How to use this calculator
- Choose Cooling (AC) or Heating.
- Enter the room’s length and width, or its floor area in square feet. For an open plan, measure the whole connected space you want to cool.
- Enter the ceiling height.
- For cooling, set the sun exposure, the number of people who regularly use the room, and whether it’s a kitchen.
- Pick your climate zone. The U.S. Department of Energy and the International Energy Conservation Code (IECC) split the country into zones by county; the examples in the list help you find yours.
The result is the capacity in BTU per hour, the unit printed on every air conditioner and heater, and the same figure in tons (for central systems) or kilowatts (for electric heat). The table shows the full ENERGY STAR chart, with your adjustments applied to every size band.
How the cooling size is worked out
The calculator follows the method ENERGY STAR publishes for room air conditioners. It starts from the chart value for your floor area and then adjusts it:
- Sun: heavily shaded rooms take 10% less; very sunny rooms 10% more (ENERGY STAR).
- People: add 600 BTU/hr for each person beyond two who regularly uses the room (ENERGY STAR).
- Kitchen: add 4,000 BTU/hr if the unit is in a kitchen (ENERGY STAR).
- Ceiling: the chart assumes 8-foot ceilings. Taller rooms hold more air and wall area, so the calculator scales up in proportion: 10-foot ceilings get × 1.25.
- Climate: ENERGY STAR’s chart has no climate adjustment. For zones 1 and 2, where summers are longest and hottest, this calculator adds 10%. Elsewhere it uses the chart as published.
To convert, 1 ton = 12,000 BTU/hr, and 1 watt = 3.412 BTU/hr.
Worked example
The default is a 16 × 20 foot living room with 8-foot ceilings, average sun, three regular occupants, in climate zone 4:
- Floor area: 16 × 20 = 320 sq ft.
- The chart band for 300 up to 350 sq ft gives 8,000 BTU/hr.
- One person beyond two adds 600: 8,600 BTU/hr, or 0.72 tons.
Room air conditioners are sold in steps, typically 5,000, 6,000, 8,000, 10,000 and 12,000 BTU/hr, so the closest size that meets 8,600 is a 10,000 BTU/hr unit.
Make the same room very sunny with 10-foot ceilings and the need rises to 11,600 BTU/hr: 8,000 × 1.25 for the ceiling = 10,000, plus 10% for sun = 11,000, plus 600 for the third person. A 12 × 15 foot kitchen (180 sq ft) needs 6,000 from the chart plus 4,000 for cooking, 10,000 BTU/hr.
ENERGY STAR room air conditioner sizing chart
| Area to be cooled | Capacity needed |
|---|---|
| 100 up to 150 sq ft | 5,000 BTU/hr |
| 150 up to 250 sq ft | 6,000 BTU/hr |
| 250 up to 300 sq ft | 7,000 BTU/hr |
| 300 up to 350 sq ft | 8,000 BTU/hr |
| 350 up to 400 sq ft | 9,000 BTU/hr |
| 400 up to 450 sq ft | 10,000 BTU/hr |
| 450 up to 550 sq ft | 12,000 BTU/hr |
| 550 up to 700 sq ft | 14,000 BTU/hr |
| 700 up to 1,000 sq ft | 18,000 BTU/hr |
| 1,000 up to 1,200 sq ft | 21,000 BTU/hr |
| 1,200 up to 1,400 sq ft | 23,000 BTU/hr |
| 1,400 up to 1,500 sq ft | 24,000 BTU/hr |
| 1,500 up to 2,000 sq ft | 30,000 BTU/hr |
| 2,000 up to 2,500 sq ft | 34,000 BTU/hr |
These are for 8-foot ceilings with average sun and up to two people, before the adjustments above. Above 2,500 square feet the calculator extends the chart at its top rate, 13.6 BTU/hr per square foot, and flags the result: a 3,000 square foot house with four people comes to 42,000 BTU/hr, about 3.5 tons. For a central system that’s a starting point for a conversation with a contractor, not a purchase decision.
