A 350 ft² bedroom with an 8 ft ceiling and average sun needs an 8,000 Btu/h air conditioner: that is the ENERGY STAR chart value for 300–350 ft². The same room with very sunny windows takes 8,800 Btu/h and moves up to a 10,000 Btu/h unit; a 500 ft² kitchen-living area with four people and lots of sun needs 18,400 Btu/h — a 2-ton (24,000 Btu/h) system. The calculator above does this for any room and adds insulation, glass area, climate and electronics.
How the calculator works
Enter the floor area, the ceiling height and how many people use the room, then pick the insulation, windows, sun, climate and the heat from electronics. The result is the cooling load in Btu/h, the next standard unit size, its power draw by EER and the energy a month.
ENERGY STAR sizing chart
| Area, ft² | Btu/h | Area, ft² | Btu/h |
|---|---|---|---|
| 100–150 | 5,000 | 450–550 | 12,000 |
| 150–250 | 6,000 | 550–700 | 14,000 |
| 250–300 | 7,000 | 700–1,000 | 18,000 |
| 300–350 | 8,000 | 1,000–1,200 | 21,000 |
| 350–400 | 9,000 | 1,200–1,400 | 23,000 |
| 400–450 | 10,000 | 1,500–2,000 | 30,000 |
Adjustments
- Heavily shaded room: −10 %; very sunny: +10 %.
- More than two people: +600 Btu/h for each extra person.
- Kitchen: +4,000 Btu/h.
- Ceiling above 8 ft: in proportion — a 10 ft ceiling adds 25 %.
- Poor insulation ×1.25, good ×0.85; few windows ×0.9, lots of glass ×1.2; mild summers ×0.9, very hot ×1.15.
Why not bigger
An oversized unit cools the air quickly and shuts off before it has pulled the moisture out, so the room feels cold and clammy and the compressor wears from short cycles. The calculator takes the next standard size up and adds no margin. One ton of cooling is 12,000 Btu/h.
Running cost
Power draw = capacity ÷ EER: an 8,000 Btu/h unit at EER 11 draws 727 W, or 175 kWh a month at 8 hours a day. Enter your electricity price to see the cost.
For a whole house, the cooling load depends on the walls, windows and attic — see the Heating BTU Calculator for the same surfaces, the Insulation Calculator for R-values and the CFM Calculator for fresh air. Room area: Square Footage Calculator.