BTU Calculator for a Room Air Conditioner

How the calculator sizes a room

The calculator follows the ENERGY STAR method for room air conditioners: the window, wall and portable units that cool one room. It works in four steps, and the working on the plate shows each step with your numbers.

1. Floor area. Length times width, or type the area. Metric sizes are turned into square feet first, because the table is in square feet.
2. The table. ENERGY STAR lists the capacity a room needs by its floor area, from 5,000 BTU/h for 100 up to 150 sq ft to 34,000 BTU/h for 2,000 up to 2,500 sq ft. Each band starts at its first figure, so a room of exactly 150 sq ft takes the 6,000 BTU/h row. The capacities assume an 8 ft ceiling.
3. Sun. A heavily shaded room takes 10% off the table figure. A very sunny room adds 10%.
4. People and kitchen. Add 600 BTU/h for each person past two who regularly uses the room, and 4,000 BTU/h if the unit cools a kitchen.

The percentage applies to the table figure only. The 600 and the 4,000 are fixed amounts, so they come after the sun adjustment and are never scaled by it.

The answer is rounded up to the next 100 BTU/h. Room units come in steps, so the plate also names the next ENERGY STAR size at or above your estimate: that is the size to look for on the label.

Tons, BTU/h and kilowatts

Air conditioners are rated in BTU per hour (BTU/h), the heat they move out of the room each hour. A ton of cooling is 12,000 BTU/h (NIST, Trane), so a 24,000 BTU/h unit is a 2 ton unit. In watts, one ton is about 3.517 kW (NIST), which makes 1 kW about 3,412 BTU/h, the figure the EIA uses.

The kilowatts on the plate are cooling power: the heat the unit removes. They are not the electricity it draws from the wall. That figure is on the unit’s label in watts or amps, and the watts to amps calculator turns one into the other.

Why bigger is not better

It is tempting to go a size or two up for hot days. ENERGY STAR and Natural Resources Canada both warn against it: a unit that is too large cools the room before it has a chance to remove the humidity, and the room feels damp and clammy.

Type the size of a unit you own or are looking at, and the pilot lamp on the plate checks it against the estimate.

Higher ceilings

The ENERGY STAR capacities are calculated for an 8 ft ceiling. With higher ceilings, ENERGY STAR says you may want a unit with more capacity, but it gives no figure for how much, and other guides disagree with each other. So the calculator adds no percentage. Tick the ceiling box and the plate shows the note while the estimate stays the same.

One room, not a whole house

The ENERGY STAR table stops at 2,500 sq ft, and so does the calculator. Central air conditioners and heat pumps that serve a whole home are sized from a load calculation: a pro looks at the walls, windows, insulation, air leaks and the local climate, which a rule per square foot cannot see. ACCA’s Manual J is the usual method for that work.

An estimate above 34,000 BTU/h, the largest size in the ENERGY STAR list, points the same way: several room units or a central system, and a pro to size them.

Heating: furnace output and electric heat

Switch the plate to Heating. A furnace label lists its input, the fuel it burns, and its output, the heat it delivers: about input × AFUE, so 100,000 BTU/h at 80% gives 80,000. Type the heat a load calculation calls for and the plate shows the input to look for. Electric heaters turn every watt into heat (DOE): 1,500 W gives 5,118 BTU/h. A heat pump gives less heat as it gets colder, so a pro sizes it from the maker’s capacity at your design temperature; running costs are on the heat pump calculator.

Common mistakes

• Using one rate per square foot. The table needs 30 BTU/h per sq ft at 200 sq ft but only about 17 at 1,750 sq ft. A flat rate undersizes small rooms or oversizes large ones.
• Reading kW as running cost. Cooling kilowatts are heat moved, not power drawn. Use the label’s watts for running cost.

Worked example

1. Inputs: a 20 ft × 10 ft room (200 sq ft), very sunny, 3 people, not a kitchen.
2. Floor area = 20 ft × 10 ft = 200 sq ft
3. 200 sq ft: 6,000 BTU/h (ENERGY STAR, 150 up to 250 sq ft)
4. Very sunny: + 10% = 6,600
5. 3 people: + 600 = 7,200
6. Tons = BTU/h ÷ 12,000 = 7,200 ÷ 12,000 = 0.60 ton
7. kW = BTU/h × 3,516.853 ÷ 12,000,000 = 7,200 × 3,516.853 ÷ 12,000,000 = 2.11 kW

Questions people ask