BTU Calculator
Find the air conditioner or heater size your room needs — based on the EnergyStar sizing chart, adjusted for your room. Runs entirely in your browser.
Base capacity from the EnergyStar room-AC chart, then adjusted for ceiling, sun, insulation, climate, occupants, and kitchens. Estimates only — for expensive equipment, confirm with an HVAC pro (Manual J).
How the BTU calculator works
A BTU calculator estimates how much heating or cooling power a room needs. BTU stands for British Thermal Unit, and an air conditioner or heater is rated by how many BTU per hour it can move. Too few and the unit runs constantly without keeping up; too many and it wastes energy and leaves the room humid. This tool uses the official EnergyStar room air-conditioner sizing chart for the base capacity, then adjusts for the details that change the load.
Works for window units, mini splits, and central AC
Use this air conditioner BTU calculator to size any cooling system: a window unit, a portable AC, a through-the-wall unit, a ductless mini split, or central air. The required cooling BTU is the same whatever the type — it's the heat the room gains that sets it, not the equipment. For winter, the heating estimate works the same way for a furnace, a heat pump, or an electric space heater. So whether you came here for an ac btu calculator or a heating BTU estimate, both live in one place.
The base: the EnergyStar sizing chart
Rather than a flat rule, EnergyStar maps room area to a recommended cooling capacity. For example, a 12 × 14 ft room is 168 sq ft, which falls in the 150–250 sq ft range and starts at 6,000 BTUbefore adjustments. Most window air conditioners are sold in standard sizes (5,000, 6,000, 8,000, 10,000, 12,000 BTU and up), so this calculator also suggests the nearest standard unit that covers your need, in BTU and tons.
| Room area (sq ft) | Cooling capacity (BTU/hr) |
|---|---|
| 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 |
The adjustments that matter
- Ceiling height — the chart assumes an 8-foot ceiling. Add about 1,000 BTU for each foot above that.
- Sun exposure — a room that bakes in afternoon sun needs about 10% more; a heavily shaded room about 10% less.
- Insulation — well-insulated rooms hold temperature better (about 10% less); poorly insulated rooms need about 10% more.
- Climate — hotter regions need a little more cooling (and less heating); colder regions the reverse.
- Occupants — add roughly 600 BTU for each person beyond two who regularly uses the room.
- Kitchens — ovens and cooktops add heat, so add about 4,000 BTU.
Why right-sizing matters
It is tempting to buy the biggest unit you can afford, but an oversized air conditioner is a common mistake. It cools the air quickly and shuts off before it has run long enough to pull moisture out, so the room ends up cold and clammy. The frequent on-off cycling also wears out the compressor and uses more electricity. An undersized unit has the opposite problem: it runs non-stop and still can't reach the target temperature on hot days. Matching capacity to the room is the comfortable, efficient middle.
What about heating?
Heating depends much more on your climate and insulation, so treat the heating number as a rough guide. A common range is 30 to 60 BTU per square foot — nearer 30 in warm regions and up to 60 where winters are cold. Set the climate option to shift the estimate, and add a margin for drafty rooms, large windows, or poor insulation.
Frequently asked questions
How many BTU do I need per square foot?▾
There is no single number — it depends on room size. The EnergyStar chart this tool uses works out to roughly 33 BTU per square foot for tiny rooms down to about 14 for large ones, and a common rule of thumb is around 20. We start from the chart, then adjust for your room.
What size air conditioner do I need for a 300 sq ft room?▾
The EnergyStar chart lists 250–300 sq ft at about 7,000 BTU. Add ~10% for a very sunny room or poor insulation, 600 BTU per person beyond two, and 4,000 BTU if it is a kitchen.
Does ceiling height affect the BTU I need?▾
Yes. The chart assumes an 8-foot ceiling. EnergyStar guidance adds roughly 1,000 BTU for each foot of ceiling above 8, and this calculator does the same.
How many BTU do I need to heat a room?▾
Heating varies more by climate and insulation, but a rough range is 30–60 BTU per square foot: about 30 in warm climates and up to 60 in cold ones. Set the climate option for an estimate.
Is a higher-BTU air conditioner always better?▾
No. An oversized unit cools fast then shuts off before removing humidity, leaving the room cold and clammy and wearing out the compressor. Right-sizing is more comfortable and efficient.