Heating: a rule of thumb by climate zone
There’s no federal chart for heaters. HVAC suppliers commonly use a rule of thumb that rises from about 30 BTU/hr per square foot in the warmest zones to 60 in the coldest, and that’s what the heating mode uses:
| Climate zone | Example places | BTU/hr per sq ft | For 320 sq ft |
|---|---|---|---|
| Zone 1 (very hot) | South Florida, Hawaii | 30 | 9,600 |
| Zone 2 (hot) | Gulf Coast, Phoenix | 35 | 11,200 |
| Zone 3 (warm) | Atlanta, Dallas, Las Vegas | 40 | 12,800 |
| Zone 4 (mixed) | Washington DC, St. Louis, Seattle | 45 | 14,400 |
| Zone 5 (cool) | Chicago, Denver, Boston | 50 | 16,000 |
| Zone 6 (cold) | Minneapolis, Vermont | 55 | 17,600 |
| Zone 7–8 (very cold) | Northern Minnesota, Alaska | 60 | 19,200 |
For the default room in zone 4, that’s 14,400 BTU/hr, or 4.22 kW of electric heat. A plug-in space heater is limited to 1,500 watts (about 5,100 BTU/hr), so one heater can take the chill off a bedroom but won’t carry a large, cold room alone. With 9-foot ceilings in zone 6, the same room needs 19,800 BTU/hr.
Tips and interpreting your result
- Don’t round up a size “to be safe.” ENERGY STAR warns that an oversized unit costs more, wastes energy and leaves the room cool but clammy, because it shuts off before it has removed the moisture. If your number falls between sizes, take the closest one at or just above it.
- Whole-house systems need a Manual J. Central air, heat pumps and furnaces should be sized with an ACCA Manual J load calculation, which accounts for insulation, windows, duct leaks and orientation. Ask any contractor quoting a system to show you theirs.
- Running costs matter too. Once you know the size, use the electricity cost calculator to see what a unit costs to run at your state’s electricity rate, or the solar calculator to see how much of that a rooftop system could cover.
- Measuring an odd-shaped room? The square footage calculator adds up L-shapes and multiple areas.
Assumptions and limitations
- The cooling chart is for single rooms and room air conditioners. Results above 2,500 square feet are extrapolated and flagged.
- The ceiling and climate adjustments are this calculator’s, not ENERGY STAR’s. The heating figures are a rule of thumb, not a heat-loss calculation, and assume reasonably insulated walls.
- The calculator doesn’t account for insulation level, window area, upstairs rooms under a hot roof, or heat from electronics. Lean toward the next size up for a top-floor room with a lot of west-facing glass, and down for a well-shaded basement.
Sources
- ENERGY STAR: Room air conditioners, sizing chart and adjustments
- U.S. Department of Energy, Building America: climate-specific guidance and climate zones
- ACCA Manual J residential load calculation
Frequently asked questions
How many BTUs do I need per square foot?
For cooling a single room, the ENERGY STAR chart works out to between about 14 and 50 BTU per hour per square foot, highest for small rooms because the smallest units are 5,000 BTU. A 320 square foot room needs 8,000 BTU/hr, or 25 per square foot. Whole houses need less per square foot because not every room is loaded at once.
What size air conditioner do I need for a 12 by 12 room?
A 12 by 12 foot room is 144 square feet, which falls in ENERGY STAR's 100 to 150 square foot band, so a 5,000 BTU/hr unit is enough. Make it 5,500 if the room is very sunny, and add 600 for each person beyond two who regularly uses the room.
How many BTUs is a ton of air conditioning?
One ton of cooling is 12,000 BTU per hour. The term dates back to ice. It is the heat needed to melt one ton of ice over 24 hours. A 3-ton central air conditioner delivers 36,000 BTU/hr.
Is it bad to buy an air conditioner that is too big?
Yes. ENERGY STAR notes that an oversized unit costs more, wastes energy and cools the room before it has a chance to remove the humidity, so the room feels damp and clammy. Pick the closest size that meets your number, not the biggest one in the store.
Do higher ceilings need a bigger air conditioner?
ENERGY STAR's capacities assume an 8-foot ceiling. This calculator scales up in proportion for taller ceilings, so a 10-foot ceiling adds 25%. It doesn't size down for low ceilings, to stay on the safe side.
How big a heater do I need for a room?
A common rule of thumb is 30 to 60 BTU per hour per square foot, depending on how cold your climate is. A 320 square foot room in a mixed climate needs about 14,400 BTU/hr, or 4.2 kW of electric heat. A plug-in space heater maxes out at 1,500 watts, about 5,100 BTU/hr.
Updated November 2026. Results are estimates for educational purposes and depend on the inputs you provide.