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BTU calculator

Enter a room's size, how sunny it is and how many people use it to get the air conditioner size ENERGY STAR's chart gives, in BTU per hour, watts and tons.

Units
Shape
Sun

Who regularly use it. ENERGY STAR adds 600 BTU for each person above two.

Not covered: furnaces, heat pumps and central air. ENERGY STAR says a contractor should size those with a load calculation (ACCA Manual J or equivalent).

Air conditioner size

6,000 BTU/h

1.76 kW · 0.5 tons of cooling

Room
192 sq ft
Chart size
6,000 BTU/h150 up to 250 sq ft
  • A rule of thumb for one room from ENERGY STAR's chart, which assumes 8 ft ceilings. It is not a load calculation.
How it's calculated
  1. Room: 192 sq ft, in ENERGY STAR's band 150 up to 250 sq ft: 6,000 BTU/h
  2. Total 6,000 BTU/h × 0.29307 = 1,758 W; ÷ 12,000 = 0.5 tons

Estimates use manufacturer data sheets and industry guides (sources). Check local codes and your supplier's figures before you order.

How to use the btu calculator

  1. Measure the room's length and width in feet, or enter its area.
  2. Pick how sunny it is: heavily shaded, average or very sunny.
  3. Enter how many people regularly use the room, and tick the box if it's a kitchen.
  4. Read the size in BTU per hour for a window, wall or portable unit for that room.

How to size a room air conditioner

Start with ENERGY STAR's capacity for the room's area
Heavily shaded: − 10%, very sunny: + 10%
Each person above two: + 600 BTU
Kitchen: + 4,000 BTU

Example: a 12 × 16 ft bedroom that gets a lot of sun, used by three people.

  • Area: 12 × 16 = 192 sq ft, in the 150 up to 250 sq ft band: 6,000 BTU
  • Very sunny: + 10% = 6,600 BTU
  • One person above two: + 600 = 7,200 BTU per hour
  • That’s 7,200 × 0.293 = 2.1 kW, or 0.6 tons

ENERGY STAR’s chart

Area to be cooled Capacity
100 up to 150 sq ft 5,000 BTU/h
150 up to 250 sq ft 6,000 BTU/h
250 up to 300 sq ft 7,000 BTU/h
300 up to 350 sq ft 8,000 BTU/h
350 up to 400 sq ft 9,000 BTU/h
400 up to 450 sq ft 10,000 BTU/h
450 up to 550 sq ft 12,000 BTU/h
550 up to 700 sq ft 14,000 BTU/h
700 up to 1,000 sq ft 18,000 BTU/h
1,000 up to 1,200 sq ft 21,000 BTU/h
1,200 up to 1,400 sq ft 23,000 BTU/h
1,400 up to 1,500 sq ft 24,000 BTU/h
1,500 up to 2,000 sq ft 30,000 BTU/h
2,000 up to 2,500 sq ft 34,000 BTU/h

The capacities assume an 8 ft ceiling. The calculator reads “100 up to 150” as 100 sq ft or more but less than 150, so a room right on a boundary takes the larger size. It applies the 10% for sun to the chart’s figure first, then adds people and the kitchen; ENERGY STAR doesn’t say in which order. Rooms under 100 sq ft get the chart’s smallest size, 5,000 BTU.

A rule of thumb, not a load calculation

The chart sizes a room unit from floor area, with only the sun, people and kitchen adjustments. For a whole house, ENERGY STAR asks contractors to calculate the heating and cooling loads with ACCA Manual J or equivalent, and to size central air, heat pumps and furnaces from that. To check your attic against ENERGY STAR’s insulation levels, use the insulation calculator.

What’s not covered

Heating and furnace sizing, and whole-house systems over 2,500 sq ft, are beyond ENERGY STAR’s room chart. Ask an HVAC contractor for a Manual J load calculation.

Questions and answers

How do I calculate BTU for a room?

Find the room's area on ENERGY STAR's chart, then adjust it: 10% less if the room is heavily shaded, 10% more if it's very sunny, 600 BTU for each person above two who regularly uses it, and 4,000 BTU more in a kitchen. A 12 × 16 ft bedroom (192 sq ft) needs 6,000 BTU; very sunny with three people, 7,200.

How many square feet does a 5,000 BTU air conditioner cool?

100 up to 150 sq ft, according to ENERGY STAR's chart. 8,000 BTU covers 300 up to 350 sq ft, 10,000 BTU 400 up to 450 and 12,000 BTU 450 up to 550, all for rooms with 8 ft ceilings.

What size air conditioner do I need for 500 square feet?

12,000 BTU per hour: ENERGY STAR's chart puts 450 up to 550 sq ft at 12,000 BTU, before the adjustments for sun, people and a kitchen. A very sunny 500 sq ft room takes 13,200.

How many tons of AC do I need?

Divide the BTU per hour by 12,000, since a ton of cooling is 12,000 BTU per hour (NIST). A 12,000 BTU room unit is 1 ton. For a whole house or central air, ENERGY STAR says the contractor should size the system with a load calculation, ACCA Manual J or equivalent, not a square-foot chart.

What is a BTU?

A British thermal unit, a unit of energy. Air conditioners are rated in BTU per hour; NIST gives 1 BTU per hour as 0.293 watts, so a 10,000 BTU unit removes about 2.9 kW of heat.

Is a bigger air conditioner better?

No. ENERGY STAR says an oversized unit costs more, wastes energy and doesn't cool better: it cools the room before it can take out the humidity, so the room feels damp and clammy. Size it to the room.

Does a high ceiling need more BTU?

ENERGY STAR's chart assumes 8 ft ceilings and gives no factor for higher ones, so the calculator doesn't add one. For a tall, open or unusual room, ask an HVAC contractor.

What size furnace or heat pump do I need?

That isn't covered here. ENERGY STAR's chart is for cooling one room. For a furnace, a heat pump or central air, ENERGY STAR says the contractor should calculate your home's heating and cooling loads with ACCA Manual J or equivalent.

Sources

Figures checked October 2026.