BTU Calculator

To size a room air conditioner, look up the floor area in the ENERGY STAR sizing chart, then adjust for sun, people and cooking. A 15 ft × 12 ft bedroom (180 sq ft) falls in the 150 to 250 sq ft row, so it needs 6,000 BTU/h, half a ton of cooling. Subtract 10% if the room is heavily shaded, add 10% if it is very sunny, add 600 BTU/h for each person beyond two and add 4,000 BTU/h for a kitchen. This BTU calculator applies the chart and every adjustment as you type.

ENERGY STAR, the federal efficiency program run by the EPA and the Department of Energy, publishes the chart for room air conditioners such as window and through-the-wall units, and it is also a reasonable first check for a portable unit or a single mini-split head. The chart assumes an 8-foot ceiling. If you are unsure of the floor area, run the room through the Square Footage Calculator first, and once you know the size, the Electricity Cost Calculator tells you what the unit will cost to run.

The ENERGY STAR sizing chart

These are the capacities ENERGY STAR lists for each floor-area band, before any adjustment. Each band runs from the lower figure up to the upper one; at an exact boundary such as 250 sq ft the calculator uses the higher band.

Source: ENERGY STAR, Room Air Conditioners, "Properly Sized Room Air Conditioners" (capacities for an 8-foot ceiling). Tons = BTU/h ÷ 12,000. The last column divides each capacity by the band's upper and lower area.

The four ENERGY STAR adjustments

After the chart, ENERGY STAR lists four corrections. The calculator applies them in this order:

The sun correction applies to the chart value only, so a very sunny 6,000 BTU/h bedroom becomes 6,600 BTU/h. The people correction counts only regular occupants beyond the first two, because the chart already assumes two. The kitchen correction is a flat 4,000 BTU/h for the heat from cooking appliances. The result is the minimum capacity to shop for; the calculator also converts it to tons at 12,000 BTU/h per ton.

Worked example: a sunny 360 sq ft kitchen-diner

Take an open kitchen and dining area of 24 ft × 15 ft = 360 sq ft, used by four people, with a wall of west-facing glass. Each row shows what the calculator adds:

The space needs a unit rated at least 15,100 BTU/h, so a 15,000 BTU/h model falls just short and you would look at the next size up. Notice that the kitchen alone adds more than a quarter of the total: two rooms with the same floor area can need very different units.

How to measure your room and use the tool

ENERGY STAR's own instructions take a tape measure and a minute:

1. Measure in feet and convert the inches. 12 ft 6 in is 12.5 ft; 9 in is 0.75 ft, 6 in is 0.5 ft and 3 in is 0.25 ft.
2. Find the floor area. For a square or rectangular room multiply length by width. For a triangular area multiply length by width and divide by two. Split an L-shaped or complex room into rectangles and triangles and add them, then switch the calculator to "Square feet" and type the total.
3. Enter the ceiling height. The chart assumes 8 feet; the calculator shows the room volume and flags taller ceilings.
4. Count the people who regularly use the room at the same time. Two are built into the chart.
5. Tick the kitchen box if the unit will cool a kitchen or an open plan that includes one.
6. Pick the sun exposure: "Very sunny" for large sunlit windows, "Shaded" for a room that stays in shade most of the day, "Normal" otherwise.

The result updates as you type: the recommended BTU/h and tons on top, then the step-by-step build-up, the ENERGY STAR chart with your row highlighted and a rough heating range.

Who this calculator is for

• Renters and homeowners shopping for a window, wall or portable air conditioner for one room.
• DIYers checking the size of a ductless mini-split head for a bedroom, sunroom or garage.
• Landlords choosing units for apartments without paying for capacity they do not need.
• Small-business owners cooling an office, a back room or a small shop floor.
• Anyone double-checking a quote who wants an independent number before talking to an installer.

Key terms explained

• BTU (British thermal unit): a unit of heat energy. Capacity is quoted in BTU per hour (BTU/h), how much heat a unit can move each hour.
• Ton: 12,000 BTU/h of cooling, about 3,517 watts (NIST). A 1.5-ton unit is 18,000 BTU/h.
• Cooling capacity: the BTU/h rating on the box or nameplate; it should be at least the calculated figure.
• EER and CEER: efficiency ratings for room air conditioners, in BTU/h of cooling per watt of electricity. Higher is cheaper to run.
• SEER2: the seasonal efficiency rating for central air conditioners and heat pumps tested under the 2023 federal procedure.
• Manual J: the residential load calculation published by ACCA, the Air Conditioning Contractors of America, used to size whole-home systems.
• Short cycling: an oversized unit switching on and off quickly, cooling the air without removing enough humidity.

Quick scenarios

Here is how the chart and the adjustments play out in common rooms:

• Small bedroom, 120 sq ft, one person, heavily shaded: 5,000 BTU/h from the chart minus 10% = 4,500 BTU/h.
• Home office, 200 sq ft, two people, normal sun: the 150 to 250 sq ft row gives 6,000 BTU/h with no adjustments.
• Living room, 500 sq ft, four people, very sunny: 12,000 + 1,200 + 1,200 = 14,400 BTU/h, about 1.2 tons.
• The same living room with an open kitchen: add 4,000 for 18,400 BTU/h, about 1.5 tons.

Floor area gets you into the right row, but a kitchen or a wall of sunny glass can push you up a size or more.

Why the "20 BTU per square foot" shortcut misleads

Many websites multiply floor area by 20. That shortcut ignores the way the ENERGY STAR chart front-loads small rooms, which have more wall and window per square foot of floor:

For bedrooms and offices the shortcut can point you to a unit half the size ENERGY STAR recommends, which will run flat out on hot afternoons without keeping up. That is why this calculator uses the chart rather than a single multiplier.

