An air conditioner or heater that is too small runs constantly and never catches up; one that is too big cycles on and off, wastes energy and, in the case of an air conditioner, leaves the room cool but clammy. This calculator gives a reasoned starting size for a single room. For cooling it uses the ENERGY STAR room air conditioner chart and its adjustments. For heating it runs a simplified heat-loss estimate in the style of a Manual J calculation: heat lost through walls, windows, ceiling and floor, plus air leakage.
How to use the BTU calculator
- Choose Cooling or Heating and your units.
- Enter the room’s length, width and ceiling height.
- For cooling, set the sun exposure, the number of people who regularly use the room and whether it is a kitchen.
- For heating, enter the length of exterior wall, the window and door area, the insulation level, whether there is an attic above or unheated space below, and the outdoor design temperature.
How the estimates work
The chart assumes 8-foot ceilings, so the calculator scales the result up for taller rooms. In the heat-loss formula, U is each surface’s heat-transfer coefficient (the inverse of its effective R-value), ΔT is the indoor minus outdoor design temperature, ACH is air changes per hour, and 0.018 BTU per cubic foot per °F is the heat capacity of air. Floors over a crawl space or garage are counted at half the temperature difference, since those spaces stay warmer than outdoors.
Worked example: a 16 × 20 ft bedroom
Cooling: 320 ft² falls in the 300–350 ft² row: 8,000 BTU/h (0.67 ton, 2.34 kW). An 8,000 BTU unit draws roughly 730 W at an EER of 11.
Heating at 10 °F outside and 70 °F inside (ΔT = 60 °F), average insulation, 36 ft of exterior wall with 30 ft² of windows, attic above:
Walls 258 ft² × 0.085 × 60 = 1,316 · windows 30 × 0.5 × 60 = 900 · ceiling 320 × 0.035 × 60 = 672 · air leakage 2,560 ft³ × 0.5 × 0.018 × 60 = 1,382.
Total ≈ 4,270 BTU/h (1.25 kW), about 13 BTU/h per ft² — a 1,500 W heater would cover it.
The same bedroom used as a home office with four people, very sunny windows and 10-foot ceilings would need about 16,200 BTU/h of cooling, pointing to an 18,000 BTU unit.
ENERGY STAR room AC sizing
| Room area (ft²) | Capacity (BTU/h) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 350 | 7,000–8,000 |
| 350 up to 450 | 9,000–10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,500 | 21,000–24,000 |
| 1,500 up to 2,500 | 30,000–34,000 |
Adjustments: reduce 10% for heavy shade, add 10% for a very sunny room, add 600 BTU/h per regular occupant beyond two, and add 4,000 BTU/h for a kitchen.
Envelope assumptions for heating
| Level | Walls (U) | Ceiling (U) | Windows (U) | Air changes/h |
|---|---|---|---|---|
| Poor | 0.25 | 0.08 | 1.0 | 1.0 |
| Average | 0.085 | 0.035 | 0.5 | 0.5 |
| Good | 0.055 | 0.021 | 0.3 | 0.3 |
These are typical values for an older uninsulated room, a code-era room with R-13 walls and double-pane windows, and a tight, well-insulated room.
These are estimates for one room. Whole-house furnaces, heat pumps and central air conditioners should be sized with an ACCA Manual J load calculation, which accounts for orientation, shading, duct losses, internal gains and local design temperatures.
Improving insulation shrinks the heating load; see the insulation calculator for recommended R-values. To see what the unit will cost to run, plug its wattage into the electricity cost calculator.
Frequently asked questions
How many BTUs do I need to cool a 12 × 12 room?
A 12 × 12 ft room is 144 ft², which falls in the 100–150 ft² row of the ENERGY STAR chart: 5,000 BTU per hour. Add 10% if the room is very sunny, 600 BTU/h for each regular occupant beyond two, and 4,000 BTU/h if it is a kitchen.
What size air conditioner do I need for 500 square feet?
The ENERGY STAR chart gives 12,000 BTU per hour — one ton of cooling — for 450 up to 550 ft² with 8-foot ceilings and average sun. Taller ceilings, strong afternoon sun or a kitchen push it higher.
How many BTUs per square foot do I need?
For cooling a single room, the ENERGY STAR chart works out to roughly 20–35 BTU/h per square foot for typical bedrooms and living rooms, more for very small rooms and less for large open areas. Heating needs vary far more with climate and insulation: a well-insulated room may need under 15 BTU/h per square foot, while a drafty one in a cold climate can need 40 or more.
What is a ton of air conditioning?
One ton of cooling is 12,000 BTU per hour, the rate needed to melt a ton of ice in 24 hours. A 24,000 BTU/h system is a 2-ton unit.
How many BTUs does a 1,500-watt space heater produce?
Electric resistance heat turns every watt into 3.412 BTU per hour, so a 1,500 W heater delivers about 5,100 BTU/h. That is enough for a modest, reasonably insulated room in moderate cold, but not for a large or drafty space on a very cold night.