Getting AC sizing wrong is expensive both ways: too small and it runs constantly without ever cooling properly, too large and it short-cycles, leaving the room cold but damp while wearing out the compressor early. Use this to get a real starting number based on ENERGY STAR and DOE guidance.
How This Calculator Works
The math behind the number is straightforward once you see each piece. Here is exactly what the calculator is doing with your inputs:
- Base calculation: 20 BTU per square foot. This is the ENERGY STAR and DOE baseline for residential cooling. A 300 sq ft room starts at 6,000 BTU before any adjustments.
- Ceiling height adjustment: +1,000 BTU per foot above 8 feet. Standard rooms are 8 feet tall. Taller ceilings mean more air volume to cool, vaulted or high ceilings add real load that flat square footage alone misses.
- Sun exposure: ±10%. A room with large south or west-facing windows takes on significant extra heat during the day, so it gets a 10% boost. A heavily shaded room gets a 10% reduction, since it never absorbs that same heat load.
- Occupants: +600 BTU per person beyond two. Bodies generate heat. The baseline assumes a typical 1-2 person room, extra regular occupants add real, if modest, cooling demand.
- Kitchen: +4,000 BTU flat. Ovens, stovetops, and other appliances generate substantially more ambient heat than a typical living space, so kitchens get a fixed bump rather than a percentage.
Add all of that together and divide by 12,000 to get tonnage, which is how central air systems are typically sized and sold. This is the same starting formula HVAC contractors use before a full, in-person Manual J load calculation, it will not catch every detail of your specific home, but it will get you within the right ballpark before you talk to anyone or buy anything.
This gives a solid starting estimate, the same baseline HVAC contractors use before a full Manual J load calculation. Older homes with poor insulation or drafty windows may need to size up. Once you know your target BTU, compare specific thermostat and climate control options in our Climate & Energy guide.
Formula based on ENERGY STAR and DOE cooling guidance. Last Updated: August 2026.