Ceiling height, climate and other factors the chart does not cover

• Ceiling height: the chart assumes 8 feet. ENERGY STAR suggests more capacity for higher ceilings; a 12-foot ceiling holds 50% more air than an 8-foot one.
• Top floors and attic rooms: a room under a hot roof behaves like a very sunny room, so treat it as one.
• Insulation and air leaks: the chart has no input for them. A drafty room or one with single-pane windows needs more than the chart suggests.
• Climate: long, humid summers favor a unit that runs steadily and dehumidifies, which is another reason not to oversize.
• Placement: ENERGY STAR advises that a unit mounted near a corner should be one that can send its airflow in the right direction.

Which type of air conditioner fits your number?

The BTU figure also points to the kind of unit that fits, because each form factor covers a typical capacity range:

• Window and wall units: the usual choice for a single room up to the mid-teens of thousands of BTU/h. They need a suitable window or a wall sleeve.
• Portable units: roll from room to room and vent through a hose to a window; compare their rated capacity, not the box's room-size claim.
• Ductless mini-splits: a wall head plus an outdoor unit, well suited to additions, garages and homes without ducts, and most of them also heat.
• Central air: for whole homes through ductwork. Here a room chart is only a sanity check; the system should be sized with a Manual J calculation.

From BTUs to watts and running cost

Capacity tells you whether a unit is big enough; its efficiency rating tells you what it costs to run. Room units are rated by EER or CEER, BTU/h per watt, so electrical draw is roughly capacity divided by the rating. A 6,000 BTU/h unit rated 11 draws about 545 W; run 8 hours a day, that is 4.36 kWh, or about $0.80 a day and $24 a month at the U.S. average residential price of 18.31 cents per kWh (EIA, July 2026). A 12,000 BTU/h unit at the same rating uses twice as much. Plug your own model's rating, hours and rate into the Electricity Cost Calculator.

For central systems, federal rules (10 CFR 430.32) have required new split-system air conditioners to reach at least 13.4 SEER2 since January 1, 2023, and 14.3 SEER2 for units under 45,000 BTU/h installed in the Southeast and Southwest. Because energy use scales with 1 ÷ SEER2, a 20 SEER2 system uses about 30% less electricity for the same cooling than a 14 SEER2 system.

Heating BTUs: what this tool can and cannot tell you

There is no official square-foot chart for heating. Heat loss depends on how cold your winters are, how well the walls, ceiling and windows are insulated, and how leaky the house is. The calculator therefore shows heating only as a rough range of 30 to 40 BTU/h per square foot, a common contractor rule of thumb for average insulation: 5,400 to 7,200 BTU/h for the 180 sq ft bedroom above. For electric heat, NIST's conversion makes the comparison easy: 1 watt is 3.412 BTU/h, so a 1,500 W space heater delivers about 5,118 BTU/h, and each kilowatt of electric heat about 3,412 BTU/h. For a furnace, boiler or heat pump, ask for a Manual J calculation; in cold climates the heating load, not the cooling load, usually decides the size.

Chart vs. a Manual J load calculation

The ENERGY STAR chart is the right tool for choosing a window, wall, portable or single mini-split unit, because a small error in one room is easy to live with. A Manual J load calculation, the residential method published by ACCA, is far more detailed: it works room by room from wall and ceiling insulation, window area, glass type and orientation, air leakage, duct losses and local design temperatures. For a whole-home central system that precision matters, because an oversized system short-cycles across the entire house and an undersized one never keeps up in a heat wave. Use this calculator for single rooms and as a sanity check: if a contractor's whole-home number looks far off, ask to see the Manual J inputs.

Tips for getting the size right

• Buy at or just above the number. ENERGY STAR is explicit that an oversized air conditioner costs more, wastes energy and does not cool better.
• Include shared open space. If the room opens to a hallway or dining area with no door, add that area.
• Mind the windows. Large sunlit windows are what the "very sunny" setting is for.
• Check the electrical. Larger room units can need a dedicated circuit or a 230/240 V receptacle; the nameplate and plug type tell you which.
• Keep heat away from the thermostat. ENERGY STAR advises keeping lamps and TVs away from the unit's thermostat and setting it as high as is comfortable, typically 78°F.

Limitations and assumptions

• It follows the ENERGY STAR room air conditioner chart, which covers 100 to 2,500 sq ft at an 8-foot ceiling; rooms under 100 sq ft use the smallest row.
• It does not model insulation, window area, glass type or air leakage beyond the shade and sun settings.
• The heating range is a rule of thumb, not an official figure, and real heating loads vary widely by climate.
• It sizes one room or open area at a time and does not replace a Manual J calculation for a whole home.

Related calculators

• Use the Square Footage Calculator to get the floor area of an irregular room.
• Turn the unit's watts and hours into dollars with the Electricity Cost Calculator.
• Find the current a unit will draw from its watts with the Watts to Amps Calculator.
• Reduce the load itself by upgrading the attic or walls; the Insulation Calculator estimates how much material reaches a target R-value.

Sources

• ENERGY STAR (U.S. EPA and DOE): Room Air Conditioners, "Properly Sized Room Air Conditioners": sizing chart, 8-foot ceiling basis, shade/sun, occupant and kitchen adjustments, oversizing and thermostat advice.
• National Institute of Standards and Technology: NIST SP 811, Appendix B.8 (1 BTU/h = 0.2930711 W; ton of refrigeration = 12,000 BTU/h = 3,516.853 W).
• Code of Federal Regulations: 10 CFR 430.32(c), energy conservation standards for central air conditioners (SEER2 minimums from January 1, 2023).
• U.S. Energy Information Administration: Electric Power Monthly, Table 5.6.A (average residential price 18.31 cents per kWh, July 2026).