What the result means
Heat flows out through every surface in proportion to its area and the temperature difference, and inversely to its insulation value. Air leaking in also has to be heated. Adding those pieces gives the heat loss at your design temperature.
Use this for quick checks, like whether a space heater or ductless head is in the right range, or to explain to a homeowner why an addition needs more heat. Equipment sizing for a whole house should use a full Manual J with local design data.
How it's calculated
- Conduction: Q = A × ΔT ÷ R (or Q = U × A × ΔT)
- Infiltration: Q = 0.018 × volume (ft³) × ACH × ΔT
- Total kW = Btu/h ÷ 3,412.142
0.018 Btu/ft³·°F is the heat capacity of air at standard density. ΔT = indoor design temperature − outdoor design temperature.
Worked example
1,000 ft² of R-13 wall and a 12,000 ft³ space at 0.5 ACH, with a 70 °F design difference.
- Walls: 1,000 × 70 ÷ 13 = 5,385 Btu/h
- Infiltration: 0.018 × 12,000 × 0.5 × 70 = 7,560 Btu/h
- Total = 12,945 Btu/h ≈ 3.79 kW
Air leakage is the bigger piece here, which is common in older homes.
Reference table
The values below are computed from the same calculator functions used above. They let you compare common sizes and operating points without replacing the interactive calculation or its assumptions.
| Assembly | Area | R-value | Temperature difference | Approx. Btu/h |
|---|---|---|---|---|
| Insulated wall | 200 ft² | R-13 | 40°F | 615 |
| Ceiling | 500 ft² | R-30 | 40°F | 667 |
| Window | 40 ft² | U-0.5 | 40°F | 800 |
Illustrative component cases, not a whole-home design or Manual J.
Sources and assumptions
0.018 Btu/ft³·°F is the heat capacity of air at standard density. ΔT = indoor design temperature − outdoor design temperature. Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box.
Last reviewed . Spot an error? Email support@tiermenu.com.
FAQ
Is this the same as Manual J?
No. Manual J includes design conditions, solar and internal gains, duct losses, and many detailed factors. This is a simplified estimate.
Should I use the insulation R-value or the whole-wall R-value?
Whole-assembly R-value is more accurate because studs and framing conduct more heat than insulation.
What ACH should I use?
A blower door test is best. Without one, 0.35 is tight new construction, 0.5 to 1.0 average, and 1.5 or more for leaky older homes.
Can I enter U-factor for windows?
Yes. Switch that row's rating to U and enter the NFRC U-factor from the window label.
Using a reference result in the field
A calculator is most useful when the inputs match the job being checked. Confirm whether a value is nominal or measured, and whether readings were taken at design or actual operating conditions. The unit label beside the result is part of the result; do not carry a number into another calculation without checking its units.
Reference rows are provided for comparison, not as universal design limits. They use the values and conditions stated with each table. Elevation, air temperature, equipment and installation details can change a field result. Where the page calls out a code check, equipment rating or manufacturer table, that check remains part of the job.
Use the interactive inputs for the exact conditions at hand, then review the displayed formula and assumptions. If a value looks unexpected, change one input at a time and compare it with a known point in the reference table. Keep the measured readings and job-specific notes with the work record so another technician can understand how the result was obtained.
TierMenu provides these free calculators without an email gate. The results stay in this browser unless you choose to copy a shareable URL or print the page. A shared URL includes the calculator inputs in its query string; do not put personal or sensitive information in calculator fields.