BTU Calculator
Use your area's 99% winter design temperature for heating, or the summer one for cooling.
Walls (net of windows)
2×4 wall with batts ≈ R-13, 2×6 ≈ R-19
4,615
Windows and doors
Double-pane ≈ R-2, triple ≈ R-3 to R-5
4,500
Ceiling / roof
Attic insulation R-30 to R-60
1,974
Floor over unheated space
Leave area 0 for slab or heated basement
0
Air leakage
Volume (ft³) and air changes per hour: tight 0.3, average 0.5, leaky 1.0+
5,400
Heat loss at design conditions
16,489 BTU/hr
- Kilowatts
- 4.83 kW
- Tons (12,000 BTU/hr)
- 1.37
- ΔT
- 50 °F
- Air leakage share
- 33%
Energy over time
- Heat
- 395,738 BTU
- Electric (resistance)
- 116.0 kWh
- Natural gas
- 3.96 therms
Before equipment efficiency: divide by about 0.95 for a condensing furnace, or by a heat pump's COP (often 2–4).
Formulas
R-values are per assembly (U = 1/R). Framing, thermal bridges and ducts in unconditioned space make real losses somewhat higher.
Estimate a building's heat loss in BTU per hour at design conditions from the area and R-value of its walls, windows, ceiling and floor plus air leakage, and the heating energy that loss adds up to over time.
Formula
Worked Example
With a 50 °F difference the walls lose 1,200 × 50 ÷ 13 = 4,615 BTU/hr, and the windows almost as much (4,500) on less than a sixth of the area. Air leakage adds 0.018 × 12,000 × 0.5 × 50 = 5,400 BTU/hr, a third of the total.
Frequently Asked Questions
What outdoor temperature should I use?
Your location's 99% winter design temperature, the outdoor temperature that is colder only 1% of the hours in a year. ASHRAE publishes it for thousands of weather stations, and ACCA Manual J tables list it by city; your local building department can also tell you the value it uses.
How many BTUs do I need per square foot?
Rules such as 30–60 BTU/hr per square foot are crude because insulation, windows and air tightness change the load by a factor of two or more. A heat loss calculation like this one, or a full Manual J, avoids an oversized furnace or heat pump that short-cycles.
What is ACH?
Air changes per hour: how many times the whole volume of indoor air is replaced by outdoor air through leaks each hour. Tight new homes are around 0.3 under natural conditions, average homes about 0.5 and leaky older homes 1.0 or more.