Free Manual J load calculator.

The guide

What a Manual J load calculation tells you

A Manual J calculation, published by ACCA (the Air Conditioning Contractors of America), estimates how much heat your home gains on a hot design day and loses on a cold one. That number, in BTU per hour, is what an air conditioner or heat pump has to remove or replace. Equipment is then chosen to match it (Manual S), and ducts are sized to deliver it (Manual D).

The load comes from many small pieces: heat through walls, ceilings and floors; sun through windows, which depends heavily on the direction they face; outside air leaking in; ducts running through a hot attic; and the people and appliances inside. The calculator above adds each of these up and shows the breakdown, so you can see which ones drive your result.

Block load vs. room-by-room

A block load treats the whole house as one zone. It’s the right tool for sizing a single central system or sanity-checking a contractor’s quote, and it’s what this calculator does. A room-by-room Manual J calculates every room separately, so each one gets the right airflow; that’s what duct design and most permit offices need. The two usually land close on the total, but only room-by-room tells you how to split the air.

Why rules of thumb oversize your AC

“One ton per 500 square feet” (or 400, or 600) ignores everything that actually sets the load: insulation, window area and orientation, glass type, air tightness and duct location. Two homes of the same size can differ by a ton or more. An oversized system costs more up front, cycles on and off in short bursts, wears out sooner and, most importantly in humid climates, doesn’t run long enough to pull moisture out of the air. The house ends up cold and clammy.

Why latent load matters in Florida

Cooling has two parts. Sensible load is temperature; latent load is moisture. In Florida and along the Gulf Coast, humid outdoor air brings a large latent load with every bit of infiltration, and people add more. The calculator reports both, and warns you when latent is more than about a quarter of the total, because then equipment selection (Manual S) has to confirm the unit can actually dehumidify, not just cool.

What a building department will accept

Most building departments, including those enforcing the Florida Building Code, ask for heating and cooling equipment sized per ACCA Manual J and Manual S, and many also want Manual D for the ducts. A DIY block load is a good starting point but is generally not accepted on its own. When you need the permit-ready version, download your inputs from the results and send them with our intake: a licensed engineer prepares a room-by-room Manual J, S and D package, signed and sealed.

How this calculator simplifies

• The footprint is assumed square, split evenly by the number of stories.
• Door area is fixed; specific shading, overhangs and room-by-room variation are not modeled.
• Ducts outside the conditioned space add a standard loss factor; ducts inside conditioned space add none.
• Choosing “container home” shows a thermal-bridging warning but does not change the math: steel walls bridge heat, so treat the result as advisory and have a container home calculated properly.
• The custom climate option uses a fixed infiltration factor; pick the nearest climate profile when you can.