HVAC Load Calculator
Get a rough whole-house heating and cooling load in BTU and tons from the floor area, climate-based factors, window count, and occupants. Calculates in your browser.
Cooling load
-
Show the math
Each load scales the floor area by a factor you choose:
heating = area × heating factor
cooling = area × cooling factor + windows × 1,000 + occupants × 600
The default factors (35 and 25 BTU per sq ft, 1,000 BTU per window, 600 BTU per person) are rough placeholders, not a published standard, and they tend to run high: ENERGY STAR advises against relying on a rule of thumb. Equipment should be sized with an ACCA Manual J load calculation, which accounts for your climate, insulation, windows and air leakage. All math runs locally in your browser.
What this does
An HVAC load calculator gives a rough whole-house heating and cooling load in BTU and tons from the floor area, climate factors, window count, and occupants.
How to use it
- Enter the floor area.
- Set the climate factors.
- Add windows and occupants.
- Read the heating and cooling load.
How it works
It multiplies the floor area by the BTU-per-square-foot factors you enter, then adds 1,000 BTU per window and 600 BTU per person for cooling. The default factors are rough placeholders, not a published standard, and tend to run high; ENERGY STAR advises against sizing equipment by a rule of thumb, and an ACCA Manual J load calculation is the real method.
load = area × factor + windows + occupants
Understanding your result
Treat this as a ballpark for whole-house sizing, not a substitute for a Manual J calculation. The common and costly mistake is oversizing, which causes short-cycling, uneven temperatures, poor humidity control, and higher bills, so before buying equipment have a proper load calculation done rather than matching or exceeding whatever unit you currently have.
Example
With the default placeholders, a 2,000 sq ft home at 25 BTU per sq ft with 10 windows and 4 occupants comes to 62,400 BTU/hr, or 5.2 tons of cooling. That is not a sizing recommendation: lower the factors for a tight, well-insulated home, and have a Manual J done before you buy equipment.
Last updated .
Frequently asked questions
How do I size a furnace or AC?
Estimate the heating and cooling load in BTU, then choose equipment that meets it without large oversizing. This tool gives a ballpark figure.
What are the heating and cooling factors?
They are BTU per square foot that you choose for your climate and insulation; milder, well-insulated homes use lower factors than cold or leaky ones. The defaults are placeholders, not a published standard, because no authoritative per-square-foot rule exists.
Is this a substitute for Manual J?
No. It is a simplified estimate; a certified Manual J calculation is the standard for equipment sizing.
Is my data saved?
No. Everything is calculated locally in your browser.
Related tools
BTU Calculator
Estimate the cooling BTU and tons an air conditioner needs for a room from its size, ceiling height, sun exposure, occupants, and whether it’s a kitchen. Calculates in your browser.
Drywall Calculator
Work out the drywall sheets a project needs from the wall and ceiling area and sheet size, plus screws, joint compound, and tape. Calculates in your browser.
Conduit Fill Calculator
Check NEC conduit fill (total conductor area versus the allowed percentage for EMT) from your wire gauges and counts, with a pass or fail. Calculates in your browser.
Wire Size Calculator
Find the wire gauge that safely carries the load and keeps voltage drop within limit, from the current, one-way distance, voltage, conductor metal, and phase. Calculates in your browser.