Enter the floor area, climate, insulation quality and ceiling type for a rough planning estimate of cooling load in BTU per hour and tons, with a plus or minus 15% range.
Important: This is a rough planning estimate only. Correct sizing requires a Manual J load calculation by an HVAC professional; oversizing and undersizing both cause problems.
The estimate multiplies floor area by a climate factor (25 BTU per sq ft for a mild climate up to 40 for a very hot one, selected from editable defaults), then adjusts for insulation quality and high ceilings, and converts at 12,000 BTU per hour to one ton of cooling. A range of plus or minus 15% is shown because a rule-of-thumb method cannot capture window area and orientation, air leakage, duct losses, occupancy or local design temperatures. Treat the output as a rough planning estimate only: correct sizing requires a Manual J load calculation by an HVAC professional, which models the actual building room by room. Oversizing and undersizing both cause problems, from short cycling and poor humidity control when too large, to rooms that never reach temperature when too small.
Is this tonnage estimate enough to buy a system? No. It is a rough planning estimate only, useful for early budgeting and for sanity-checking a quote. Correct sizing requires a Manual J load calculation by an HVAC professional, because window area, orientation, leakage, ducts and local design temperatures all change the real load.
How many BTU are in a ton of cooling? One ton of cooling is 12,000 BTU per hour, a unit inherited from the cooling effect of melting a ton of ice over a day. A 36,000 BTU per hour system is therefore a 3 ton system.
What goes wrong with the wrong size? Oversizing and undersizing both cause problems. An oversized system short-cycles, controls humidity poorly and wears faster; an undersized system runs constantly and still cannot hold temperature on the hottest days. That is why a Manual J calculation, not a square-foot rule, is the standard for final sizing.