HVAC load calculator
A first-pass heating and cooling estimate for a whole house in a few inputs. Useful for a ballpark before the site visit; not a substitute for a room-by-room Manual J.
- Free, no sign-up
- Runs in your browser
- Estimate: shows its formulas
Climate
99% heating and 1% cooling dry bulb, from ASHRAE Handbook—Fundamentals design conditions via Climate.OneBuilding.org. Your local code may set its own values.
House
Envelope
Whole-house estimate
Not a Manual J · updates as you type
- Heating
- 29.9k
- BTU/h · 8.8 kW
- Cooling
- 1.7 tons
- 20,570 BTU/h
- Sensible / latent
- 96 / 4
- % of cooling
- ft² per ton
- 1,167
- from this estimate, not a rule
- Windows9,540 · 32%
- Walls9,320 · 31%
- Air leakage7,060 · 24%
- Ceiling2,230 · 7%
- Floor / foundation1,770 · 6%
Estimate, not a Manual J. It assumes a simple box-shaped house and average sun on the glass, so a particular house can differ by 25% or more. Don't select equipment from it.
An estimate is not a Manual J
This calculator turns a few inputs into a whole-house number with textbook physics: conduction through a box-shaped envelope, air leakage, sun on the windows and heat from people and appliances. A Manual J calculation follows the same physics through every room as it is actually built. The difference matters when you pick equipment and design ducts.
| This estimate | Room-by-room Manual J | |
|---|---|---|
| Geometry | A box from floor area and stories | Every room, wall and window as built |
| Windows | A share of floor area, average sun | Each window's size, orientation, glass and shading |
| Construction | One insulation level for the house | Each assembly as built |
| Air leakage | A tightness class | A blower-door test or a standard estimate |
| Ducts | A flat allowance | Location, insulation and leakage |
| Output | Whole-house heating and cooling | Room-by-room loads for equipment and duct design |
What goes into the numbers
- Heating: U × A × ΔT for walls, windows and ceiling, the foundation (slab edge, a buffered floor, or basement walls against the ground), and 1.08 × CFM × ΔT for air leakage.
- Cooling: walls and ceiling with an allowance for sun-heated surfaces and a hot attic, window conduction plus solar gain (area × SHGC × an average solar gain), people and appliances, and air leakage, both the temperature (sensible) and the moisture (latent, 0.68 × CFM × Δgrains).
- Typical assemblies for each insulation level and generic leakage for each tightness class. They are reasonable averages, not your house.
For a single number you can trust more, enter the real areas and R-values in the heat loss calculator, then check equipment with the balance point calculator. Read what a Manual J calculation is and how to size a heat pump.
Load calculation questions
- Is there a free Manual J calculator?
- A real Manual J follows ACCA's procedure room by room, with construction details, orientation, duct location and local design conditions. A quick calculator like this one can give a reasonable ballpark, but it is not a Manual J and shouldn't be submitted as one.
- How many square feet per ton of air conditioning?
- There is no reliable number. Real homes range from well under 500 to over 1,500 square feet per ton depending on insulation, windows, air tightness, orientation and climate. Square-foot rules of thumb are a big reason so many systems are oversized.
- How accurate is this estimate?
- It can be off by 25% or more for a particular house, because it assumes a box-shaped house, typical assemblies for the chosen insulation level and average sun on the windows. Use it to sanity-check, not to pick equipment.
- What does the latent load mean?
- Latent load is the moisture the air conditioner has to remove, from outdoor air leaking in and from people. It matters most in humid climates, where an oversized system can cool the air without running long enough to dry it.
When the estimate isn't enough, do the Manual J in the house
calc.blue does the room-by-room calculation on an iPad Pro: LiDAR captures the rooms, windows and doors as you walk, sensible defaults fill the gaps, and every room gets heating and cooling loads with a likely range and the inputs worth checking. Then it matches equipment and writes the reports. Built on ACCA Manual J 8th edition procedures.