Free AC size calculator. Find the right BTU and tonnage for any room - with adjustments for sun, ceiling height, occupants and kitchens.
Your results will appear here.
Enter your values and press Calculate.
The calculator applies the industry rule-of-thumb: 20 BTU per square foot for a standard 8 ft ceiling, scaled by actual ceiling height and sun exposure (shaded 0.9x, sunny 1.1x), plus 600 BTU per occupant beyond the first two and 4,000 BTU for kitchens. Tons = BTU / 12,000. The recommendation snaps up to the nearest standard unit size you can actually buy. Results are planning estimates, not professional HVAC advice - get a Manual J calculation before buying central AC.
The calculator follows the rule-of-thumb HVAC retailers use, then adjusts for what actually changes the load:
BTU/hr = area × 20 × (ceiling height / 8) × sun factor + max(0, occupants - 2) × 600 + (4000 if kitchen)
Sun factor: 0.9 shaded, 1.0 normal, 1.1 sunny. Tons = BTU / 12,000. The result rounds to the nearest 100 BTU, and the recommendation snaps up to the nearest standard unit size (5,000 / 6,000 / 8,000 / 9,000 / 10,000 / 12,000 / 14,000 / 18,000 / 24,000 / 30,000 / 36,000 BTU...).
Worked example - 600 sq ft room, 9 ft ceilings, sunny, 4 occupants, kitchen: 600 × 20 = 12,000; × 9/8 = 13,500; × 1.1 = 14,850; + 2 × 600 = 16,050; + 4,000 = 20,050, rounded to 20,100 BTU/hr = 1.68 tons. Nearest standard size: 24,000 BTU (2 tons).
BTU (British Thermal Unit) measures how much heat an AC can remove per hour - the higher the BTU, the more cooling power. Tons measure the same thing on a bigger scale: 1 ton of cooling = 12,000 BTU/hr. So an 18,000 BTU unit is a 1.5-ton system and a 36,000 BTU unit is 3 tons.
Window and portable units are sold by BTU; mini-splits and central systems are sold by tons. The calculator shows both, so you can shop either aisle with the right number.
Quick reference for standard 8 ft ceilings, average sun, two occupants:
Up to 150 sq ft: 5,000 BTU (0.5 ton) - small bedroom, office 150-250 sq ft: 6,000 BTU - medium bedroom 250-400 sq ft: 8,000-10,000 BTU - large bedroom, studio 400-550 sq ft: 12,000 BTU (1 ton) - master bedroom 550-800 sq ft: 14,000-18,000 BTU (1.5 ton) - apartment 800-1,100 sq ft: 18,000-24,000 BTU (2 ton) - small home 1,500-2,000 sq ft: 30,000 BTU (2.5 ton) - 3-bed home
For a 1,500 sq ft house in a moderate climate, expect 2.5-3 tons (30,000-36,000 BTU); hot-humid climates push toward 3-3.5 tons.
The 20 BTU/sq ft baseline assumes a standard room. Reality adjusts it:
Ceiling height: the baseline assumes 8 ft - taller rooms hold more air, so the formula scales by (height/8). Add ~1,000 BTU per foot above 8 as a rough check. Sun exposure: heavy afternoon sun +10%; a shaded room -10%. Occupants: the first two people are included in the baseline; each extra person adds ~600 BTU. Kitchens: ovens and cooktops add serious heat - add ~4,000 BTU. Insulation and climate: well-insulated homes need ~10% less; hot-humid regions run ~25 BTU/sq ft instead of 20.
These adjustments can shift the answer 20% or more - which is exactly why a flat per-square-foot number is only a starting point.
Bigger is NOT better. An oversized AC cools the room too fast and shuts off before it has dehumidified the air - the result is a cold, clammy room, 10-20% wasted energy, temperature swings and extra wear from constant short-cycling.
Slightly undersized beats significantly oversized: a right-sized unit runs longer cycles, pulls out humidity steadily and holds temperature even. For bedrooms, err slightly small on purpose - longer run cycles dehumidify better while you sleep. Match the unit to the room, not to your optimism.
Enter the room's floor area in square feet (measure the actual room, not the whole house), ceiling height, sun exposure, how many people regularly use the room, and whether it is a kitchen.
The calculator returns the BTU/hr needed, the equivalent tonnage, and the nearest standard unit size to buy. Window units come in fixed sizes (5,000 / 6,000 / 8,000 / 10,000 / 12,000...); mini-split heads too - so the recommendation always lands on something you can actually purchase.
Up to ~550 sq ft: a window unit or portable AC in the recommended BTU does the job and costs least. 550-1,100 sq ft: a mini-split (9,000-24,000 BTU heads) is the efficient choice, especially without ductwork. Whole home (1,200+ sq ft): central AC sized in tons (2-5 tons typical).
Whatever type you buy, check the SEER rating - higher SEER means cheaper to run. A SEER 20 unit uses roughly 30% less energy than SEER 14, often paying back in 4-8 years. Look for Energy Star models to minimize running costs.
This calculator is a planning estimate using the industry rule-of-thumb. For a single room or a window unit purchase, it is all most people need.
For central AC, new construction or major renovations, get a Manual J load calculation - the HVAC industry standard. Manual J does a detailed room-by-room analysis including insulation, windows, orientation, ductwork and local climate. It is what reputable contractors use, and it is worth it before spending thousands on a central system.
Results are estimates for planning, not professional HVAC advice.
The standard rule is 20 BTU per square foot for a standard 8 ft ceiling, adjusted for sun exposure, ceiling height, occupants and kitchens. Hot-humid climates run closer to 25 BTU/sq ft; cool northern regions closer to 15-18.
A 12x12 room is 144 sq ft - about 5,000 BTU, covered by a small window unit or mini-split. Enter your exact room details in the calculator above for a precise number.
1 ton of cooling = 12,000 BTU/hr. Divide BTU by 12,000 to get tons: an 18,000 BTU unit is a 1.5-ton system. Window units are sold by BTU; mini-splits and central AC by tons.
It short-cycles: cools too fast, shuts off before dehumidifying, and leaves the room cold but clammy. It also wastes 10-20% more energy and wears components faster. Slightly undersized is better than significantly oversized.
In a moderate climate, typically 2.5-3 tons (30,000-36,000 BTU). Hot-humid climates may need 3-3.5 tons; cooler regions 2-2.5 tons. Insulation, windows and ceiling height move the number - run the calculator with your details.
Generally yes if you run AC a lot. A SEER 20 unit uses about 30% less energy than SEER 14, saving roughly $150-400 a year with payback in 4-8 years. Many high-SEER units also qualify for rebates and tax credits.
Manual J is the HVAC industry's standard load calculation - a detailed room-by-room analysis of insulation, windows, orientation, ducts and climate. Get one before buying central AC, building new, or doing major renovations; a rule-of-thumb is fine for a single room or window unit.
Add about 600 BTU per person beyond the first two, who are already included in the baseline. Four regular occupants means +1,200 BTU.
To calculate air conditioner tonnage, divide BTU by 12,000, one ton of cooling equals 12,000 BTU/hour. A quick air conditioner BTU chart: 6,000 BTU = 0.5 ton (up to 150 sq ft), 12,000 BTU = 1 ton (400-550 sq ft), 18,000 BTU = 1.5 tons (600-800 sq ft), 24,000 BTU = 2 tons (up to 1,000 sq ft), 36,000 BTU = 3 tons (up to 1,500 sq ft). For a 1500 sq ft home you typically need around 36,000 BTU (3 tons), adjusted for climate and insulation. The calculator above does this math automatically.