What the BTU / air conditioner calculator does
Air conditioners are rated in BTU per hour (or kilowatts), and buying the wrong size is the most common mistake: too small never cools the room, too large cycles on and off, leaves the air humid and wastes money. Enter the room's length, width and ceiling height and whether it is sunny, normal or shaded, and the calculator gives a recommended cooling capacity in BTU/h and kW. Runs in your browser.
The calculation
Base load = floor area (ft²) × 20 BTU/h (≈ 215 BTU/h per m²)
Height = adjusted for ceilings above 8 ft (2.4 m), proportionally
Sun = +10% for a sunny room, −10% for a heavily shaded one
Room 15 × 20 ft, 8 ft ceiling, sunny:
300 ft² × 20 = 6,000 BTU/h × 1.10 = 6,600 BTU/h → choose a 7,000–8,000 BTU unit (≈ 2.1–2.3 kW)The 20 BTU per square foot rule is the standard residential starting point (Energy Star uses a similar table). It assumes normal insulation, one or two occupants and no major heat sources; the adjustments below refine it.
Adjustments to make by hand
| Condition | Adjust |
|---|---|
| Each person beyond two in the room regularly | +600 BTU/h |
| Kitchen | +4,000 BTU/h |
| Home office with several computers, or a server closet | +1,000–2,000 BTU/h |
| Top-floor room under an uninsulated roof | +10–20% |
| Large west-facing windows | +10% |
| Very hot climate (design temperature above 35 °C / 95 °F) | +10–20% |
| Well-insulated modern build | −10–20% |
Common sizes
| Room area | BTU/h | kW | Typical unit |
|---|---|---|---|
| Up to 150 ft² (14 m²) | 5,000 | 1.5 | Small window unit / 9,000 BTU mini-split at low setting |
| 150–250 ft² (14–23 m²) | 6,000 | 1.8 | Window or portable |
| 250–350 ft² (23–33 m²) | 8,000 | 2.3 | Window / portable / 9,000 BTU split |
| 350–450 ft² (33–42 m²) | 10,000 | 2.9 | 12,000 BTU split |
| 450–550 ft² (42–51 m²) | 12,000 | 3.5 | 12,000 BTU split |
| 550–700 ft² (51–65 m²) | 14,000 | 4.1 | 18,000 BTU split |
| 700–1,000 ft² (65–93 m²) | 18,000 | 5.3 | 18,000–24,000 BTU split |
| 1,000–1,400 ft² (93–130 m²) | 24,000 | 7.0 | 24,000 BTU split or multi-zone |
1 kW of cooling = 3,412 BTU/h. Mini-split ratings are often quoted in both; a “12,000 BTU” unit is about 3.5 kW.
Why oversizing is a real problem
An air conditioner removes humidity as a side effect of cooling, but only while it runs. An oversized unit cools the air to the set point in a few minutes and shuts off before it has dehumidified, leaving the room cold and clammy — and the frequent starts wear the compressor and use more electricity than steady running. Inverter (variable-speed) mini-splits tolerate mild oversizing better because they can run slowly, but even they should be within about 20% of the calculated load.
Beyond the rule of thumb
For whole-house systems, heat pumps or unusual buildings, a proper load calculation (ACCA Manual J in the US, or a CIBSE / local equivalent) accounts for insulation levels, window orientation and glazing, infiltration, internal gains and local climate data. A good installer will do one; treat this calculator as the sanity check for a single room or a first quote.