What size air conditioner for this room?

People type this into search every day: air conditioner sizing calculator.

A room has a cooling load. The equipment has a capacity. If you pick from the floor area alone, you skip the sun, the people, and the appliances that also dump heat into the room.

The air conditioner room sizing calculator is a sketch of that load. You type the watts per square metre, a percent for the room, and the extra people and equipment. This page runs that sketch on labeled inputs. It is not an ACCA Manual J, and it is not a shopping list.

Area, then the loads the area misses

The formula is:

length × width × watts per square metre × (1 + adjustment) + extra people × watts each + appliance watts

The total is the approximate cooling capacity, printed in kilowatts. The tool also multiplies the watts by 3.412141633 and prints BTU/h. A planning range is 85% to 115% of the same total. That band is part of this calculator. It is not a safety factor from a load manual.

The assumptions say the watts-per-area figure and the percent are yours to set for the climate and the room. Ceiling height is treated as standard. Humidity, ventilation, ducts, infiltration, and the rooms next door are not in the model. The result note says to use a qualified room-by-room load calculation before you select or install equipment.

The Department of Energy’s Building America fact sheet on right-sizing says to beware of casual sizing estimates based on home floor area. The procedures it points to for US homes are ACCA Manual J for the load and Manual S for the equipment. Manual J uses wall, ceiling, window, and floor area, insulation, leakage, orientation, and occupancy. The site’s own source page on proper HVAC sizing is the short version of that warning. Use the examples here to see what each input does. Use Manual J when you are buying a whole-home system.

For a window unit, ENERGY STAR publishes a different room method on how to choose the right size. Example 9 follows that page. It does not feed this calculator.

Example 1: the published defaults

• length: 5 m
• width: 4 m
• base capacity: 125 W/m²
• sun, glazing, and insulation adjustment: 15%
• additional occupants beyond two: 1
• watts per additional occupant: 100 W
• lighting and appliance load: 300 W

Floor area: 5 × 4 = 20 m². The card prints that as 215.3 ft², because 20 × 10.763910417 = 215.278.

Room load: 20 × 125 × 1.15 = 2,875 W.

People: 1 × 100 = 100 W.

Appliances: 300 W.

Total: 2,875 + 100 + 300 = 3,275 W, which prints as 3.28 kW.

BTU/h: 3,275 × 3.412141633 = 11,174.76, which prints as 11,175 BTU/h.

Planning range: 3,275 × 0.85 = 2,783.75 W, printed as 2.78 kW. 3,275 × 1.15 = 3,766.25 W, printed as 3.77 kW.

People and appliances together: 400 W.

The area term is 2,875 ÷ 3,275 = 87.8% of the total. The extra person is 3.1%. The appliance line is 9.2%. A floor-area-only number leaves out that 400 W, which is example 2.

Example 2: stopping at the floor area

Same 5 m by 4 m room and the same 125 W/m². Set the adjustment to 0, extra people to 0, and appliances to 0.

Room load: 20 × 125 = 2,500 W, which prints as 2.5 kW.

BTU/h: 2,500 × 3.412141633 = 8,530.35, which prints as 8,530 BTU/h.

Range: 2.13 to 2.88 kW. People and appliances: 0 W.

Against the default, this shortcut is 3,275 − 2,500 = 775 W lower, or 11,175 − 8,530 = 2,645 BTU/h. Of that gap, 375 W is the 15% adjustment on the 2,500 W room term. The other 400 W is the person and the appliances. Both are easy to leave out if you only multiply length by width.

A related edge: length 0, everything else at the defaults. Area is 0, so the room term is 0. The 400 W of people and appliances remains, which prints as 0.4 kW and 1,365 BTU/h. The range is 0.34 to 0.46 kW. The formula does not require a floor in order to keep the extra lines. A real room with no floor area is a bad input. The result is there so you can see those lines are added after the area, not inside it.

