Heat Loss Calculator (BTU/h)

Estimate steady-state heat loss through entered walls, windows, doors and suspended floors, plus natural air leakage. Review component losses and explicit exclusions.

Indoor temperature (°F)

Outdoor design temperature (°F)The 99% heating design temperature for your location, not the record low.

Wall area (excluding windows and doors) (sq ft)

Wall R-valueUse a whole-assembly imperial R-value. Framing can materially reduce resistance below the insulation batt rating.

Window area (sq ft)

Window U-factorFrom the NFRC label. Lower is better.

Exterior door area (sq ft)A standard door is about 20 sq ft.

Door U-factor

Ceiling area (under attic or roof) (sq ft)

Ceiling R-value

Floor area over unheated space (sq ft)

Floor R-value

Temperature below the floor (°F)Temperature of the unheated space below a suspended floor. Ground slabs need a different perimeter/ground model.

Heated volume (cu ft)Floor area × ceiling height.

Air changes per hour (natural leakage)Rough guide: about 0.2–0.35 for a tight newer house, 0.5–1 or more for an older one.

Conditioned floor area for intensity (optional) (sq ft)0 omits the per-square-foot result. Enter actual served floor area; envelope floor/ceiling area can cover only part of a multistory building.

This calculator estimates steady-state heat loss through the building components you enter, plus natural air leakage, at a chosen indoor/outdoor temperature difference. Its breakdown helps compare thermal assumptions and identify larger losses. It does not rank retrofit payback, establish a complete building peak or certify that a heating appliance will meet the load.

How to use this tool

Enter the indoor temperature and your winter design temperature.
Enter the net area and R-value of walls, ceiling and floor, and the area and U-factor of windows and doors.
Set the temperature under the floor.
Enter heated volume and natural leakage; optionally enter actual conditioned floor area for an intensity result.
Read the total and check the table for the largest losses.

Formula

Q = Σ (A × ΔT ÷ R) + Σ (U × A × ΔT) + 0.018 × V × ACH × ΔT

Q is BTU/h, A is ft², R is ft²·°F·h/BTU and U is its reciprocal. The leakage factor 0.018 equals assumed air density 0.075 lb/ft³ × specific heat 0.24 BTU/lb·°F. The suspended floor uses its own temperature difference; warmer space below gives zero loss. Metric R/U values require conversion.

Example

With 70 °F inside and 10 °F outside (ΔT = 60 °F): 1,200 sq ft of R-13 wall loses 5,538 BTU/h, 200 sq ft of U-0.30 windows 3,600, two doors totalling 40 sq ft at U-0.40 960, a 1,500 sq ft R-38 ceiling 2,368, and a 1,500 sq ft R-19 floor over a 50 °F basement 1,579. Air leakage at 0.5 ACH in 12,000 cu ft adds 6,480. The total is 20,526 BTU/h (6.02 kW), and air leakage is the largest single item at 32%.

Frequently asked questions

What design temperature should I use?

Use the local 99% heating design temperature: colder conditions occur during roughly 88 hours of an average year. It is a design percentile, not the historical record low.

How do I find my air changes per hour?

Use a natural leakage estimate for the design conditions. ACH50 is a pressure-test result, not natural ACH; climate, shielding, height and wind change the conversion. A universal divide-by-20 rule cannot reliably produce design leakage.

Which R-value should I use for walls?

Use whole-assembly R-values or reciprocal U-factors, including framing and relevant layers. Insulation batt ratings alone can materially overstate a wall’s resistance. Window U-factors should be whole-product imperial ratings.

Does this include duct losses?

Ducts, mechanical ventilation, basement walls and ground-slab edges are excluded. The floor calculation applies to a suspended floor over a space of known temperature. A complete load calculation must address omitted components before equipment selection.

Sources

Last updated: 2026-10-30