Heat loss calculator
The heat loss of a house is the heat that escapes through its walls, ceiling, floor, windows and doors, plus the heat needed to warm the outside air that leaks in, on a cold design day. Enter the areas, the insulation and the temperatures, and the calculator adds the parts up and shows which one loses the most.
- Heat loss on the design day32,400 BTU/h
- Per square foot16.2 BTU/h
- Walls, 16%5,000 BTU/h
- Ceiling, 10%3,200 BTU/h
- Floor, 19%6,300 BTU/h
- Windows, 25%8,100 BTU/h
- Doors, 4%1,200 BTU/h
- Air leakage, 27%8,600 BTU/h
Temperature difference: 70 °F indoors minus 10 °F outdoors = 60 °F. Each part is its area times its U-factor, or its area divided by its R-value, times that difference.
Outside wall length was estimated at 179 ft from a square footprint, which gives 1,091 sq ft of wall after windows and doors. A real house has more wall than a square one, so measure the length for a better answer.
Air leakage: 10.0 ACH50 divided by 20 gives 0.50 natural air changes per hour, which is 133 CFM of outside air to heat.
A furnace that only heats should deliver between 100% and 140% of the heat loss as output: 32,400 BTU/h to 45,400 BTU/h.
This is an estimate of the heating load. It is not a Manual J calculation.
Results update as you type. Nothing you enter leaves your browser.
What outdoor temperature should I use?
Use the heating design temperature for your county: the outdoor temperature that the local weather stays above for 99% of the hours in a year. Heating equipment is sized for that temperature, not for the coldest night on record. ENERGY STAR publishes a design temperature limit for every US county: the coldest 99% heating temperature among the weather stations in the county or within 40 miles of its center. A designer of an ENERGY STAR home may not use a colder temperature than that. The figure for the weather station nearest you can be a few degrees warmer.
Design temperature by county
3,146 counties, from the ENERGY STAR Design Temperature Limit Reference Guide, 2019 edition.
The indoor temperature for sizing is 70 °F. ACCA Manual J and the ENERGY STAR design report both use 70 °F for heating and 75 °F for cooling.
How is heat loss calculated?
Each part of the outside shell loses heat in proportion to its area and to the temperature difference across it:
Heat loss (BTU/h) = area (sq ft) × temperature difference (°F) ÷ R-value
Windows and doors are rated by U-factor, which is 1 divided by the R-value, so their loss is U-factor × area × temperature difference. A slab floor loses its heat at the edge, so it uses a factor per foot of perimeter, the F-factor: F × perimeter × temperature difference.
Outside air that leaks in has to be heated as well:
Heat loss (BTU/h) = 1.08 × CFM × temperature difference (°F)
The 1.08 is the heat one cubic foot of sea-level air holds per degree, 0.075 pounds times 0.24 BTU per pound, times 60 minutes. Air is thinner at elevation, so the calculator lowers the 1.08 in step with air pressure when you enter an elevation.
What R-values and U-factors should I enter?
Enter the R-value of the whole wall, ceiling or floor, not only of the insulation in it. These published figures give the range:
- A 2x4 wall with no insulation is about R-4. Pacific Northwest National Laboratory modeled one at R-4.2.
- A house from the 1970s or 1980s: ENERGY STAR’s baseline for that era has R-11 batts in the walls and R-13 to R-15 in the ceiling.
