The Room BTU Calculator answers “what size air conditioner (or heater) do I need?” for a single room. Cooling capacity is based on the Energy Star room air-conditioner sizing chart, adjusted for sun exposure, number of people and kitchens. Heating output uses a floor-area rule of thumb by climate and ceiling height.
Room BTU Calculator (Heating & AC Size)
Air conditioner or heater size for a room in BTU/h, watts, kW and tons, from room size, sun, occupants and climate.
Results are shown in BTU/h, kW and — for cooling — tons. For whole-house systems, a Manual J load calculation by an HVAC professional is the reliable method.
Formula
Cooling: BTU/h = chart size × sun factor + 600 × (people − 2) + 4,000 if kitchen
Heating: BTU/h = floor ft² × climate factor × (ceiling height ÷ 8 ft)
Conversions: 1 BTU/h = 0.29307 W; 1 ton of cooling = 12,000 BTU/h.
Energy Star Room Size Chart
| Room area (ft²) | Capacity (BTU/h) |
|---|---|
| up to 150 | 5,000 |
| 150–250 | 6,000 |
| 250–300 | 7,000 |
| 300–350 | 8,000 |
| 350–400 | 9,000 |
| 400–450 | 10,000 |
| 450–550 | 12,000 |
| 550–700 | 14,000 |
| 700–1,000 | 18,000 |
| 1,000–1,200 | 21,000 |
| 1,200–1,400 | 23,000 |
| 1,400–1,500 | 24,000 |
| 1,500–2,000 | 30,000 |
| 2,000–2,500 | 34,000 |
Adjustments: heavily shaded −10%; very sunny +10%; +600 BTU/h for each regular occupant beyond two; +4,000 BTU/h if the unit serves a kitchen. Rooms over 2,500 ft² are extrapolated by the calculator.
How the Calculation Works
Step 1: Room area in ft² picks the base size from the chart.
Step 2: Apply the sun factor, then add occupant and kitchen allowances.
Step 3: For heating, multiply area by the climate factor and scale by ceiling height.
Example Calculation
Air conditioner
Given: 15 × 12 ft bedroom (180 ft²), average sun, 2 people, not a kitchen
Chart size = 6,000 BTU/h (150–250 ft²) ≈ 1.76 kW ≈ 0.5 ton
Heater
Given: same room, 8 ft ceiling, moderate climate (≈40 BTU/h per ft²)
180 × 40 × 1 = 7,200 BTU/h ≈ 2.1 kW
Engineering Assumptions
- The cooling chart is intended for room air conditioners; insulation, windows and climate also affect real load.
- The heating factors are rules of thumb, not a load calculation.
- An oversized AC can cycle on and off without dehumidifying well; an undersized one may not keep up.
Related: Electric Bill Per Square Foot Calculator.
Frequently Asked Questions
How many BTUs do I need for my room?
Find the room’s square footage and use the chart, then adjust for sun, people and kitchen use.
What size AC for a 12 × 12 room?
144 ft² falls in the up-to-150 ft² band: 5,000 BTU/h before adjustments.
How many BTUs per square foot?
The chart is not linear; small rooms need more per ft² than large ones. For heating, the calculator uses about 30–60 BTU/h per ft² depending on climate.
How do I convert BTU to tons?
Divide BTU/h by 12,000.
How do I convert BTU/h to watts?
Multiply by 0.293.
Does ceiling height matter?
It is used in the heating estimate; the Energy Star cooling chart is based on floor area.
