HVAC Load Calculator
Get a fast, rough estimate of the cooling load your space needs — in BTU per hour and tons of air conditioning — before you shop for a system.
Reviewed by the CalcCafe editorial team · Last updated 1 July 2026 · How we test our tools
Example
A 1,500 sq ft home at 25 BTU/sq ft gives 37,500 BTU, plus 3 occupants × 400 = 1,200 BTU and 8 windows × 1,000 = 8,000 BTU, for a total of 46,700 BTU/hr. Dividing by 12,000 BTU per ton gives about 3.89 tons of cooling — you would likely shop for a 4-ton system.
How it works
BTU/hr = area × BTU-per-sq-ft factor + occupants × 400 + windows × 1,000. Tons of cooling = BTU/hr ÷ 12,000, since one ton of air conditioning equals 12,000 BTU per hour.
Good to know
Sizing an air conditioner or heat pump comes down to matching its cooling capacity to how much heat your space gains on a hot day. This tool gives a back-of-the-envelope version of that: it multiplies your floor area by a BTU-per-square-foot factor, then adds a fixed allowance for the people inside (each body sheds roughly 400 BTU/hr of heat) and for windows (about 1,000 BTU/hr each, since glass lets in far more solar heat than a wall). The result is a cooling load in BTU per hour and its equivalent in tons.
The BTU-per-sq-ft factor is the biggest lever. A common starting point is 20–30 BTU/sq ft, but a well-insulated, shaded home in a mild climate sits at the low end while a sunny, poorly insulated, or hot-climate space pushes higher. Ceiling height, kitchen appliances, sun exposure, and how many people gather in a room all shift the real number, which is why the occupants and windows fields nudge the total up.
One ton of cooling equals 12,000 BTU/hr, a unit that dates back to the ice-block era. Manufacturers sell systems in half- and whole-ton increments, so your calculated tonnage tells you which size bracket to look at rather than an exact model. Bigger is not better: an oversized unit short-cycles, cools unevenly, and leaves humidity behind, while an undersized one runs constantly and struggles on peak days.
Treat this as a planning ballpark, not a spec. Professional sizing uses an ACCA Manual J calculation that accounts for insulation R-values, air leakage, orientation, duct losses, and local design temperatures — details this simple formula cannot see. Use the estimate to sanity-check a quote, then let a licensed contractor run the full load calculation before you buy.
Frequently asked questions
How many tons of AC do I need per square foot?
A rough rule is one ton (12,000 BTU) per 400–600 sq ft, but that varies widely with climate, insulation, and sun exposure. This tool lets you set your own BTU-per-sq-ft factor and adds occupant and window loads for a closer ballpark.
What is a Manual J load calculation?
Manual J is the industry-standard method (from ACCA) for calculating a building's heating and cooling loads using insulation, air leakage, window details, orientation, and local design temperatures. It is far more accurate than any square-footage rule and is what a contractor should use before installing equipment.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser. Your inputs never leave your device, and it works offline once loaded.
Is this calculator free?
Yes, completely free with no sign-up and no limits.
People also ask
How do I calculate HVAC size for my home?
Multiply your square footage by a BTU-per-sq-ft factor (often 20–30), add roughly 400 BTU per occupant and 1,000 BTU per window, then divide the total by 12,000 to convert BTU/hr into tons of cooling.
How many BTU is a ton of cooling?
One ton of air conditioning equals 12,000 BTU per hour. So a 3-ton system provides 36,000 BTU/hr and a 4-ton system provides 48,000 BTU/hr of cooling capacity.
Is it bad to oversize an air conditioner?
Yes. An oversized unit cools the air quickly then shuts off before removing humidity, leaving the space cold and clammy, and the frequent on-off cycling wastes energy and wears out the compressor faster.
Can I calculate the HVAC load online?
Yes, this calculator gives a quick online estimate from floor area, occupants and windows: BTU/hr = area x BTU per sq ft + occupants x 400 + windows x 1,000, then tons = BTU/hr / 12,000. It is a rule-of-thumb figure good for shopping and sanity-checking a quote, not a substitute for a full Manual J calculation, which also accounts for insulation, orientation, climate and duct losses. Use it to get in the right range, then have a contractor confirm the size before buying.
What is the $5,000 rule for HVAC?
The $5,000 rule is a repair-versus-replace rule of thumb: multiply the age of your system in years by the estimated repair cost, and if the result is over $5,000, replacement is usually the better value. For example, a $700 repair on a 12-year-old unit gives 12 x $700 = $8,400, which points toward replacing, while the same repair on a 4-year-old unit gives $2,800 and favors repairing. It is a guideline, not a rule; efficiency gains from a new unit and your local energy prices also matter.
Is there an app that can calculate HVAC load?
This web calculator works on any phone or tablet browser and gives a rough load in BTU/hr and tons in seconds, with nothing to install. Professionals use ACCA-approved Manual J software, which models each room's walls, windows, insulation and local design temperatures for a precise load. A quick estimate here plus a contractor's Manual J is the usual path to a correctly sized system.
What is the best HVAC load calculation software?
For an official sizing, look for software approved by ACCA to perform Manual J residential load calculations, since that is the method most building codes and equipment warranties expect. Those tools model every room's walls, windows, insulation, orientation and local climate, which is far more detailed than a per-square-foot estimate. This calculator is the quick version for getting a ballpark tonnage before you talk to a contractor.
How many tons of cooling for 1,000 sq ft?
At 25 BTU per square foot, 1,000 sq ft needs a base of 25,000 BTU/hr, which is 25,000 / 12,000 = about 2.1 tons. Adding two occupants (800 BTU) and six windows (6,000 BTU) brings it to 31,800 BTU/hr, or about 2.65 tons, so a 2.5 to 3 ton unit would be typical. A very sunny space or a hot climate pushes toward 3 tons, while a shaded, well-insulated one can get by with 2 tons.
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Sources & references
These tools follow our methodology and provide educational estimates only — verify important figures with a qualified professional.