AC Sizing Sheet
Work the number out, then resist rounding up. Oversizing is the mistake that makes a room clammy.
BFCBrilliance · bfcbrilliance.com/tools/ac-btu-calculator
The room
- Room
- Date
- Length
- Width
- AREA
- Ceiling height
- BTU per sq ft used
- Sun exposure
- Extra people
- Kitchen?
- BTU NEEDED
- Unit chosen
- EER
The rule and the adjustments
- Base rule
- 20 BTU/hr per sq ft (range 20-25)
- Assumes a ceiling of
- about 8 ft
- Heavily shaded
- −10%
- Very sunny
- +10%
- Each person beyond two
- +600 BTU
- Kitchen
- +4,000 BTU — large, and not a typo
- Source of the adjustments
- ENERGY STAR
- Common unit sizes
- 5,000 · 6,000 · 8,000 · 10,000 · 12,000
- ⚠ Between two sizes?
- take the NEARER, not the bigger
- Efficiency
- ENERGY STAR label, EER 10 or above
- ⚠ Whole house?
- needs a Manual J load calculation, not this
Room by room
| Room | Area | Ceiling | Conditions | BTU needed | Unit chosen |
|---|---|---|---|---|---|
Before buying
- Floor area measured, not estimated
- Ceiling height accounted for if it is not about 8 ft
- Sun exposure judged honestly — which way does it face?
- Kitchen surcharge applied if there is a cooker in the room
- NEARER unit size chosen, not automatically the bigger one
- EER checked — 10 or above, ENERGY STAR labelled
- Window size and type checked against the unit's fitting requirements
- Electrical circuit checked for the unit's draw
- Drainage and where condensate goes considered
- Whole-house system? Ask for a Manual J, not a rule of thumb
Oversizing makes the room worse
Removing humidity takes RUNNING TIME. A unit that is too big hits the thermostat setting fast and shuts off, never running long enough to dehumidify — so the room ends up cold and clammy, and the compressor wears from constant cycling. Rounding up feels like buying safety and is buying that problem.
The rule ignores the third dimension
20 BTU per square foot assumes roughly an 8-foot ceiling. A 12-foot ceiling means half as much air again for the same footprint, and an air conditioner cools air, not floor. Scaling by the height ratio is the correction the basic rule leaves out.
This sizes a room unit, not a system
For a central or whole-house system the right tool is a Manual J load calculation — insulation, window area and orientation, air leakage, local design temperatures. If someone quotes a central system without one, that is worth noticing.