Heat Loss Calculator
Estimate your home's design heat loss. Enter the floor area, ceiling height and the indoor and outdoor design temperatures, then pick your insulation and air-sealing level — and get the heat loss in BTU/hr, the loss per square foot, and a recommended furnace size at your efficiency. Includes a full breakdown, by-temperature and by-insulation tables, and a downloadable PDF.
| Outdoor | ΔT | Heat loss | BTU / ft² |
|---|---|---|---|
| Enter a space to compare | |||
| Insulation | U-factor | Heat loss | BTU / ft² |
|---|---|---|---|
| Enter a space to compare | |||
How to use the heat loss calculator
Four quick steps turn your home’s size and winter conditions into a design heat loss in BTU per hour and a recommended furnace size — with by-temperature and by-insulation tables and a downloadable report.
Enter the space
Give the heated floor area and the average ceiling height.
Set the temperatures
Enter your indoor setpoint and your area’s outdoor winter design temperature.
Pick insulation & sealing
Choose your insulation level and how tight or drafty the home is, then add your furnace AFUE.
Read results & save a PDF
Get heat loss, BTU per sq ft and a furnace size, compare temperatures and insulation — then download a report.
How the heat loss is calculated
Floor area times a U-factor for your insulation, adjusted for air sealing and ceiling height, times the indoor-to-outdoor temperature difference — then divided by furnace efficiency for the input size. Here is the exact math the calculator runs.
Worked example — 1,500 sq ft home, 8 ft ceilings, 70°F inside, 20°F outdoor design, average insulation and sealing, 80% AFUE
ΔT: 70 − 20 = 50°F · Adjusted U: 0.7 × 1.0 = 0.7 · Ceiling factor: 8 ÷ 8 = 1.0
Heat loss: 1,500 × 0.7 × 1.0 × 50 = 52,500 BTU/hr · Per sq ft: 52,500 ÷ 1,500 = 35
Furnace input: 52,500 ÷ 0.80 ≈ 65,625 BTU · Recommended: round up → 80,000 BTU
≈ 52,500 BTU/hr heat loss → an 80,000 BTU furnace (about 64,000 BTU/hr output at 80% AFUE).
Heat loss reference charts
Handy lookups for how outdoor temperature and insulation change the heat loss, plus the adjustments the calculator applies. Always confirm equipment size with a Manual J.
| Outdoor | ΔT | Heat loss | BTU / sq ft | Furnace (80% AFUE) |
|---|---|---|---|---|
| 40°F | 30°F | 31,500 | 21 | 40,000 BTU |
| 30°F | 40°F | 42,000 | 28 | 60,000 BTU |
| 20°F | 50°F | 52,500 | 35 | 80,000 BTU |
| 10°F | 60°F | 63,000 | 42 | 80,000 BTU |
| 0°F | 70°F | 73,500 | 49 | 100,000 BTU |
| −10°F | 80°F | 84,000 | 56 | 120,000 BTU |
| −20°F | 90°F | 94,500 | 63 | 120,000 BTU |
1,500 sq ft, 8 ft ceilings, 70°F indoor, average insulation and air sealing. Heat loss in BTU/hr.
| Insulation | U-factor | Heat loss | BTU / sq ft |
|---|---|---|---|
| Well insulated | 0.5 | 37,500 | 25 |
| Average | 0.7 | 52,500 | 35 |
| Poorly insulated | 1.0 | 75,000 | 50 |
1,500 sq ft, ΔT 50°F, average air sealing. Going from poor to well insulated halves the heat loss.
| Factor | Adjustment |
|---|---|
| Air sealing | Tight −10% · Average 0% · Leaky +15% |
| Ceiling height | Heat loss × (height ÷ 8 ft) |
| Furnace efficiency | Input = heat loss ÷ AFUE |
| Furnace sizes | 40k, 60k, 80k, 100k, 120k, 140k BTU input |
Rule-of-thumb factors. A Manual J models each wall, window, ceiling and floor separately.
Everything the calculator works out
Floor area and a few winter conditions give you the full heating picture — heat loss, loss per square foot and a furnace size — with side-by-side comparisons.
Key figures behind the estimate
Built for anyone heating a home
From replacing a furnace to planning upgrades, the same calculator turns a home’s size and climate into a clear heating load.
Getting quotes for a new furnace and wants to know whether the size contractors suggest makes sense.
- Enter your floor area
- Use your design temperature
- Compare to your quotes
Weighing insulation or air sealing and wants to see how much heat loss each upgrade would save.
- Switch insulation levels
- Try tight vs leaky
- Read the by-insulation table
Considering a heat pump and needs the design heat loss to check a model’s capacity at cold temperatures.
- Set your coldest design temp
- Note the BTU/hr loss
- Match it to cold-climate output
7 things to know about heat loss
A few details keep a heat loss estimate realistic and the home warm without wasting energy.
Heat loss calculator FAQ
The heat loss, insulation and furnace sizing questions people ask most about heating a home.
