Measure the conditioned room, not the whole floor by default
Use finished floor length, width and ceiling height of the space you mean. Garages, porches and unconditioned rooms stay off the line unless they are heated or supplied.
- BTU per square foot is a climate and construction factor you type, not a hidden constant.
- Air changes per hour is a planning ACH, not a ventilation-code path.
- Pipe volume on the utilities trade is capacity, not airflow.
BTU first, CFM second, never a furnace SKU
Open the furnace BTU calculator for a planning load from area and a stated factor. Open CFM for room airflow from volume and ACH. Do not buy equipment from either page. Adjacent utilities help with pipe volume and a 3-4-5 layout check.
These pages do not create an equipment order
A 36,000 BTU/h planning load is not a 3-ton furnace. A 240 CFM room is not a duct size. Carry the number into a conversation with the HVAC designer.
- Replace the default BTU-per-square-foot factor with the number you were given.
- Replace ACH with the occupancy you actually have.
- Keep equipment, refrigerant, registers and labour as separate estimate lines.
Planning boundary
These tools estimate a planning load or airflow. They do not design equipment, ducts, refrigerant charge, combustion air or a code path.