Heat loss calculator
Areas and R-values in, BTU/h out, for one room or the whole house. Conduction is U × A × ΔT, air leakage is 1.08 × CFM × ΔT, and the breakdown shows where the heat goes.
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- Runs in your browser
- Estimate: shows its formulas
What are you calculating?
Switching loads an example you can edit.
Design temperatures
99% heating and 1% cooling dry bulb, from ASHRAE Handbook—Fundamentals design conditions via Climate.OneBuilding.org. Your local code may set its own values.
Surfaces
U = 1 ÷ R Enter an R-value or switch the row to a U-factor. Whole-assembly values (framing included) are more accurate than nominal insulation R-values.- ft²R-value9,540 BTU/h
- ft²U-factor6,110 BTU/h
- ft²R-value510 BTU/h
- ft²R-value1,670 BTU/h
- ft²R-value1,670 BTU/h
Air leakage
Volume 16,000 ft³ · 94 CFM of outside air
Design heat loss
Whole house · estimate
- Total heat loss
- 24,800 BTU/h
- 7.3 kW · ΔT 63.6°F
- Per ft² of floor
- 12.4
- BTU/h·ft²
- UA (whole)
- 390
- BTU/h·°F; load at any temp = UA × ΔT
Where the heat goes
- Walls9,540 · 38%
- Windows6,110 · 25%
- Air leakage5,300 · 21%
- Ceiling / roof1,670 · 7%
- Floor1,670 · 7%
- Doors510 · 2%
Steady-state conduction plus air leakage. No credit for sun or internal gains, no duct losses, and no below-grade detail.
How to calculate heat loss
Design heat loss is the heat a building loses on a cold day at the outdoor design temperature, with no help from the sun, people or appliances. It is steady-state physics, as set out in the ASHRAE Handbook—Fundamentals, and it comes down to two terms.
Conduction through each surface
Every wall, window, door, ceiling and floor gets its own line. Windows are rated by U-factor already; for insulated assemblies, a whole-assembly R-value (framing included) is more honest than the insulation's label. A slab on grade loses most of its heat at the edge, so it is counted per foot of exposed perimeter with an F-factor instead of per square foot.
Air leakage
CFM is outside air leaking in. From a blower-door test, natural leakage is roughly ACH50 ÷ N (the LBL N-factor, about 10 to 25), and CFM = ACH × volume ÷ 60. The 1.08 factor assumes sea-level air; at altitude the air is thinner and carries less heat per CFM, so the calculator scales it with the standard atmosphere.
A worked example
A 1,000 ft² wall at R-13 with 70°F inside and 0°F outside loses 1 ÷ 13 × 1,000 × 70 ≈ 5,385 BTU/h. 200 ft² of U-0.30 windows add 0.30 × 200 × 70 = 4,200 BTU/h: a fifth of the area, nearly as much heat. That is why windows and air leakage so often top the breakdown.
From heat loss to equipment
A whole-house number is a good start, but equipment and ducts are sized room by room, and a Manual J also counts duct losses, below-grade walls and construction details in a standard way. Take this total to the heat pump balance point calculator, try the quick HVAC load calculator for cooling, or read what a Manual J calculation covers.
Heat loss questions
- How do you calculate heat loss?
- Add conduction through each surface, U × A × ΔT, where U = 1/R, A is the area in square feet and ΔT is indoor minus outdoor design temperature. Then add air leakage, 1.08 × CFM × ΔT, where CFM is the outside air that leaks in. The total is the design heat loss in BTU/h.
- What outdoor temperature should I use?
- Use the 99% heating design dry bulb for the nearest representative weather station: the temperature the site is colder than for only about 88 hours in an average year. Building departments may publish their own design temperatures; use theirs if they do.
- How do I turn a blower door number into CFM?
- Divide ACH50 (or CFM50) by an N-factor of roughly 10 to 25 to estimate natural air leakage. Around 15–20 suits a typical one- or two-story house; use a lower N for tall or exposed houses, where leakage on a cold windy day is higher. For ACH, CFM = ACH × volume ÷ 60.
- Why does my R-value give a higher loss than I expected?
- Nominal R-values ignore framing. Studs, plates and headers conduct more heat than the cavity insulation, so a 2×4 wall with R-13 batts performs closer to R-10 or R-11 overall. Use whole-assembly U-factors if you have them.
- Is heat loss the same as a Manual J heating load?
- It is the core of it. A Manual J heating load also accounts for ducts outside the conditioned space, below-grade surfaces, construction details and local conditions in a standard way, room by room, which this calculator does not.
Every room's heat loss, without the spreadsheet
calc.blue measures the house with an iPad Pro and LiDAR, so the areas come from the scan instead of a tape. You get heating and cooling for every room, with a likely range and the inputs that would move it most, built on ACCA Manual J 8th edition procedures.