What is a BTU?
A BTU (British Thermal Unit) is the amount of energy required to raise the temperature of one pound of water by one degree Fahrenheit. In HVAC, capacity is always stated in BTU per hour — often shortened to just "BTU." A 10,000 BTU window AC removes 10,000 BTU of heat every hour. Higher BTU = more cooling or heating capacity.
How to calculate BTUs for a room
BTU = sq ft × 20 × ceiling_factor × adjustments
+ 600/occupant (above 2) + 1,000/window + 4,000 if kitchen
Start with the 20 BTU per sq ft base rule for cooling, then apply multipliers:
- Ceiling height — multiply by (height ÷ 8). Vaulted ceilings add 20–30%.
- Insulation — poor +30%, average 0%, good −10%, excellent −20%.
- Climate — hot & humid +20%, moderate 0%, cold −10% for cooling.
- Sun exposure — heavy sun +10%, full shade −10%.
- Occupants — +600 BTU per person beyond 2.
- Windows — +1,000 BTU per window.
- Kitchen — +4,000 BTU for cooking heat.
For heating, use 30 BTU/sq ft in mild climates, up to 45 BTU/sq ft in very cold climates.
BTU chart by room size
| Room size | Cooling BTU | Heating BTU | AC tons |
|---|---|---|---|
| 100 sq ft | 5,000 | 3,500 | 0.4 |
| 150 sq ft | 5,000 | 5,500 | 0.5 |
| 200 sq ft | 6,000 | 7,000 | 0.6 |
| 300 sq ft | 8,000 | 10,500 | 0.7 |
| 400 sq ft | 9,000 | 14,000 | 0.8 |
| 500 sq ft | 12,000 | 17,500 | 1.0 |
| 700 sq ft | 14,000 | 24,500 | 1.2 |
| 1,000 sq ft | 18,000 | 35,000 | 1.5 |
| 1,500 sq ft | 24,000 | 52,500 | 2.0 |
Assumes 8-ft ceilings, moderate climate, average insulation, 2 occupants, 2 windows. Adjust up for adverse conditions.
AC tonnage chart
| House size | BTU/hr | Tons | Typical use |
|---|---|---|---|
| 450–550 sq ft | 12,000 | 1.0 | Studio, small apartment |
| 550–850 sq ft | 18,000 | 1.5 | Small 1-BR home |
| 850–1,100 sq ft | 24,000 | 2.0 | 2-BR condo, large apartment |
| 1,100–1,400 sq ft | 30,000 | 2.5 | Small house |
| 1,400–1,700 sq ft | 36,000 | 3.0 | Medium house |
| 1,700–2,100 sq ft | 42,000 | 3.5 | Medium-large house |
| 2,100–2,500 sq ft | 48,000 | 4.0 | Large house |
| 2,500–3,300 sq ft | 60,000 | 5.0 | Very large house |
Window AC vs central AC vs mini-split
Window AC
- 5,000–25,000 BTU
- Single room only
- $150–$800
- Easy install
- Noisy
Central AC
- 24,000–60,000 BTU
- Whole home
- $5,000–$12,000
- Needs ductwork
- Most efficient for big homes
Mini-split
- 9,000–36,000 BTU
- 1–5 zones
- $3,000–$10,000
- No ducts required
- Quiet & efficient
Oversizing vs undersizing
Too big (oversized)
- Short-cycles (on/off rapidly)
- Doesn't remove humidity — room feels clammy
- Higher purchase cost
- More wear on compressor
Too small (undersized)
- Runs constantly
- Can't hit target temperature
- Higher electric bills
- Overworked, shorter lifespan
Right-size within 10%. When in doubt, go slightly up — not 50% over. For central systems, always have a professional do a Manual J load calculation.
SEER and EER ratings
SEER (Seasonal Energy Efficiency Ratio) is the average cooling output divided by electricity used over a full cooling season. EER is the same ratio at peak conditions (no seasonal averaging). Higher numbers = more efficient.
- Minimum (federal): SEER 14 (South) / 15 (North & Southwest), as of 2023
- Good value: SEER 16–18
- Premium: SEER 20+ (best for hot climates, high energy costs)
- Window units: look for EER 10+ (EER roughly = SEER − 2 to − 3)
Every extra SEER point ≈ 7% less energy use. Moving from SEER 14 to SEER 18 saves ~25% on cooling costs.
