ACCA Manual J is the residential load calculation standard, and it is referenced by building codes in most of the United States. It answers one question: how much heat enters and leaves this specific house at design conditions.
What it counts
Envelope conduction — walls, ceiling, floor, each by area and assembly R-value.
Glazing, separately by orientation. A west window and a north window of the same size are not the same load, and this is one of the largest terms in a house with a lot of glass.
Infiltration — air leakage, ideally from a blower door test rather than an assumed value.
Duct gains and losses, which matter enormously when ducts run through an unconditioned attic.
Internal gains — occupants, lighting, appliances.
Latent load — moisture from occupants, cooking, bathing, and infiltrating humid outdoor air.
The output is a heating load and a cooling load in BTU/h, and in a proper job, a room-by-room breakdown rather than a single house figure.
Why the room-by-room part matters
The whole-house number sizes the equipment. The per-room numbers size the ducts, and they are what determine whether the back bedroom is comfortable.
A calculation delivered as a single total is half a calculation, and it is the half that does not prevent the most common complaint after a new system goes in.
Why it comes out lower than the rule of thumb
The square-foot rules were developed for the housing stock of the 1960s and 1970s: minimal wall insulation, single glazing, high air leakage.
A house built to any modern energy code has a fraction of that envelope load. Applying an old rule to a new house produces a system 30% or more oversized, and that shows up as the humidity and short-cycling problems described elsewhere on this site.
The same applies to an older house that has since been insulated, air-sealed or re-glazed.
What a real one requires
Plan dimensions or a measured survey. Wall, ceiling and floor assemblies. Window count, size, orientation, and U-factor and SHGC where known. Foundation type. Duct location and insulation. Local design conditions.
That is an hour or two of measurement and data entry, using software such as Wrightsoft, Elite or CoolCalc.
What is not Manual J
"I've done a thousand of these" — experience is genuinely valuable and it is not a calculation.
A tablet app that asks for square footage and a zip code. That is the rule of thumb with a user interface.
Matching the existing equipment. This propagates the previous contractor's error forward, and since oversizing has been the industry default for decades, the error usually points the same way.
Sizing from the old unit plus half a ton "for safety." There is no safety in extra capacity; there is humidity.
The number to ask for
Ask for the printed report — the load in BTU/h for heating and cooling, the room-by-room table, and the design conditions used.
A contractor who has run one will hand it over. A contractor who has not will explain why it is unnecessary for your house, and that answer is itself the information you were looking for.
Cost and whether it pays
A standalone Manual J typically runs a few hundred dollars, and many contractors include it with a quote for a full system replacement.
Against an equipment and installation cost in the thousands, and against the running cost of an oversized system for the next fifteen years, it is the cheapest part of the project — and it is the part that determines whether the rest of the money is well spent.