Estimate the cooling BTU/hr and air-conditioner size in tons for a room or home from floor area, climate, occupants, sun exposure and kitchen load.
Enter the floor area and details to estimate the cooling BTU/hr and tonnage needed.
Screening estimate only — not a substitute for a full ACCA Manual-J load calculation.
About this tool
The AC Tonnage / Cooling Load Calculator is a free, in-browser HVAC tool that estimates how many BTU/hr of cooling — and therefore how large an air conditioner in tons — a space needs. It uses a quick screening version of a Manual-J load calculation: base load = floor area (ft²) × a climate factor (about 20 BTU/hr·ft² in cool zones, 25 moderate, 30 warm, and 35 in hot, humid climates), scaled by ceiling height ÷ 8 for rooms taller or shorter than a standard 8-foot ceiling.
Every calculation runs locally in your browser, so nothing you enter is uploaded. On top of the base load the tool adds 600 BTU/hr for each occupant beyond two, 4,000 BTU/hr when a kitchen is included, and a ±10% sun adjustment for very sunny/west-facing or heavily shaded rooms. The total BTU/hr is divided by 12,000 (one ton of refrigeration) to get tonnage, and the recommended unit is rounded up to the next half-ton.
Results show the total cooling BTU/hr, the exact tonnage, the recommended standard unit size, and the effective BTU per square foot so you can sanity-check the number. This is a fast estimate for shopping and rough sizing — a licensed installer should still run a full ACCA Manual-J for a final specification, since real loads depend on windows, insulation, air leakage and duct layout.
Frequently asked questions
How many BTU do I need per square foot?
A common rule of thumb is 20–35 BTU/hr per square foot: about 20 in cool climates, 25 moderate, 30 warm, and 35 in hot, humid regions. This tool applies your chosen climate factor, then adjusts for ceiling height, occupants, kitchens and sun exposure.
How do I convert BTU to tons for an AC?
Divide the cooling BTU/hr by 12,000, because one ton of refrigeration equals 12,000 BTU/hr. So 24,000 BTU/hr is a 2-ton unit and 36,000 BTU/hr is 3 tons. The calculator rounds the recommendation up to the nearest half-ton.
What size AC do I need for a 500 sq ft room?
At a moderate factor of 25 BTU/hr·ft², a standard-height 500 ft² room needs about 12,500 BTU/hr, roughly 1 ton, before occupancy, kitchen or sun adjustments. Add 600 BTU per extra person and 4,000 for a kitchen.
Is this a real Manual-J calculation?
No — it is a quick screening estimate using rule-of-thumb factors. A full ACCA Manual-J accounts for window area, orientation, insulation R-values, infiltration and duct losses. Use this for shopping and rough sizing, then have an installer confirm.