Frequently asked questions
How many BTUs do I need per square foot?
The general rule is 20 BTU per square foot for cooling in average climates with average insulation. For heating, figure 30–45 BTU/sq ft depending on climate (30 in mild zones, 45 in very cold). These are baseline figures — adjust up for poor insulation (+30%), heavy sun exposure (+10%), high ceilings (ceiling_height ÷ 8), extra occupants (+600 BTU each beyond 2), windows (+1,000 each), and kitchens (+4,000).
What size AC do I need for a 12x12 room?
A 12×12 room is 144 square feet, which needs about 5,000 BTU of cooling (144 × 20 = 2,880 base, rounded up to nearest standard size). A 5,000 BTU window AC is the standard choice. If the room has heavy sun exposure, high ceilings, or poor insulation, step up to 6,000 BTU. For kitchens of the same size, add 4,000 BTU so you'd want 8,000 BTU.
What is a ton of cooling?
One ton of cooling equals 12,000 BTU per hour. The term originated from the amount of cooling produced by melting one ton of ice in 24 hours. Central AC systems are sized in tons: a 3-ton AC = 36,000 BTU/hr, typical for a 1,500–1,700 sq ft home. Window units use BTU directly since they're all under 1 ton.
How many BTUs do I need to heat a room?
Heating requires more BTU than cooling because temperature differences are larger. Rule: 30 BTU/sq ft in mild climates, 35 in moderate, 40 in cold, 45 in very cold climates. A 500 sq ft room in a cold climate needs 20,000 BTU; in a mild climate, 15,000 BTU. Furnaces and heat pumps are typically sized in BTU/hr or tons the same as AC.
What is the difference between BTU and BTU/hr?
BTU is a unit of energy (British Thermal Unit — the heat to raise 1 lb of water 1°F). BTU/hr is a unit of power — the rate of energy transfer. AC and heater specs are in BTU/hr, but it's often shortened to just 'BTU' in consumer labeling. A '10,000 BTU' AC means it can remove 10,000 BTU of heat per hour.
How do I convert BTU to watts?
Divide BTU/hr by 3.412 to get watts. Example: 12,000 BTU/hr ÷ 3.412 = 3,517 watts (3.52 kW). Conversely, multiply watts by 3.412 to get BTU/hr. A 1,500W space heater produces ~5,118 BTU/hr. A 1-ton AC (12,000 BTU/hr) uses ~1,200W input but produces 3.5 kW equivalent cooling (because AC moves heat rather than generating it — EER 10 = 10 BTU per watt).
Is it better to oversize or undersize an AC?
Neither — get the right size. Oversized AC problems: short-cycling (turns on/off too quickly), poor humidity removal (leaves room cold and damp), higher upfront cost, more wear on compressor. Undersized AC problems: runs constantly, can't reach set temperature on hot days, higher electric bills. A properly sized AC matches your cooling load. When in doubt, go 10% over — not 50%.
How many BTUs for a 1,500 sq ft house?
About 24,000–30,000 BTU (2–2.5 tons) for cooling a 1,500 sq ft home in a moderate climate. For heating, expect 45,000–60,000 BTU in cold climates. This is whole-home sizing — typically met by a 2- or 2.5-ton central AC. Mini-split systems for zonal cooling often total 30,000–36,000 BTU spread across 3–4 indoor heads.
What SEER rating should I look for?
Minimum federal efficiency is SEER 14 (South) or SEER 15 (North/Southwest) as of 2023. SEER 16–18 is a good sweet spot for value. SEER 20+ is premium efficiency — worth it in hot climates with high electricity rates. Every extra SEER point = ~7% lower energy use. For window AC, look for EER 10+ (EER is the non-seasonal equivalent, ~SEER − 2 to − 3).
What size window AC do I need?
Window AC sizing by room: 150 sq ft → 5,000 BTU, 250 sq ft → 6,000 BTU, 350 sq ft → 8,000 BTU, 450 sq ft → 10,000 BTU, 550 sq ft → 12,000 BTU, 700 sq ft → 14,000 BTU, 1,000 sq ft → 18,000 BTU. Above 18,000 BTU, most window units need a 240V outlet (not standard 120V). Beyond ~1,400 sq ft, central AC or multi-zone mini-splits become more efficient than window units.