward the higher factor. Count window area from the rough sizes of each window (width × height) and include sliding and French doors as glass. The insulation R-value chart lists target levels by climate zone, and what R-value means explains how each layer contributes.

Turning heat loss into equipment size

The heat loss is the output the heating system must deliver at the design temperature.

• Gas or propane furnace: divide by AFUE to get the input rating, then pick the next standard size. The furnace sizing calculator does this step.
• Heat pump: check the unit's capacity at your design temperature, not its 47 °F rating, and plan backup heat for any shortfall.
• Electric resistance or boiler: divide BTU/h by 3,412 to get kW for baseboard or an electric furnace.

The heat-loss figure is also the starting point for estimating seasonal fuel use. UA × 24 × heating degree days gives a rough annual load, which the heating cost calculator turns into dollars.

Where the heat goes in a typical house

For an average existing home, a rough breakdown of design heat loss looks like this, though every house differs:

That is why air sealing usually gives the best return before new equipment: closing gaps around attic penetrations, rim joists and doors lowers the loss factor for every hour of the heating season. Attic insulation is next, as it is usually the cheapest surface to reach. Re-run the calculator with a lower factor after upgrades to see how much smaller the replacement system can be.

Limits of a whole-house estimate

A single loss factor cannot see a cold bonus room over a garage or a basement that sits partly below grade. Use this result to check a contractor's quote or to compare upgrades, not to size ducts room by room. If your estimate and a quote differ by more than about 25%, ask to see the Manual J inputs; wrong window counts and default infiltration settings are common sources of error.

Frequently asked questions