Example 3: shade, a flat adjustment, and a sunny room

Hold the default 5 m by 4 m, 125 W/m², one extra person at 100 W, and 300 W of appliances. Change only the percent.

The room term before the percent is 20 × 125 = 2,500 W. People and appliances stay 400 W.

• Adjustment 0%: room 2,500 W. Total 2,900 W, printed as 2.9 kW. BTU/h 9,895. Range 2.47 to 3.33 kW.
• Adjustment −10% (shade): room 2,500 × 0.90 = 2,250 W. Total 2,650 W, printed as 2.65 kW. BTU/h 9,042. Range 2.25 to 3.05 kW.
• Adjustment 15% (the default): 3.28 kW and 11,175 BTU/h.
• Adjustment 25%: room 2,500 × 1.25 = 3,125 W. Total 3,525 W, printed as 3.53 kW. BTU/h 3,525 × 3.412141633 = 12,027.8, printed as 12,028. Range: 2.996 kW prints as 3, and the top prints as 4.05 kW.

From −10% to +25% the capacity moves by 3,525 − 2,650 = 875 W, which is 12,028 − 9,042 = 2,986 BTU/h. The 400 W of people and appliances is in every row. Only the room term moved.

Example 4: six people in the default room

“Additional occupants beyond two” is the field name. A room with six people has four additional people if you are using the field the way it is labeled.

Hold every default except that count. Set additional occupants to 4.

People: 4 × 100 = 400 W.

Appliances stay 300 W. Together that is 700 W, which is what the card prints.

Room term stays 2,875 W.

Total: 2,875 + 700 = 3,575 W, printed as 3.58 kW.

BTU/h: 12,198. Range: 3.04 to 4.11 kW.

Against one additional person, three more people add 300 W, or 12,198 − 11,175 = 1,023 BTU/h. Each 100 W person is 100 × 3.412141633 = 341 BTU/h inside this conversion. ENERGY STAR’s window-unit page uses 600 BTU/h per person beyond two. Those are different adders. Type 100 W here only when 100 W is the load you mean.

Example 5: a small bedroom

• length: 3 m
• width: 3 m
• base: 125 W/m²
• adjustment: 0%
• additional occupants: 0
• appliance load: 200 W

Area: 9 m², printed as 96.9 ft².

Room: 9 × 125 = 1,125 W.

Total: 1,125 + 200 = 1,325 W, printed as 1.33 kW.

BTU/h: 4,521. Range: 1.13 to 1.52 kW. People and appliances: 200 W.

The default room was 20 m² and 3.28 kW. This one is under half the floor and under half the capacity, because the appliance line was cut from 300 W to 200 W as well as the area. 1,325 ÷ 3,275 = 40.5% of the default watts, on 9 ÷ 20 = 45% of the floor.

Example 6: a larger room with a heavier equipment line

• length: 6 m
• width: 5 m
• base: 125 W/m²
• adjustment: 10%
• additional occupants: 2
• appliance load: 800 W

Area: 30 m², printed as 322.9 ft².

Room: 30 × 125 × 1.10 = 4,125 W.

People: 2 × 100 = 200 W.

Appliances: 800 W. Together 1,000 W.

Total: 4,125 + 1,000 = 5,125 W, printed as 5.13 kW.

BTU/h: 17,487. Range: 4.36 to 5.89 kW.

The appliance line alone is larger than the entire small bedroom in example 5. If that 800 W is a kitchen, ENERGY STAR’s window-unit note adds a flat 4,000 BTU/h for a kitchen instead. Example 9 shows their adder on their own example room. I am not going to add 4,000 BTU/h on top of this 17,487. That would mix two methods.

Example 7: what if the watts per square metre change

Hold the default room, the 15% adjustment, one extra person, and 300 W. Change only the base rate. The area term is 20 × rate × 1.15 = 23 × rate. Then add 400 W.

