Recommended AC Unit
Heat Load Factors Explained
How this BTU Calculator solves real problems
Most online BTU calculators apply a single 20 BTU per square foot rule of thumb. They ignore ceiling height, sun exposure, insulation, climate, occupancy, kitchen appliance heat and attic heat. The result is an undersized AC that runs constantly or an oversized unit that short cycles without dehumidifying. A homeowner who follows a generic 20 BTU rule undersizes their sunroom by 4,000 BTU, their kitchen by 6,000 BTU, and their attic by 12,000 BTU. This calculator applies 8 heat load factors deterministically using the same methodology professional HVAC contractors use for residential Manual J load calculations. The math is reproducible: same inputs always produce the same BTU recommendation.
Frequently Asked Questions
How many BTU do I need to cool a room?
Multiply the room area in square feet by 20 to get a baseline BTU. Then adjust for ceiling height (taller rooms need more), sun exposure (shaded 0.85x, average 1.0x, very sunny 1.15x), insulation quality (good 0.9x, average 1.0x, poor 1.15x), climate zone (cold 0.9x, temperate 1.0x, hot 1.15x, very hot 1.25x), occupancy (first 2 included, plus 600 BTU per additional person), windows (plus 1000 BTU per window, capped at 5 windows), and room type (kitchen plus 4000, attic plus 25 percent). A 12x12 bedroom with 8 foot ceilings, average sun, good insulation, temperate climate, 2 people and 1 window needs about 5,200 BTU.
What size air conditioner do I need for a 12x12 room?
A 12 foot by 12 foot room is 144 square feet. With 8 foot ceilings, average insulation, shaded exposure, temperate climate and 2 occupants, the base heat load is about 2,880 BTU. With 1 window (plus 1000) and standard adjustments, the recommended size is about 4,000 to 5,000 BTU which is a small window AC (5,000 BTU unit). For hotter rooms (sunny, hot climate, kitchen, attic) the recommendation rises to 6,000 to 8,000 BTU.
How do I convert BTU to tons?
One ton of cooling equals 12,000 BTU per hour. To convert BTU to tons divide by 12,000. A 24,000 BTU system is 2 tons. A 36,000 BTU system is 3 tons. The ton rating comes from the heat needed to melt one ton of ice in 24 hours which is exactly 12,000 BTU per hour. This calculator shows the tonnage next to the BTU so you can pick the right central AC unit.
How many BTU do I need per square foot?
Industry rule of thumb is 20 BTU per square foot for residential cooling. This works for a typical room with 8 foot ceilings, average insulation, average sun and 2 occupants. Very sunny rooms need 25 to 30 BTU per square foot. Heavily shaded rooms need 17 to 20. Kitchens need 30 to 40 because of appliance heat. Attics need 25 to 35 because heat rises into them. AI chatbots often give a single 20 BTU per square foot number for every room which undersizes kitchens, attics and sunny rooms.
What is the difference between cooling BTU and heating BTU?
Cooling BTU removes heat from a room. Heating BTU adds heat to a room. Both use the same base heat load formula but the climate multiplier is inverted. In cooling a hot climate needs more BTU. In heating a cold climate needs more BTU. A 200 square foot room in Phoenix Arizona needs about 4,500 BTU cooling but only 2,000 BTU heating. The same room in Minneapolis Minnesota needs about 3,500 BTU cooling but 7,500 BTU heating. This calculator switches the climate multiplier when you change from Cooling to Heating mode.
How do I size a mini split air conditioner?
Mini split sizes are usually 9,000, 12,000, 18,000, 24,000, 30,000 and 36,000 BTU. Pick the smallest BTU size that covers your calculated heat load. A room needing 8,000 BTU should use a 9,000 BTU mini split. A room needing 15,000 BTU should use an 18,000 BTU mini split. Oversizing a mini split causes short cycling which wastes energy and reduces dehumidification. Undersizing means the unit runs constantly without reaching the set temperature. This calculator recommends the matching mini split size for your calculated BTU.
How many BTU do I need to heat a room?
Heating BTU follows the same formula as cooling but with inverted climate multipliers. Base is room area times 20 BTU per square foot for an 8 foot ceiling. Then multiply by ceiling height adjustment, sun exposure (shaded rooms need 15 percent more heat because they get less passive solar), insulation quality, occupancy (each person gives off about 400 BTU of heat so fewer occupants means more heat needed), and heating climate zone (cold regions like Boston need 1.3x base, hot regions like Houston need 0.8x base). For a 200 square foot bedroom in Boston with poor insulation, the recommended furnace size is about 8,000 to 10,000 BTU.
What size space heater do I need?
Space heaters are usually rated 1,500 watts (about 5,000 BTU) for small rooms, 3,000 to 5,000 watts (10,000 to 17,000 BTU) for medium rooms, and 8,000 to 12,000 watts (27,000 to 41,000 BTU) for large rooms. Use the calculated BTU from this calculator to pick the right size. For safety, electric space heaters should never exceed 1,500 watts on a 15 amp 120 volt household circuit. For larger heating needs use a gas furnace or central heating system.
Can AI chatbots calculate BTU correctly?
No. AI text models almost always give a single 20 BTU per square foot number for every room. They ignore ceiling height (a 10 foot ceiling needs 12 percent more BTU than 8 foot), ignore sun exposure (a sunny room needs 15 percent more), ignore insulation quality (a poorly insulated room needs 15 percent more), ignore climate zone (Phoenix needs 25 percent more than Boston for cooling), ignore occupancy (4 people in a bedroom need 1,200 more BTU than 2 people), ignore kitchen appliance heat (plus 4,000 BTU for stoves and ovens), and ignore attic heat (plus 25 percent for attic rooms). The result is usually undersized AC or oversized heaters that short cycle. This calculator applies all 8 factors deterministically.
Does this work in metric?
Yes. Toggle the unit switch at the top to Metric and every input switches to metres and centimetres. Room area switches to square metres (1 sq m equals 10.7639 sq ft). BTU stays the same because it is a unit of energy. Tonnage stays the same. Watts stay the same. A 4x4 metre room with 2.4 m ceiling is 16 sq m or 172 sq ft, so the BTU calculation uses the same formula with metric dimensions.
Is my room data private?
Yes. Everything runs in your browser using JavaScript. No room dimensions, BTU calculations, or AC size recommendations are sent to any server. Close the tab and your data is gone. The only data persisted is the last room and settings in localStorage so the form reloads when you come back. There is no account, no email capture, no analytics tied to your project.
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