BTU Calculator

Calculate how many BTUs of air conditioning a room needs from its size, climate, sun exposure and occupants to size a window unit or mini-split.

By CalcBuilt Editorial TeamUpdated September 11, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To size an air conditioner, multiply the room area by 20 to 35 BTU per square foot depending on climate, then adjust for sun, ceiling height, occupants and kitchens. A 15 x 12 ft room with 8 ft ceilings in a moderate climate needs about 4,500 BTU per hour, so a 5,000 BTU window unit or 0.4 tons of cooling.

Results

Cooling load: 4,500 BTU per hour; 25 BTU per sq ft plus adjustments
Air conditioner size: 0.4 tons; 1 ton of cooling = 12,000 BTU per hour
Window or portable unit: 5,000 BTU; Smallest common size that meets the load
Room area: 180 sq ft; 8 ft ceiling

BTU formula

Base load = Area (sq ft) x BTU per sq ft  (20 mild, 25 moderate, 30 hot, 35 very hot)
Adjusted load = Base load x sun factor (0.9 shaded, 1.1 sunny) x Ceiling height / 8
Total BTU = Adjusted load + 600 x (people - 2) + 4,000 if kitchen
Tons = Total BTU / 12,000

Worked example

A 20 x 18 ft living room with 10 ft ceilings in a hot climate, very sunny, with 4 people and an open kitchen: 20 x 18 = 360 sq ft. 360 x 30 = 10,800 BTU. Times 1.1 for sun and 10 / 8 for ceiling height: 10,800 x 1.1 x 1.25 = 14,850 BTU. Add 2 x 600 = 1,200 for the extra people and 4,000 for the kitchen: 20,050 BTU per hour.

That is about 1.7 tons of cooling. The smallest common window unit that covers it is 24,000 BTU, but a room this size is usually better served by an 18,000 to 24,000 BTU mini-split.

Quick reference

Cooling load for common room sizes (moderate climate, 25 BTU per sq ft, 8 ft ceiling, 2 people)
Room size	Area	BTU per hour	Window unit
10 x 10 ft	100 sq ft	2,500	5,000 BTU
10 x 12 ft	120 sq ft	3,000	5,000 BTU
12 x 12 ft	144 sq ft	3,600	5,000 BTU
12 x 15 ft	180 sq ft	4,500	5,000 BTU
15 x 20 ft	300 sq ft	7,500	8,000 BTU
20 x 20 ft	400 sq ft	10,000	10,000 BTU
20 x 25 ft	500 sq ft	12,500	14,000 BTU
25 x 30 ft	750 sq ft	18,750	24,000 BTU

How to calculate BTUs for a room

Start with the floor area in square feet and multiply by a climate factor: about 20 BTU per square foot in a mild northern climate, 25 in most of the country, and 30 to 35 in the hot South and Southwest. Then adjust for the things that add heat. A very sunny room needs 10 percent more and tall ceilings hold more air; each extra person adds about 600 BTU and a kitchen adds about 4,000 BTU for the range and refrigerator.

These are the Energy Star rules of thumb for room air conditioners: window units, portables and single-zone mini splits. Whole-house systems, sunrooms, garages and rooms with a west-facing glass wall need an ACCA Manual J room-by-room load. A permit for a new system usually requires Manual J and S (IRC M1401.3).

Picking the right size

Round up to the next common size, but not further. An oversized unit short-cycles: it runs 5 to 8 minutes, satisfies the thermostat and shuts off before the coil condenses much moisture, so the room feels cold and damp. A slightly small unit runs longer and keeps the room drier.

Window units run 5,000 to about 25,000 BTU. NEC 440.62 limits a plug-in unit to 50 percent of a circuit shared with lights and receptacles. So 15,000 BTU and up (12 to 14 A) gets a dedicated 20 A circuit, and most units above 15,000 BTU are 208/230 V. A window unit is typically $150 to $700; a single-zone mini split is typically $2,500 to $5,000 installed, runs SEER2 20 or better against CEER 11 to 15, and heats too. Heads come in 9,000, 12,000, 18,000, 24,000 and 36,000 BTU and modulate down to about 30 percent. A 12,000 head handles a 4,500 BTU room without much cycling; an 18,000 does not.

What this calculator leaves out

Not included: the circuit (a dedicated 20 A or a new 240 V run) and a window support bracket. A mini split also needs a line set, pad, condensate drain, a disconnect within sight (NEC 440.14) and mechanical and electrical permits. Mini splits made since 2025 use R-32 or R-454B refrigerant, not R-410A.

Mistakes that cost money

A 15,000 BTU unit draws 12 to 14 A; NEC 440.62 caps a shared circuit at 50 percent, so it needs a dedicated 20 A.
Two sizes up buys short cycling: an 18,000 BTU unit in a 4,500 BTU room runs 5 minutes, never dehumidifies, and feels clammy at 72°F.
Units over 15,000 BTU are mostly 208/230 V on a 6-15 or 6-20 plug; a new 240 V circuit typically runs $300 to $600 installed.
The smallest single-zone mini split head is 6,000 to 9,000 BTU, so a 120 sq ft bedroom is oversized; inverters modulate down to 30 percent.
Portable units: the box's ASHRAE number overstates cooling; the DOE SACC on the yellow label is 30 to 50 percent lower, 12,000 becomes about 7,000.
A sunroom or garage sized by square feet misses the glass and slab; run a Manual J, often 40 to 60 BTU per sq ft.

Key facts

One ton of air conditioning removes 12,000 BTU per hour, the heat needed to melt one ton of ice in 24 hours.
Energy Star's rule of thumb is about 20 BTU per square foot for an average room, before adjustments.
Energy Star suggests reducing capacity 10% for a heavily shaded room and increasing it 10% for a very sunny one.
Add about 600 BTU for each person beyond two who regularly occupies the room, and 4,000 BTU for a kitchen.
An oversized air conditioner cools the air quickly but shuts off before it removes humidity, leaving the room clammy.

Frequently asked questions

How many BTUs do I need for a 12 x 12 room?

A 144 sq ft room needs about 3,600 BTU per hour in a moderate climate, so the smallest 5,000 BTU window unit is plenty. In a very sunny room in a hot climate it rises to about 4,750 BTU.

How many square feet does a 12,000 BTU air conditioner cool?

About 450 to 550 square feet in a moderate climate, or 350 to 400 square feet in a hot, sunny location.

Do I need a dedicated circuit for a window air conditioner?

Under NEC 440.62 a plug-in unit may take only 50 percent of a circuit it shares with lights and receptacles, which is 7.5 A on a 15 A bedroom circuit. Anything above about 8,000 BTU technically needs its own circuit, and 15,000 BTU and up always does. Units over 15,000 BTU are usually 208/230 V on a 6-15 or 6-20 receptacle, which is a new circuit from the panel.

Is it better to oversize an air conditioner?

No. An oversized unit cycles on and off too quickly to dehumidify, wastes energy and wears out faster. Size it to the load and round up only to the next available size.

Sources and references

Room Air Conditioners: sizing chart and adjustments (ENERGY STAR)
Purchasing Energy-Efficient Room Air Conditioners (U.S. Department of Energy)
Manual J Residential Load Calculation (ACCA)

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