• 80 W/m²: room 1,840 W. Total 2,240 W, printed as 2.24 kW. BTU/h 7,643. Range 1.9 to 2.58 kW.
• 100 W/m²: room 2,300 W. Total 2,700 W, printed as 2.7 kW. BTU/h 9,213. Range 2.3 to 3.1 kW.
• 125 W/m² (the default): 3.28 kW and 11,175 BTU/h.
• 150 W/m²: room 3,450 W. Total 3,850 W, printed as 3.85 kW. BTU/h 13,137. Range 3.27 to 4.43 kW.
• 175 W/m²: room 4,025 W. Total 4,425 W, printed as 4.43 kW. BTU/h 15,099. Range 3.76 to 5.09 kW.

Each extra 25 W/m² adds 20 × 25 × 1.15 = 575 W to the total, which is 4,425 − 3,850 = 575, and 3,850 − 3,275 = 575. The 125 W/m² default is a field value, not a code requirement. If you have a load per square metre from a designer or from a product guide you trust, type that.

Example 8: the common mistake of treating cooling watts as plug watts

3.28 kW is a cooling capacity in this model. A window unit’s nameplate watts are the electricity it draws. Those are different numbers. A machine that moves 3,275 W of heat does not have to draw 3,275 W from the outlet.

The appliance electricity calculator uses the Department of Energy’s running-cost shape, which the appliance post spells out:

watts ÷ 1,000 × hours × days × price per kWh

Here the nameplate is an input I am choosing so the steps are visible: 900 W, 4 hours a day, 30 days, not a claim about how long anyone’s unit runs.

Energy: 0.900 × 4 × 30 = 108 kWh a month.

At the calculator’s $0.32 / kWh planning rate: 108 × 0.32 = $34.56 a month, $1.15 a day, and $414.72 a year.

The EIA Electricity Monthly Update puts US residential average revenue at 18.31 cents / kWh in July 2026. That is utility revenue per kilowatt-hour, not the line on one bill. The same 108 kWh at $0.1831 is $19.77 a month, $0.66 a day, and $237.30 a year.

If someone multiplies the 3,275 W cooling result by those hours, they get 3.275 × 4 × 30 = 393 kWh, which is 393 ÷ 108 = 3.6 times the nameplate example. Use the nameplate, or the rated input watts, in the cost tool. Leave the sizing result in kilowatts and BTU/h.

The heat pump post is a payback question for a different machine. It is not a room-sizing method.

Example 9: ENERGY STAR’s window-unit steps, on their example room

Their page works in feet, then a BTU table, then adjustments, then a round-down.

Their worked room is 14 ft 9 in by 10 ft. 14.75 × 10 = 147.5 sq ft. They say that area needs 5,000 BTU/h from the table. I am not going to invent the table row for this site’s 215.3 ft² default. Their printed example stops at 147.5.

Adjustments they list, applied to that 5,000:

• Very sunny: 5,000 × 1.10 = 5,500. Round down to the nearest thousand: 5,000.
• Heavily shaded: 5,000 × 0.90 = 4,500. Round down: 4,000.
• Two people beyond the first two: 5,000 + 2 × 600 = 6,200. Round down: 6,000.
• Kitchen: 5,000 + 4,000 = 9,000. That figure is already on a thousand.

They also say an oversized unit can cool the room before it has removed the humidity, and that the efficient choice is the one sized to the space. Their round-down is why 5,500 and 5,000 land on the same shopping number. This site’s calculator does not round to the nearest thousand. 11,175 BTU/h stays 11,175.

Our default room and their 147.5 sq ft room are different rooms, on different methods. The wallpaper post uses the perimeter of a 5 m by 4 m floor, which is 18 m of wall, not the 20 m² this cooling sketch multiplies.

The sizing runs

Open the room sizing calculator with the watts per square metre and the adjustment you intend to model. For a whole-home system, take the room list to a Manual J calculation before you buy the equipment.