Multiply the heated floor area by a U-factor for your insulation level, adjust for air sealing and ceiling height, then multiply by the temperature difference between inside and outside on a cold design day. The result is the heat loss in BTU per hour.
For example, a 1,500-square-foot home with average insulation (a U-factor of about 0.7), 8-foot ceilings, a 70°F indoor setpoint and a 20°F outdoor design temperature loses about 1,500 × 0.7 × 50, or 52,500 BTU per hour. That is the amount of heat your furnace or heat pump must supply to hold 70°F on the coldest typical day.
Treat this as a planning estimate. A Manual J calculation by a licensed HVAC contractor models each wall, window and ceiling separately for an exact figure.
The outdoor design temperature is the coldest temperature your area typically sees in winter, usually the 99 percent design temperature, which is only colder about 1 percent of the hours in a year. Heating systems are sized for this temperature rather than the all-time record low.
Roughly speaking, it is around 40°F in the Deep South, about 20°F in mid-latitude states and 0°F or below in the far North and mountain regions. Your HVAC contractor or local building department can give you the exact figure.
The colder the design temperature, the bigger the gap from your indoor setpoint, and the higher the heat loss.
It depends on your insulation and climate, which is why a single number is misleading. Using this calculator’s method with a 50°F temperature difference, a well-insulated home loses about 25 BTU per square foot per hour, an average home about 35, and a poorly insulated home about 50.
In colder climates the figure rises: an average home with a 70°F difference loses about 49 BTU per square foot, and with a 90°F difference about 63. Taller ceilings and leaky construction push it higher, while good air sealing brings it down.
Enter your own details in the calculator to see the figure for your home.
Start with the design heat loss, then divide by the furnace efficiency to get the input rating, and round up to the next common size.
A 1,500-square-foot home with average insulation and a 50°F temperature difference loses about 52,500 BTU per hour. An 80 percent AFUE furnace needs about 65,600 BTU of input to cover that, so the calculator recommends an 80,000 BTU furnace, which puts out about 64,000 BTU per hour. Furnaces are commonly sold in 40,000, 60,000, 80,000, 100,000, 120,000 and 140,000 BTU input sizes.
Avoid oversizing, which causes short, inefficient cycles and uneven heat, and confirm the size with a Manual J calculation.
AFUE stands for Annual Fuel Utilization Efficiency. It is the percentage of the fuel a furnace burns that ends up as heat in your home over a typical season.
An 80 percent AFUE furnace turns 80 percent of its fuel into heat and loses the rest up the flue, so a 100,000 BTU input furnace delivers about 80,000 BTU per hour. High-efficiency condensing furnaces reach 90 to 98 percent AFUE.
Because the heat loss has to be covered by the furnace output, a higher-efficiency furnace needs a smaller input rating to heat the same home. That is why this calculator asks for your AFUE when recommending a furnace size.
Insulation has one of the largest effects on heat loss. In this calculator, a well-insulated home uses a U-factor of about 0.5, an average home about 0.7, and a poorly insulated home about 1.0.
For a 1,500-square-foot home with a 50°F temperature difference, that works out to about 37,500 BTU per hour well insulated, 52,500 average and 75,000 poorly insulated, so going from poor to well insulated cuts the heat loss in half. Attic insulation is usually the cheapest place to start, since heat rises and attics are easy to reach.
Lower heat loss means lower heating bills and often a smaller, cheaper furnace or heat pump.
Yes. Warm air escaping through gaps around windows, doors, outlets, attic hatches and plumbing penetrations is replaced by cold outside air that has to be heated.
This calculator reduces the heat loss by about 10 percent for a tight, well-sealed home and increases it by about 15 percent for a leaky, drafty one. Air sealing is often one of the most cost-effective upgrades, and it works best done before adding insulation.
A blower door test from an energy auditor can measure exactly how leaky your home is and find the biggest gaps.
No. This calculator uses a single rule-of-thumb U-factor that bundles the walls, roof, floor, windows and air leakage together, which makes it fast and good for planning.
A Manual J calculation, the industry-standard method from the Air Conditioning Contractors of America (ACCA), measures each surface separately, including window sizes and types, wall and attic R-values, foundation type, duct location and your exact local design temperature. Many building codes and rebate programs require a Manual J before a new heating system is installed.
Use this calculator for a ballpark figure, then ask your HVAC contractor for a Manual J to confirm the equipment size.
Estimate only — not a substitute for a Manual J. This calculator estimates design heat loss in US units (°F, BTU/hr): floor area × U-factor × ceiling factor × (indoor − outdoor temperature). The U-factor is a rule-of-thumb value of 0.5, 0.7 or 1.0 by insulation level, adjusted −10% for a tight home or +15% for a leaky one, and the ceiling factor is height ÷ 8 ft. The furnace input is the heat loss divided by AFUE, rounded up to a common 40,000–140,000 BTU size. Real heat loss depends on windows, wall and attic R-values, foundation, ductwork and your exact local design temperature. Have a licensed HVAC contractor perform an ACCA Manual J calculation before you buy or install heating equipment — oversizing causes short-cycling, wasted fuel and uneven heat. For planning purposes only.