Room Air Conditioner BTU Calculator
Uses the ENERGY STAR room air conditioner sizing chart and its four adjustments, exactly as published.
Size a room air conditioner
Recommended cooling capacity
Room area: 175.0 sq ft (chart row 150 up to 250 sq ft).
Chart and adjustments from energystar.gov, checked 2026-10-05.
A room air conditioner works best when its capacity matches the room. ENERGY STAR puts it plainly: "An oversized air conditioner costs more to purchase, wastes energy and does not provide better cooling." The calculator measures the floor area the way ENERGY STAR describes, looks up the capacity in the official chart, and then applies the four adjustments ENERGY STAR lists: shade, sun, extra occupants, and kitchens. It does not add rules of its own. As on the ENERGY STAR chart, cooling capacity is shown in British thermal units (BTUs) per hour.
Worked examples
| Room | Area (sq ft) | Chart BTU | Adjustments | Result |
|---|---|---|---|---|
| Bedroom, 12 ft 6 in x 14 ft, normal sun, 2 people | 175.0 | 6,000 | none | 6,000 |
| Living room, 18 ft x 20 ft, very sunny, 4 people | 360.0 | 9,000 | +900 sun, +1,200 people | 11,100 |
| Kitchen, 10 ft x 16 ft, heavily shaded, 2 people | 160.0 | 6,000 | -600 shade, +4,000 kitchen | 9,400 |
| Triangular loft, 20 ft x 16 ft, normal sun, 3 people | 160.0 | 6,000 | +600 people | 6,600 |
The triangle is 20 x 16 / 2 = 160 square feet. ENERGY STAR: "Multiply the length of the area by the width and divide by two."
The ENERGY STAR sizing chart
| Area to be cooled (square feet) | Capacity needed (BTU per hour) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
How it works
- Area. "Multiply the full length of the room by its width." For triangles, multiply length by width and divide by two. Inches become fractions of a foot, so 12 ft 6 in is 12.5 ft.
- Chart row. Each row runs from its first number up to its second. A room that lands exactly on a boundary uses the row that starts there, and the result also shows the row that ends there. The chart covers 100 to 2,500 square feet; outside that range the calculator says so and gives no number.
- Adjustments, in the order ENERGY STAR lists them. "If the room is heavily shaded, reduce capacity by 10 percent." "If the room is very sunny, increase capacity by 10 percent." "If more than two people regularly occupy the room, add 600 BTUs for each additional person." "If the unit is used in a kitchen, increase capacity by 4,000 BTUs." The 10 percent is taken on the chart capacity.
Formula: BTU per hour = chart capacity x (1 - 0.10 if heavily shaded, 1 + 0.10 if very sunny) + 600 x (people - 2, if above 2) + 4,000 if kitchen.
Frequently asked questions
What size air conditioner do I need for my room?
ENERGY STAR sizes room air conditioners by floor area. For example, 150 up to 250 square feet calls for 6,000 BTU per hour and 450 up to 550 square feet calls for 12,000. Then adjust for sun, shade, occupants, and kitchens.
How many BTU do I need for a 250 square foot room?
On the ENERGY STAR chart, 250 square feet starts the row "250 up to 300," which lists 7,000 BTU per hour. The row ending at 250 lists 6,000, so the calculator shows both when a room lands exactly on a row boundary.
Does a sunny room need a bigger air conditioner?
ENERGY STAR says: "If the room is very sunny, increase capacity by 10 percent." For a heavily shaded room it says to reduce capacity by 10 percent.
How do I size an air conditioner for a kitchen?
ENERGY STAR says: "If the unit is used in a kitchen, increase capacity by 4,000 BTUs."
Is a bigger air conditioner better?
No. ENERGY STAR says an oversized air conditioner costs more to purchase, wastes energy and does not provide better cooling. If the unit is too large, it will cool the room before it has a chance to remove the humidity.
What about high ceilings?
The ENERGY STAR capacities are calculated based on an 8-foot ceiling. The chart gives no formula for taller ceilings, so the calculator flags it rather than inventing an adjustment.
More HVAC tools
- HVAC pay by state and metro
Wage percentiles for any state or metro area, your pay compared, and an apprentice earnings planner. - Salary by state table
Sortable table of every state and DC with a two-state comparison. - EPA 608 certification checker
Find the Section 608 type you need from the EPA rule text. - HVAC license by state
Official licensing board quotes by state, with unverified states marked. - SEER2 savings calculator
DOE minimum SEER2 by region and yearly cooling cost at your price. - Job outlook 2025 to 2035
BLS projections, openings per year, and HVAC jobs in each state.