FreeConstruction · HVAC

HVAC load rule of thumb

Ballpark cooling tons, heating BTU and airflow for a house or small space by area and climate.

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Your numbers

Conditioned area (sq ft)Floor area served by the system.ClimateRoughly matches IECC climate zones 1–2, 3, 4, 5, 6–7.Ceiling height (ft)Volume scales the load above 8 ft.Insulation and windowsTight new homes need less capacity.OccupantsEach person beyond two adds about 600 BTU/h.

For the equipment schedule and running cost8 more, filled with typical values

AC efficiency (SEER2) (Btu per Wh)From the AHRI certificate or the yellow EnergyGuide label.Electricity price ($ per kWh)From the customer's bill. EIA's July 2026 US residential average was about 18 cents.Cooling full-load hours a year (hours)Hours the AC would run flat out to meet a year's cooling. Use your local figure.HeatingWhat makes the heat.Furnace efficiency (AFUE) (%)From the furnace label; 80% for a non-condensing furnace, 90%+ for condensing.Heat pump efficiency (HSPF2) (Btu per Wh)From the AHRI certificate. Used only for a heat pump.Gas price ($ per therm)From the gas bill. Used only for a furnace.Heating full-load hours a year (hours)Hours the heat would run flat out to meet a year's heating. Use your local figure.

HVAC load rule of thumbForge by Norventus

How it works

Press Calculate to see the cooling load, the nominal air conditioner size in tons, the heating output and the design airflow.

The condenser: the size the dial readsDial: nominal size in tons, one tick per half tonHouse, air handler, duct and thermometerThe drawing follows your numbers as you type: a bigger floor for more area, taller walls for higher ceilings, thicker walls and smaller windows for a tighter house, one pawn for each occupant and a warmer thermometer for a hotter climate. Press Calculate and the dial swings to the air conditioner size worked out by the tool, with the cooling load, the heating output and the airflow beside it.

Result

Ready when you are.Enter your numbers and press Calculate.

References

Check the work against these

An equipment schedule with square feet per ton and heating equipment size, a yearly running-cost estimate from SEER2, AFUE or HSPF2 and local prices, and the Manual J inputs to collect. Numbers in the result and in the plain-words read point here.

1
Manual J: Residential Load CalculationCode
Air Conditioning Contractors of America (ACCA)
The ANSI-recognized standard for sizing loads for homes; ACCA notes a compliant calculation is required by national building codes and most state and local jurisdictions.
2
British thermal units (Btu): sample conversion factorsData
US Energy Information Administration
1 kilowatthour = 3,412 Btu, 1 therm = 100,000 Btu and 1 cubic foot of natural gas is about 1,036 Btu: the conversions behind the running-cost estimate.
3
Electric Power Monthly, Table 5.6.A: average price of electricity by sector and stateData
US Energy Information Administration
The average residential price per kWh in each state, updated monthly. Use the customer's own bill when you have it.
4
Natural Gas Prices (monthly, by sector)Data
US Energy Information Administration
US and state residential natural gas prices in dollars per thousand cubic feet. Divide by about 10.36 therms per thousand cubic feet, or use the price per therm on the gas bill.
5
Manual J Load Calculations vs. Rules of ThumbStudy
ACCA HVAC Blog (Allison Bailes, Energy Vanguard, Oct 2016)
Contractors often size at one ton per 400 to 600 sq ft; 40 Manual J loads for hot and mixed climates averaged 1,431 sq ft a ton, and the lowest was 624.
6
Room air conditioners: sizing chartGuide
ENERGY STAR (US EPA and DOE)
The capacity chart is calculated for an 8 ft ceiling and adds 600 Btu/h for each regular occupant beyond two, the same allowances this tool uses.

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HVAC load rule of thumb

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