Free · Equipment sizing
HVAC Load Calculator
Square footage, climate and construction in, cooling and heating BTUs, tonnage and equipment size out. A Manual J-style block load you can run from the driveway, with every factor shown.
The house
Construction
Each person over two adds 600 BTU/h of cooling load.
Cooling load
44,600BTU/h
Recommended AC size
4 ton
48,000 BTU/h
Heating load
90,000
BTU/h output needed
Furnace at 80% AFUE
113,000
BTU/h input
Furnace at 96% AFUE
94,000
BTU/h input
Manual S check
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How this load calculator works
It is a block load: a base BTU-per-square-foot rate for the climate, multiplied by factors for ceiling height, insulation, window type, sun exposure and stories, plus a fixed gain per occupant. Those are the same variables a Manual J weighs, reduced to the ones you can answer from the driveway.
The base rates and factors the calculator uses:
| Climate | Cooling BTU/sq ft | Heating BTU/sq ft | Typical sq ft per ton |
|---|---|---|---|
| Hot (Zones 1–2: FL, Gulf Coast, AZ, S. TX) | 30 | 30 | 400–500 |
| Warm (Zone 3: GA, Carolinas, OK, N. TX, S. CA) | 25 | 40 | 500–600 |
| Mixed (Zone 4: VA, KY, MO, NJ, Pacific NW) | 22 | 45 | 550–650 |
| Cool (Zone 5: PA, OH, IL, CO, NE) | 18 | 50 | 650–750 |
| Cold (Zones 6–7: MN, WI, ME, MT, ND) | 15 | 60 | 750–900 |
- Ceiling height: +5% per foot above 8 ft.
- Insulation: poor +15% cooling and +25% heating; good −12% cooling and −20% heating.
- Windows: single pane +12% cooling and +15% heating; low-E −6% cooling and −5% heating.
- Sun: shaded −10%, full sun +10% on cooling only.
- Two or more stories: −7% cooling, since less of the floor area sits under the roof.
Reading the result
Divide cooling BTU/h by 12,000 to get tons. The recommended size is the standard unit (1.5 to 5 tons) that lands in the ACCA Manual S window of 95–115% of the load. Oversizing is the common mistake: a unit that short-cycles never runs long enough to pull humidity out of the house, so the customer is cold and clammy at the same time.
For heating, the load is the output you need. Furnaces are sold by input, so the calculator divides by AFUE: an 80,000 BTU input furnace at 96% AFUE delivers 76,800 BTU/h. Pick the smallest furnace whose output covers the load.
When you need a full Manual J
Any time the equipment is being ordered, a permit requires it, or the house is unusual: large glass areas, vaulted ceilings, additions over garages, or a climate at the edge of a zone. Use this tool to sanity-check a Manual J, to size a quick replacement of like-for-like equipment, or to explain to a homeowner why the 5-ton unit their neighbor recommended is too big.
Sizing a ductless system instead? The mini split calculator runs the same math room by room. For the airflow the equipment needs, use the CFM calculator.
Frequently asked questions
How many BTUs do I need per square foot?
It depends on climate more than anything else. Roughly 30 BTU per square foot of cooling in hot climates like Florida and Arizona, 20–25 in the middle of the country, and 15–18 in the northern states. Heating runs the other way: about 30 BTU per square foot in the South and 50–60 in cold climates. Insulation, windows and ceiling height move those numbers by 10–25% either way.
How many square feet does a 3-ton AC cool?
A 3-ton unit delivers 36,000 BTU/h. That covers about 1,200–1,500 square feet in a hot climate, 1,600–2,000 square feet in a mixed climate, and 2,000–2,400 square feet in a cool northern climate, assuming average insulation and 8-foot ceilings.
Is this a Manual J calculation?
No. Manual J is a room-by-room heat transfer calculation using wall areas, orientation, window specifications, infiltration and design temperatures. This is a block load that uses the same variables in simplified form. It is accurate enough for a like-for-like replacement or a sales conversation, and it will flag when a Manual J result looks wrong, but it does not replace one for permits or new construction.
Why does the calculator recommend a smaller unit than the load?
ACCA Manual S allows cooling equipment from 95% to 115% of the calculated load, so a 3.67-ton load is correctly served by a 3.5-ton unit. Slightly undersized equipment runs longer cycles, dehumidifies better and costs less to operate. Grossly oversized equipment short-cycles and leaves the house humid.
What size furnace do I need for 2,000 square feet?
In a mixed climate with average insulation the heating load is about 90,000 BTU/h of output. That is a 100,000 BTU input furnace at 96% AFUE, or a 120,000 BTU input furnace at 80% AFUE. In a warm climate the same house needs closer to 80,000 BTU input; in a cold climate, 120,000 or more at high efficiency.