❄️BTU Calculator
Calculate the BTUs needed to cool or heat a room. The AC BTU calculator estimates air conditioner size from room dimensions, ceiling height, occupants, room type, insulation, sun exposure, and climate. The general BTU calculator estimates heating or cooling loads from room volume and temperature difference.
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Required BTU/hr
5,959 BTU/hr
BTU Breakdown by Factor
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BTU Calculator: How Many BTUs Do You Need for Your Room?
A BTU (British Thermal Unit) is the energy needed to raise 1 pound of water by 1°F. Air conditioner capacity is rated in BTU/hr. The general rule is 25 BTU per square foot for a standard 8-foot ceiling, adjusted for height, sun exposure, insulation, climate, and occupancy. 1 ton of refrigeration = 12,000 BTU/hr.
Quick sizing: A 150 ft² room needs ~4,500 BTU; 300 ft² → ~8,000 BTU; 550 ft² → ~12,000 BTU (1 ton)
| Room Size | BTU/hr needed | Typical AC unit |
|---|---|---|
| Up to 150 ft² (14 m²) | 5,000 BTU | Small window unit |
| 300–350 ft² (28–32 m²) | 8,000 BTU | Medium window unit |
| 450–550 ft² (42–51 m²) | 12,000 BTU (1 ton) | Large window / mini-split |
Choosing the right BTU rating for an air conditioner is important. An undersized unit will run continuously without cooling effectively. An oversized unit will cool quickly but cycle on and off too frequently, failing to dehumidify the air properly and causing uncomfortable temperature swings.
Key Factors in BTU Calculation
Room area is the primary factor, but several adjustments matter. Ceiling height: a 10-foot ceiling has 25% more air volume than an 8-foot ceiling and requires proportionally more BTUs. Sun exposure: a south- or west-facing room with large windows can receive 30–50% more solar heat gain than a shaded room. Insulation: a poorly insulated room with drafty windows may require 25% more capacity. Climate: hot and humid regions require more cooling. Kitchens add roughly 4,000 BTU of heat from cooking appliances.
BTU, Tons, and Kilowatts
Air conditioning capacity can be expressed in three units. BTU/hr is the US standard. Tons of refrigeration (TR) are used for commercial equipment: 1 ton = 12,000 BTU/hr, derived from the heat absorption of melting one ton of ice in 24 hours. Kilowatts (kW) are used internationally: 1 kW ≈ 3,412 BTU/hr, so 1 ton ≈ 3.517 kW. Energy efficiency is rated as EER (Energy Efficiency Ratio, BTU/watt-hr) or SEER (Seasonal EER). Higher SEER means lower operating cost: a 16 SEER unit uses roughly half the electricity of an 8 SEER unit.
Frequently Asked Questions
How many BTUs do I need for my room?
Use 25 BTU per square foot as the starting point for a room with 8-foot ceilings and average conditions. Adjustments: add 10% for very sunny rooms; subtract 10% for shaded rooms; add 25% for poor insulation; add 600 BTU for each person regularly in the room beyond 2; add 4,000 BTU for kitchens. Example: A 200 ft² sunny bedroom with average insulation and 2 people: 200 × 25 = 5,000 × 1.10 = 5,500 BTU. The nearest standard size is 6,000 BTU. When in doubt, round up to the next standard size rather than down.
What is the difference between BTU and tons of refrigeration?
1 ton of refrigeration equals 12,000 BTU/hr. The "ton" unit originates from the heat absorbed by melting one short ton (2,000 pounds) of ice over 24 hours: 2,000 lbs × 144 BTU/lb ÷ 24 hours = 12,000 BTU/hr. Central air conditioning systems are typically sized in tons (1 ton, 1.5 tons, 2 tons, 3 tons, 5 tons). Window and portable units are sized in BTU/hr (5,000–25,000 BTU). Mini-split systems may use either. To convert: tons = BTU/hr ÷ 12,000.
Can an AC unit be too big for a room?
Yes — oversizing an AC unit causes "short cycling," where the unit reaches the target temperature quickly and shuts off before it has run long enough to dehumidify the air. High humidity makes a room feel warmer and clammy even at the correct temperature. Short cycling also increases wear on the compressor and reduces efficiency. An oversized window unit running 10 minutes on, 20 minutes off is less efficient and less comfortable than a properly sized unit running continuously at lower capacity. Always size for the room — not for the maximum possible use case.
How much does it cost to run an air conditioner?
Hourly cost = BTU/hr ÷ EER × (electricity rate in $/kWh) ÷ 1,000 watt-hours. Example: 10,000 BTU/hr unit with EER of 10 uses 1,000 watts = 1 kW. At $0.13/kWh, that is $0.13 per hour. For 8 hours/day in summer: $0.13 × 8 × 90 days ≈ $93 per cooling season. Newer units with SEER ratings of 15–20+ are significantly more efficient. Multiply the SEER rating by 1,000 to get approximate BTU per watt-hour under seasonal average conditions.