Window Air Conditioner Size Calculator
Calculate the right BTU window AC unit. Adjust for room size, ceiling height, sun exposure, insulation, kitchen use, occupants, and see electrical requirements and monthly cost.
π‘οΈ Window AC Size Calculator
ROOM
BTU SIZING
ELECTRICAL & COST
π‘ How to Size a Window Air Conditioner
Window air conditioners are the most common room-level cooling solution in the US, with over 10 million units sold annually. Choosing the right BTU size is critical β too small and the unit runs constantly without cooling adequately; too large and it short-cycles, leaving the room cold and humid.
The calculator above uses the Energy Star BTU guidelines with 6 real-world adjustment factors: ceiling height, sun exposure, insulation, kitchen heat, occupants, and electric rate β plus it shows electrical requirements and estimated monthly cost.
For heating, use our furnace BTU calculator. Calculate airflow with our CFM calculator. Measure room area with our square footage calculator.
Energy Star BTU Size Chart
The DOE/Energy Star baseline assumes 8 ft ceilings, average insulation, and 2 occupants.
| Room Size (sq ft) | Recommended BTU | Typical Unit |
|---|---|---|
| 100β150 | 5,000 | Small bedroom |
| 150β250 | 6,000 | Bedroom / office |
| 250β350 | 8,000 | Large bedroom |
| 350β450 | 10,000 | Living room |
| 450β550 | 12,000 (1 ton) | Large living room |
| 550β700 | 14,000 | Open plan room |
| 700β1,000 | 18,000 (1.5 ton) | Large open area |
| 1,000β1,200 | 21,000 | Multi-room / studio |
| 1,200β1,400 | 23,000 | Large apartment |
| 1,400+ | 25,000+ | Consider central AC |
Adjustment Factors
1. Ceiling Height
Standard ceilings are 8 ft. Taller ceilings mean more air volume to cool. Add 5% per foot over 8 ft. A 10 ft ceiling needs 10% more BTU; a 12 ft ceiling needs 20% more.
2. Sun Exposure
Heavy sun (south-/west-facing, large windows): add 10%. Heavy shade (north-facing, tree cover): subtract 10%. This is one of the most significant factors β a sun-baked room can need 20% more cooling than a shaded one.
3. Insulation Quality
Poor insulation (old homes, single-pane windows, no wall insulation): add 15%. Good insulation (new construction, double-pane, insulated walls): subtract 10%. Insulation quality dramatically affects cooling load.
4. Kitchen Use
Cooking appliances (oven, stove, dishwasher) generate significant heat. Add 4,000 BTU for kitchens. This is a fixed addition regardless of room size.
5. Occupants
The baseline assumes 2 people. Each additional person adds roughly 600 BTU of body heat. A room regularly occupied by 4 people needs 1,200 BTU more than baseline.
6. Electronics & Appliances
Rooms with many computers, monitors, or other heat-generating equipment may need an additional 10% BTU. Server rooms and home offices with multiple screens are common examples.
Electrical Requirements
| BTU Range | Voltage | Amperage | Circuit Type |
|---|---|---|---|
| 5,000β12,000 | 115V | 5β12 amps | Standard outlet (shared OK) |
| 12,000β14,000 | 115V | 12β15 amps | Dedicated 15A circuit recommended |
| 14,000β18,000 | 230V | 7β9 amps | Dedicated 230V outlet required |
| 18,000β25,000 | 230V | 9β13 amps | Dedicated 230V/20A circuit |
Important: Units above 14,000 BTU require a dedicated 230V outlet β most homes don't have these in bedrooms. Check your electrical panel before purchasing. Installing a 230V outlet costs $150β$400.
Estimated Operating Cost
Monthly cost = (BTU Γ· 10) Γ· 1,000 Γ hours/day Γ 30 days Γ electric rate.
| BTU | Watts | 8 hrs/day @ $0.16/kWh | Annual (5 months) |
|---|---|---|---|
| 5,000 | 500W | $19/mo | $96 |
| 8,000 | 800W | $31/mo | $154 |
| 10,000 | 1,000W | $38/mo | $192 |
| 12,000 | 1,200W | $46/mo | $230 |
| 14,000 | 1,400W | $54/mo | $269 |
| 18,000 | 1,800W | $69/mo | $346 |
Tip: Look for Energy Star certified units β they use 10β15% less energy than standard models. CEER (Combined Energy Efficiency Ratio) of 12+ is excellent; 10+ is good.