- A house built to the 2021 energy code may not exceed the U-factors in the table below. The R-value next to each one is 1 divided by the U-factor.
| Climate zone | Wall | Ceiling | Floor | Window |
|---|---|---|---|---|
| 1 | U 0.084 (R-12) | U 0.035 (R-29) | U 0.064 (R-16) | U 0.5 |
| 2 | U 0.084 (R-12) | U 0.026 (R-38) | U 0.064 (R-16) | U 0.4 |
| 3 | U 0.06 (R-17) | U 0.026 (R-38) | U 0.047 (R-21) | U 0.3 |
| 4, except marine | U 0.045 (R-22) | U 0.024 (R-42) | U 0.047 (R-21) | U 0.3 |
| 5 and 6, and marine 4 | U 0.045 (R-22) | U 0.024 (R-42) | U 0.033 (R-30) | U 0.3 |
| 7 and 8 | U 0.045 (R-22) | U 0.024 (R-42) | U 0.028 (R-36) | U 0.3 |
Maximum assembly U-factors for wood-frame construction, 2021 International Energy Conservation Code, Table R402.1.2. Your building department can tell you the climate zone and the code edition in force.
Windows, doors and slabs
| Choice in the calculator | Factor | Source |
|---|---|---|
| Single pane | U 1.13 | DOE Building America Solution Center |
| Double pane, clear glass | U 0.45 | DOE Building America Solution Center |
| Double pane, low-e (2021 energy code maximum) | U 0.3 | 2021 IECC, climate zones 3 to 8 |
| ENERGY STAR for the northern zone | U 0.22 | ENERGY STAR window criteria |
| ENERGY STAR solid door | U 0.17 | ENERGY STAR door criteria, maximum for an opaque door |
| ENERGY STAR door with glass | U 0.26 | ENERGY STAR door criteria, more than half glass, northern zones |
| Older door, not insulated (our assumption) | U 0.5 | No government figure was found for old doors. This is our assumption. |
| Slab on grade, no edge insulation | F 0.73 | 2024 International Energy Conservation Code |
| Slab on grade, R-10 edge insulation 2 ft deep | F 0.54 | 2024 International Energy Conservation Code |
| Slab on grade, R-10 edge insulation 4 ft deep | F 0.48 | 2024 International Energy Conservation Code |
ENERGY STAR’s baseline for an existing double-pane window in a wood frame is U 0.52, a little worse than the U 0.45 that the calculator uses for clear double glass.
How leaky is my house?
Air leakage is measured with a blower-door test and stated in air changes per hour at a pressure of 50 pascals, written ACH50. If you have a test report, enter its number. If not, these figures give the range:
- 3 ACH50: the limit for new houses in climate zones 3 to 8 under the 2012 to 2021 editions of the energy code. It is 5 in zones 1 and 2.
- About 17 to 18 ACH50: ENERGY STAR’s baseline for a house built from 1970 to 1989 that has not been air sealed.
- 3 to 40 ACH50: the spread across measured US single-family houses, from the 5th to the 95th percentile, in Lawrence Berkeley National Laboratory’s leakage database.
The test number is far above the leakage of a house in normal weather. The calculator divides it by 20 to estimate the natural air changes per hour, and by less for a taller house: 18 for one and a half stories, 16 for two stories, 14 for three stories. The rule comes from Lawrence Berkeley Laboratory.
Treat this part of the result as rough. The rule gives a year-round average, and a house leaks more than its average on the coldest, windiest days. A later review by NIST and Berkeley Lab calls the divide-by-20 rule a rule of thumb with no physical basis that suits rough estimates only.
How big should the furnace be?
Close to the heat loss, and not far above it. ENERGY STAR’s design report for new homes limits a furnace that only heats to between 100% and 140% of the heat loss, measured as output. The US Department of Energy’s sizing guide gives the same 140% ceiling. A furnace’s output is its input times its efficiency, so use the furnace size calculator to turn the output into the input rating printed on a furnace. To read the input of the furnace you have now, use the model number decoder.
Is this a Manual J load calculation?
No. It uses the same physics as the heating side of ACCA Manual J, but it treats the house as one box. A Manual J calculation works room by room, uses tested construction data for every wall, window and door, and adds duct losses and ventilation. It also produces the cooling load, which depends on sun, shading and humidity and cannot be reduced to a few areas and R-values. No government source publishes a simple whole-house cooling method.
