Room volume: — cu ft | Est. electricity: — kWh/yr | BTU per sq ft: —
Result Summary
| Cooling load | — |
| Recommended unit | — |
| Room volume | — |
| Estimated electricity | — |
| Operating cost | — |
BTU by Room Size (baseline, average conditions)
| Room area | Approx. BTU/hr |
|---|---|
| 150 – 250 sq ft | 5,000 – 6,000 |
| 250 – 350 sq ft | 7,000 – 8,000 |
| 350 – 450 sq ft | 10,000 |
| 450 – 550 sq ft | 12,000 |
| 550 – 700 sq ft | 14,000 |
| 700 – 1,000 sq ft | 18,000 |
| 1,000 – 1,400 sq ft | 21,000 – 24,000 |
Climate Adjustment
| Climate | Load multiplier |
|---|---|
| Cold | 0.85x |
| Moderate | 1.00x |
| Warm | 1.15x |
| Hot | 1.30x |
| Very Hot | 1.45x |
What Size Air Conditioner Do I Need?
Air conditioner sizing starts with the room area and a baseline of about 20 BTU per square foot, then adjusts for the things that add or shed heat: climate, insulation, sun exposure, windows, ceiling height, occupants, and heat-producing appliances. A shaded, well-insulated bedroom needs far less cooling than a sunny west-facing kitchen of the same size. The calculator above combines these into a cooling load in BTU per hour, converts it to tons, and recommends a window, mini-split, or central unit. The goal is to match the load, not to overshoot it.
BTU vs Tons Explained
BTU per hour measures how much heat the air conditioner removes each hour. A ton of cooling equals 12,000 BTU per hour, a holdover from the days when cooling was compared to melting a ton of ice in a day. Window units are rated in BTU, while central systems are usually rated in tons. To convert, divide BTU by 12,000: an 18,000 BTU unit is 1.5 tons. Knowing both lets you compare a window unit to a central system on equal footing.
Why Oversized AC Is Bad
It is tempting to buy a bigger unit for insurance, but oversizing backfires. A too-large AC cools the air to the thermostat setpoint quickly and shuts off, before it has run long enough to remove humidity. The result is a space that feels cold and damp, with the compressor short-cycling on and off, which wastes energy and shortens its life. A right-sized unit runs in longer, steadier cycles that both cool and dehumidify, which is why matching the load matters more than maximizing capacity.
Frequently Asked Questions
How many BTU per square foot? Around 20 as a baseline, adjusted up for sun, poor insulation, and hot climates, and down for shade and good insulation.
What size for a 300 sq ft room? Roughly 7,000 to 8,000 BTU in average conditions, more if sunny or in a hot climate.
Window, mini-split, or central? Window units suit single rooms, mini-splits cool one or a few rooms efficiently, and central systems cool a whole house through ducts.
Does a higher SEER save money? Yes, it lowers running cost, though the payback depends on how many hours you run the AC each year.