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ACCA Manual J
Validated against the standard.
PDF export
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$2.99 Pro
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How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact ACCA Manual J reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
A heat load (or load calculation) tells you how many BTU per hour a space gains in summer and loses in winter, so you can size the heating and cooling equipment correctly. Oversize it and the system short-cycles and never dehumidifies; undersize it and it can't keep up. This app follows the ACCA Manual J approach used by code officials across North America.
What drives the load
- Floor area and volume of the conditioned space.
- Insulation levels in walls, ceiling, and floor (R-values).
- Windows and doors — area, orientation, and U-factor / SHGC.
- Infiltration — how leaky the envelope is.
- Design temperature difference between indoors and the local outdoor design conditions.
- Internal gains — people, lighting, and appliances (and latent load for humidity).
Worked example
A well-insulated 1,500 ft² home in a moderate climate often lands near 18,000–24,000 BTU/h cooling (about 1.5–2 tons) once Manual J is run room by room.
The rule of thumb of "1 ton per 400–600 ft²" gets you in the ballpark, but Manual J accounts for windows, orientation, and infiltration — which is why a load calc, not a rule of thumb, is what passes inspection.
Related calculators
Convert the load to airflow with the airflow CFM calculator, then size the ducts with the duct sizing calculator.
Heat load FAQ
What method does the calculator use?
It follows the ACCA Manual J residential load calculation method, accounting for area, insulation, windows, infiltration, and the local design temperature difference for both heating and cooling.
Why not just use a rule of thumb?
Rules of thumb like one ton per 500 ft² ignore window orientation, insulation, and infiltration, which often leads to oversized equipment that short-cycles and fails to dehumidify. A Manual J load calculation is what most jurisdictions require.
Does it calculate both heating and cooling?
Yes. It reports the winter heating load and the summer cooling load, including the sensible and latent split that matters for humidity control.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Heat Load on your phone
Free with rewarded ads · $2.99 lifetime Pro · iOS & Android. Works 100% offline on the job site.