What this calculator leaves out or simplifies:
- The walls and floor of a heated basement are not counted.
- A framed floor is charged with the full indoor-to-outdoor temperature difference, although a crawl space is usually warmer than the outside air. That makes the floor figure high.
- Heat lost from ducts in an attic, garage or crawl space is not counted.
- Heat given off by people, appliances and the sun is ignored, as it is in a design heating load.
For a first estimate of cooling size, use the AC tonnage calculator.
Sources
- ENERGY STAR, Design Temperature Limit Reference Guide, 2019 edition, revised April 27, 2021: https://www.energystar.gov/sites/default/files/asset/document/Design%20Temperature%20Limit%20Reference%20Guide%20%282019%20Ed%29%20-%20ENERGY%20STAR%20SFNH%20Rev11_MFNC%20Rev02.pdf
- ENERGY STAR, National HVAC Design Report: indoor design temperatures and furnace sizing limits: https://www.energystar.gov/sites/default/files/2025-01/National%20HVAC%20Design%20Report_Rev%2014.pdf
- US Department of Energy, Building America Solution Center, introduction to heat flow: the heat loss equation, R-value and U-factor, window U-factors: https://basc.pnnl.gov/information/building-science-introduction-heat-flow
- US Department of Energy, Building America, Strategy Guideline: HVAC Equipment Sizing: furnace within 140% of the heating load, air constant falls with elevation: https://www1.eere.energy.gov/buildings/publications/pdfs/building_america/strategy_guide_hvac_sizing.pdf
- Texas A&M University, course notes on heat loss from outside air: BTU/h = 1.08 × CFM × temperature difference, and CFM from air changes: https://people.tamu.edu/~i-choudhury/335_5.html
- National Renewable Energy Laboratory, BESTEST-EX: specific heat and sea-level density of air: https://www.osti.gov/servlets/purl/1218840
- International Code Council, 2021 International Energy Conservation Code, chapter 4: maximum U-factors and air leakage limits: https://codes.iccsafe.org/content/IECC2021P2/chapter-4-re-residential-energy-efficiency
- International Code Council, 2024 International Energy Conservation Code, chapter 4: slab F-factors: https://codes.iccsafe.org/content/IECC2024V1.0/chapter-re-4-residential-energy-efficiency
- ENERGY STAR, baseline home assumptions for existing homes: insulation, window and air leakage of a 1970 to 1989 house: https://energystar.gov/sites/default/files/asset/document/Baseline_Home_Assumptions.pdf
- Pacific Northwest National Laboratory, wall retrofit study: uninsulated 2x4 wall at R-4.2: https://www.osti.gov/servlets/purl/1890229
- ENERGY STAR, certification criteria for residential windows: https://www.energystar.gov/sites/default/files/asset/document/ES_Windows_Table_Oct2023.pdf
- ENERGY STAR, certification criteria for residential doors: https://www.energystar.gov/sites/default/files/asset/document/ES_Doors_Table_Oct2023.pdf
- Lawrence Berkeley National Laboratory, analysis of air leakage measurements of US houses: https://eta-publications.lbl.gov/sites/default/files/chan_analysis_of_air_leakage.pdf
- Sherman, Lawrence Berkeley Laboratory, estimation of infiltration from leakage and climate indicators (1987): https://eta-publications.lbl.gov/sites/default/files/estimation_of_inflitration_from_leakage_and_climate_indicators.pdf
- National Institute of Standards and Technology, review of the divide-by-20 rule: https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=920716
- NOAA, NASA and US Air Force, US Standard Atmosphere 1976, for air pressure at elevation: https://ntrs.nasa.gov/citations/19770009539
Related
For a quick answer from floor area alone, use the BTU calculator. To find the airflow a furnace needs, use the CFM calculator. To date the furnace you have, use the furnace age lookup. For the full method a contractor uses, read what a Manual J load calculation is.
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Calculator last reviewed on October 5, 2026. Method and sources.