How HomeBTU calculators estimate loads, equipment sizes and costs. Last reviewed October 9, 2026.
All calculators group locations by the International Energy Conservation Code (IECC) climate zones. Selecting a state sets the zone that covers most of its population; users can change it. Each zone has a typical heating design temperature (colder than 99% of hours in a year) and cooling design temperature (exceeded only 1% of hours):
| Zone group | Heating design | Cooling design | Heating degree days |
|---|---|---|---|
| Hot · IECC 1–2 | 35–40 °F | 95 °F | 1,200 |
| Warm · IECC 3 | 25–28 °F | 95 °F | 2,500 |
| Mixed · IECC 4 | 15 °F | 92 °F | 4,200 |
| Cold · IECC 5 | 2–5 °F | 90 °F | 6,000 |
| Very cold · IECC 6–7 | −10 to −12 °F | 87 °F | 8,000 |
Whole-home sizing calculators start from square feet per ton of cooling by zone: 450 (hot), 525 (warm), 575 (mixed), 625 (cold) and 650 (very cold). Results are then adjusted for ceiling height, insulation (−10% to +15%), sun exposure (±10%) and window area (−5% to +10%). Room calculators follow the ENERGY STAR room air conditioner chart, adding 600 BTU/h per occupant beyond two and 4,000 BTU/h for kitchens.
These rules include a margin compared with a full Manual J. The Manual J calculator instead adds up conduction, air leakage, solar gain, internal gains, duct losses and latent load from the building's actual characteristics.
Design heat loss is estimated at 18, 22, 26, 32 and 38 BTU/h per square foot from hot to very cold zones for an average home, then adjusted for insulation (−15% to +25%) and ceiling height. Furnace sizing adds a 20% margin, inside the 140% limit in ACCA Manual S. The BTU calculator uses higher heating rules of thumb (30 to 60 BTU/h per square foot) that already include a margin for space heaters and small-room equipment.
Heat pump output at the design temperature is taken as a share of rated capacity at 47 °F: 85%, 70%, 60%, 50% and 35% for standard models, and 95%, 90%, 85%, 75% and 65% for cold-climate models, from hot to very cold zones. These shares reflect typical manufacturer extended-performance data and the NEEP cold-climate heat pump specification. Backup heat is the remaining shortfall, rounded up to standard strip heater sizes.
Annual heating energy is heat loss per degree × heating degree days × 24 hours × 0.65, a correction for internal and solar gains. Heat pump seasonal COP is HSPF2 ÷ 3.412, adjusted from +10% in hot zones to −20% in very cold zones. Fuel heat content: 100,000 BTU per therm of natural gas, 91,500 BTU per gallon of propane, 138,500 BTU per gallon of heating oil. Default prices of $0.17/kWh and $1.50/therm are close to recent U.S. residential averages reported by the Energy Information Administration; users should enter their own rates.
Each system type has a national installed-price range built from published 2025–2026 cost guides from contractors, manufacturers and cost-estimating sites. Ranges are expressed as a fixed amount plus an amount per ton or per BTU, split 55% equipment and 45% labor. Equipment is scaled for efficiency (+18% high, +40% premium or modulating) and labor for access (−8% to +18%). Both are multiplied by a state labor index from 0.90 to 1.30. The typical figure sits at 45% of the range, reflecting mid-tier brands and standard installations.
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