HVAC Load Calculator

How the estimate works

room base   = length × width × height (m) × 500   (600 with sun)
+ occupants = 600 BTU/h × (people − 1)
+ kitchen   = 4,000 BTU/h

total load  = Σ rooms          tons = total ÷ 12,000

The base term is the same hot-climate volumetric formula used across this site and in the HVAC Calculator app; the occupancy and kitchen adders are standard quick-calculation allowances. Fitting the result to equipment? Continue to the AC size calculator for unit sizes or the ductulator to size the duct runs.

FAQ

How is the load calculated?

Each room's base load is its volume in cubic meters × 500 BTU/h (600 when sun-exposed). On top of that, the calculator adds 600 BTU/h per occupant beyond the first and 4,000 BTU/h for a kitchen. The total across rooms is the block cooling load in BTU/h and tons.

Is this a Manual J calculation?

No — it is a quick block estimate. A full ACCA Manual J models insulation R-values, window areas and orientation, infiltration, and local design temperatures. Use this page to get in the right range and sanity-check quotes; use a full Manual J for permits and final equipment selection.

Why calculate room by room instead of the whole house?

Room-by-room results show how the load is distributed, which is what you need to size branch ducts and check that each room gets its share of airflow. A single whole-house number hides rooms that will run hot.

How do I turn room loads into airflow (CFM)?

Give each room its proportional share of the blower's airflow: room CFM = room BTU ÷ total BTU × system CFM. With 400 CFM per ton, a room carrying 25% of the load on a 3-ton system should get about 300 CFM.

What about the heating load?

A rough heating check for moderate climates is 130–175 BTU/h per cubic meter of heated space (about 30–40 BTU per sq ft with 8-foot ceilings), the higher end for cold climates and older insulation. Heating is sized separately from cooling — do not reuse the cooling tonnage.