AC BTU Calculator

Size room air conditioners, mini-splits, and window AC units using official ENERGY STAR thermal factor guidelines. Capacity is expressed in BTU/hr (co

HVAC

Room Length (ft)

Interior length of the room to be air conditioned.

Room Width (ft)

Interior width of the room to be air conditioned.

Ceiling Height (ft)

Standard residential ceiling height is 8 ft.

Sun Exposure

ENERGY STAR guideline: adjust by ±10% based on direct sun exposure.

Number of Occupants

people

ENERGY STAR adds 600 BTU/hr for each person above 2 regular occupants.

Is This a Kitchen?

ENERGY STAR adds 4,000 BTU/hr for cooking heat in kitchens.

Formula & Methodology

AC Cooling Load (BTU/hr) = (Area × 20 × Ceiling/8 × Sun Factor) + ((Occupants - 2) × 600) + Kitchen Heat

The formula starts with the standard ENERGY STAR residential cooling benchmark of 20 BTU/hr per square foot, scaled by ceiling height (Height / 8). Sun exposure adds or subtracts 10% (1.10× for direct sun, 0.90× for heavy shade). If more than 2 people regularly occupy the space, 600 BTU/hr is added per additional person. Kitchens incur a 4,000 BTU/hr heat addition to counteract stoves, ovens, and refrigerators. The final cooling requirement is rounded to the nearest 100 BTU/hr and converted to refrigeration tons using 12,000 BTU/hr per ton.

Required AC Cooling Capacity

3900 BTU/hr

Overview

Size room air conditioners, mini-splits, and window AC units using official ENERGY STAR thermal factor guidelines. Capacity is expressed in BTU/hr (cooling power per hour), where exactly 12,000 BTU/hr equals one refrigeration ton. Disclaimer: This calculator provides an estimate for preliminary sizing, budgeting, and equipment comparison based on standard industry rules of thumb; it does not replace an accredited ACCA Manual J residential load calculation performed by a licensed HVAC contractor.

Worked Examples

14 ft × 14 ft Master Bedroom (2 Occupants, 8 ft Ceiling)

Dimensions 14 ft × 14 ft (196 sq ft), 8 ft ceiling, average sunlight (1.00×), 2 occupants, non-kitchen.

Calculate room area: 14 ft × 14 ft = 196 sq ft.
Calculate ceiling height ratio: 8 ft / 8 ft = 1.00×.
Compute base cooling load: 196 sq ft × 20 BTU/hr/sq ft × 1.00 × 1.00 = 3,920 BTU/hr.
Calculate occupant addition: 2 occupants = 0 extra people (0 BTU/hr).
Calculate kitchen addition: 0 BTU/hr.
Round to nearest 100 BTU/hr: 3,900 BTU/hr.
Calculate tonnage: 3,900 BTU/hr ÷ 12,000 BTU/hr/ton = 0.33 Tons.
Recommend standard equipment size: 5,000 or 6,000 BTU/hr window or mini-split unit.

Final Answer: 3,900 BTU/hr (0.33 Tons | 5,000–6,000 BTU/hr unit recommended)

18 ft × 16 ft Sunny Kitchen with 4 Regular Occupants

Dimensions 18 ft × 16 ft (288 sq ft), 8 ft ceiling, very sunny (1.10×), 4 occupants, kitchen space.

Calculate room area: 18 ft × 16 ft = 288 sq ft.
Compute base load with sunlight: 288 sq ft × 20 BTU/hr/sq ft × 1.10 = 6,336 BTU/hr.
Add occupant heat: (4 - 2) × 600 BTU/hr = 1,200 BTU/hr.
Add kitchen appliance heat: +4,000 BTU/hr.
Sum all cooling loads: 6,336 + 1,200 + 4,000 = 11,536 BTU/hr.
Round to nearest 100 BTU/hr: 11,500 BTU/hr.
Calculate tonnage: 11,500 ÷ 12,000 = 0.96 Tons (recommended 12,000 BTU/hr / 1.0 Ton unit).

