Why BTU sizing matters
BTU (British Thermal Unit) measures how much heat your AC can remove each hour. Too small and it runs constantly without catching up; too large and it short‑cycles—cooling the air quickly but failing to dehumidify, which feels clammy and wastes energy.
Whole‑home systems deserve a pro Manual J load calculation. For a single room (window, portable, or 1‑zone mini‑split), a chart plus realistic modifiers is accurate enough to buy with confidence—especially if you choose a variable‑speed (inverter) model that can modulate output.
Room size → base BTU
| Room size (ft²) | Base BTU (window AC / mini‑split) |
|---|---|
| 100–150 | 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 | 30,000–36,000 (mini‑split) |
Smaller rooms often need more BTU per square foot (30–35 BTU/ft²) due to fixed loads (windows, electronics). As room size increases, BTU/ft² trends down (18–25 BTU/ft²).
Ceiling height multiplier (volume matters)
| Ceiling height | Multiplier vs 8 ft |
|---|---|
| 7 ft | × 0.88 |
| 8 ft | × 1.00 (baseline) |
| 9 ft | × 1.13 |
| 10 ft | × 1.25 |
| 12 ft | × 1.50 |
Multiply the base BTU by (ceiling height ÷ 8) to correct for room volume.
Climate zone modifiers (cooling intensity)
| Climate (cooling) | Multiplier |
|---|---|
| Hot‑Humid (tropics, Gulf Coast) | +10% to +20% (× 1.10–1.20) |
| Hot‑Dry (deserts) | +5% to +10% (× 1.05–1.10) |
| Mixed/Marine (temperate) | Baseline (× 1.00) |
| Cool/Cold (northern, high altitude) | −5% to −10% (× 0.95–0.90) |
Insulation, sun, occupancy & other adjusters
| Factor | Adjustment |
|---|---|
| Insulation: Excellent (tight) | −15% |
| Insulation: Good | −10% |
| Insulation: Poor | +20% to +30% |
| Sun exposure: Shaded | −10% |
| Sun exposure: Very sunny / west‑facing | +10% to +20% |
| Occupancy | +600 BTU per person beyond two |
| Kitchen / heavy appliances | +4,000 BTU |
| Home office (PC/monitors) | +1,000 to +2,000 BTU |
| Large west/south glazing | +10% |
| Poor ducting (if ducted air handler) | +10% |
Stack percentage factors multiplicatively (×1.10 then ×0.90), then add absolute BTU values (people, appliances) at the end.
“BTU per square foot” sanity check
| Scenario | Typical BTU/ft² |
|---|---|
| Small rooms (≤150 ft²), average | 30–35 |
| 150–300 ft², average | 25–30 |
| 300–500 ft², average | 20–25 |
| Well‑insulated & shaded | 15–20 |
| Hot‑humid, sunny, high glass | 22–30 |
| Open‑plan living, tall ceilings | 20–28 |
Worked examples
- 12 × 15 bedroom (180 ft²), 9 ft ceiling, hot‑humid, sunny west window, 3 people, good insulation
Base: 6,000 → ×1.13 → ×1.10 → ×0.90 → ×1.10 → +600 ≈ ~7,983 BTU ⇒ pick 8,000 (or 9k mini‑split inverter). - 12 × 12 study (144 ft²), 8 ft ceiling, cool climate, shaded, excellent insulation, 2 people
Base: 5,000 → ×0.95 → ×0.85 → ×0.90 ≈ ~3,634 BTU ⇒ smallest common size: 5,000 (6k inverter ok). - 28 × 20 living/kitchen (560 ft²), 10 ft ceiling, mixed climate, sunny, 4 people, oven use
Base: 14,000 → ×1.25 → ×1.10 → +1,200 → +4,000 ≈ ~24,450 BTU ⇒ pick 24k (2‑ton) inverter. - 14 × 18 living (252 ft²), 12 ft ceiling, hot‑humid, west glazing, poor insulation, 2 people
Base: 7,000 → ×1.50 → ×1.20 → ×1.15 → ×1.10 ≈ ~15,939 BTU ⇒ pick ~15k–16k BTU inverter. - 20 × 15 basement media (300 ft²), 8 ft, cool climate, shaded, good insulation, gaming PC + TV, 3 people
Base: 8,000 → ×0.95 → ×0.90 → ×0.90 → +1,500 → +600 ≈ ~8,256 BTU ⇒ pick 8k–9k inverter.
BTU chart for common room sizes
| Room (ft) | Area (ft²) | Base BTU |
|---|---|---|
| 10 × 10 | 100 | 5,000 |
| 10 × 12 | 120 | 5,000 |
| 11 × 12 | 132 | 5,000 |
| 12 × 12 | 144 | 5,000 |
| 12 × 14 | 168 | 6,000 |
| 12 × 15 | 180 | 6,000 |
| 13 × 16 | 208 | 6,000 |
| 14 × 16 | 224 | 6,000 |
| 15 × 15 | 225 | 6,000–7,000 |
| 15 × 20 | 300 | 7,000–8,000 |
| 16 × 20 | 320 | 8,000 |
| 18 × 20 | 360 | 9,000 |
| 20 × 20 | 400 | 9,000–10,000 |
| 22 × 20 | 440 | 10,000–12,000 |
| 25 × 22 | 550 | 12,000–14,000 |
Metric (m²) cheat sheet
| Room size (m²) | Base BTU |
|---|---|
| 9–14 | 5,000 |
| 14–23 | 6,000 |
| 23–28 | 7,000 |
| 28–33 | 8,000 |
| 33–37 | 9,000 |
| 37–42 | 10,000 |
| 42–51 | 12,000 |
| 51–65 | 14,000 |
| 65–93 | 18,000 |
| 93–112 | 21,000 |
Special cases & pro tips
- Window vs portable vs mini‑split: Mini‑split inverter > window > portable. If portable, prefer dual‑hose.
- Noise & comfort: Oversized fixed‑speed units short‑cycle and dehumidify poorly. Inverters run long and low.
- Electrical: 12k+ window units may need 15–20A circuits; confirm plug type.
- Placement: Avoid blasting air at beds/seats; don’t block airflow with curtains or furniture.
- Two rooms: Open doors most of the time? Size for combined area but bias to hotter room; otherwise use separate units.
- Dehumidification: In hot‑humid climates, slightly smaller inverters can be more comfortable thanks to better moisture removal.
Buying checklist
Inverter (variable‑speed) compressor
High CEER/SEER
Low noise (dB) ratings
Strong dehumidification rate
Easy filter access / washable
Smart schedules / eco mode
Warranty & service network
Voltage, amperage, plug type
FAQ
Most rooms land between 18–30 BTU per sq ft. Small rooms often need 30–35; large rooms trend 20–25. Always apply ceiling, climate, insulation, sun, and occupancy modifiers.
A 12×12 room is 144 ft² → 5,000 BTU base. Tall ceilings, hot‑humid climates, west sun, or extra people can push to 6,000–8,000 BTU—ideally an inverter model.
No. Oversizing causes short cycling, poor dehumidification, and more noise. Choose an inverter near the lower appropriate size so it can modulate.
Yes—cooling is about volume. Correct the base BTU by multiplying with (ceiling height ÷ 8). 10 ft ceilings increase load ~25%.
Add roughly 600 BTU per person beyond two. Kitchens add about 4,000 BTU. Home office PCs/monitors add 1,000–2,000 BTU.
Mini‑split inverter > window AC > portable. If portable is your only option, choose a dual‑hose model and consider slight upsizing.
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