Final Answer: 11,500 BTU/hr (0.96 Tons | 12,000 BTU/hr unit recommended)

12 ft × 12 ft Shaded Home Office (1 Occupant, 9 ft Ceiling)

Dimensions 12 ft × 12 ft (144 sq ft), 9 ft ceiling, heavily shaded (0.90×), 1 occupant, non-kitchen.

Calculate room area: 12 ft × 12 ft = 144 sq ft.
Calculate ceiling height ratio: 9 ft / 8 ft = 1.125×.
Compute base cooling load with shade: 144 sq ft × 20 BTU/hr/sq ft × 1.125 × 0.90 = 2,916 BTU/hr.
Occupants (1 person ≤ 2) and kitchen: 0 additional BTU/hr.
Round to nearest 100 BTU/hr: 2,900 BTU/hr.
Calculate tonnage: 2,900 ÷ 12,000 = 0.24 Tons (recommended 5,000 BTU/hr minimum unit).

Final Answer: 2,900 BTU/hr (0.24 Tons | 5,000 BTU/hr unit recommended)

How to Use This Calculator

Enter your room length and width in feet to determine total floor area.

Enter ceiling height (standard residential ceilings are 8 feet).

Select whether the room receives heavy shade, normal sun, or intense direct sunlight.

Specify the typical number of occupants regularly present in the room.

Indicate whether the room is a kitchen with cooking appliances.

View required cooling capacity in BTU/hr and find the recommended standard unit size.

Reference Table

Room Area (sq ft)	Standard AC Size	Cooling Tonnage	Typical Room Application
100 to 150 sq ft	5,000 BTU/hr	0.42 Tons	Small bedroom, home nursery, or home office
150 to 250 sq ft	6,000 BTU/hr	0.50 Tons	Standard bedroom or small guest room
250 to 350 sq ft	8,000 BTU/hr	0.67 Tons	Large master bedroom or small living room
350 to 450 sq ft	10,000 BTU/hr	0.83 Tons	Mid-sized family room or kitchen area
450 to 550 sq ft	12,000 BTU/hr	1.00 Ton	Large living room or studio apartment
550 to 700 sq ft	14,000 BTU/hr	1.17 Tons	Open concept living/dining combination
700 to 1,000 sq ft	18,000 BTU/hr	1.50 Tons	Great room, open floor plan, or loft space

Frequently Asked Questions

Can I size an air conditioner by square footage alone?

While square footage provides a useful baseline, sizing solely by area often causes errors. Sunlight, ceiling height, body heat from multiple occupants, and heat from cooking appliances can alter cooling demand by 30% to 50%. Our calculator incorporates all four ENERGY STAR adjustment factors.

What happens if I buy an AC unit that is too large?

An oversized air conditioner cools the room too quickly without running long enough to remove moisture. This leaves the room feeling humid, sticky, and clammy. It also causes excessive compressor cycling, which spikes electrical consumption and wears out mechanical parts prematurely.

How many BTU/hr is one ton of air conditioning?

One refrigeration ton is defined as exactly 12,000 BTU/hr. A 2.0-ton central AC or heat pump delivers 24,000 BTU/hr of cooling capacity, and a 3.0-ton unit provides 36,000 BTU/hr.

What do SEER2 and CEER ratings mean for air conditioners?

Seasonal Energy Efficiency Ratio 2 (SEER2) measures cooling efficiency for central AC and ductless mini-splits over a typical season. Combined Energy Efficiency Ratio (CEER) measures window and portable AC efficiency including standby power. Higher numbers mean lower electric bills.

Does this calculator replace an official Manual J load calculation?

No. This calculator is a screening estimate based on ENERGY STAR rules of thumb. An accredited ACCA Manual J load calculation accounts for insulation R-values, window glass coatings (Low-E), ductwork efficiency, and local design temperatures.