BTU Calculator for Air Conditioning (USA)
Free AC size calculator and BTU cooling calculator: get an instant air conditioner sizing estimate in BTU/h, tons, and watts. Adjust for insulation, sun exposure, people, and electronics, then download a simple PNG report.
How to use this AC size calculator
Enter your room width and length in feet, then choose the number of people, electronics, insulation quality and sun exposure. The BTU calculator updates instantly and shows the BTU/h you need, the equivalent cooling tons, and the approximate watts of cooling capacity. If you have been wondering how many BTU you need for a bedroom, living room or office, this is the quickest way to calculate the BTU needed before you shop for an air conditioner.
How BTU sizing works
BTU/h (British Thermal Units per hour) measures how much heat an air conditioner can remove. A common reference is that 1 ton of cooling = 12,000 BTU/h. Proper sizing improves comfort and energy efficiency.
An air conditioning sizing calculator estimates the cooling load of a room: the heat that enters through walls, windows and the roof, plus the heat added by people and electronics. This BTU load calculator multiplies floor area by 22, 31 or 45 BTU per square foot (excellent, average or poor insulation), adds 500 BTU/h per person and per electronic device, and applies a sun exposure factor of 1.0 to 1.18.
Undersizing may run constantly. Oversizing can short-cycle and remove less humidity. Use this tool as a starting point and verify final sizing with a licensed professional.
BTU chart for AC: room size and air conditioner size
This room BTU chart shows example results from the calculator for average insulation, medium sun, two people and one electronic device. Use it as a quick guide to air conditioner size by room size, then enter your own numbers above.
| Room size (sq ft) | BTU/h needed | Cooling tons | Nearest AC unit size |
|---|---|---|---|
| 150 | 6,800 | 0.6 | 7,000 |
| 250 | 10,200 | 0.9 | 10,000–12,000 |
| 350 | 13,600 | 1.1 | 14,000 |
| 500 | 18,700 | 1.6 | 18,000–24,000 |
| 750 | 27,300 | 2.3 | 28,000 |
| 1,000 | 35,800 | 3.0 | 36,000 |
Window, portable, wall, mini-split and central air
Window AC. Use this window AC size calculator for a single room. Window air conditioner units commonly range from about 5,000 to 25,000 BTU/h, so pick the nearest size at or slightly above your result.
Portable air conditioner. The same math applies to a portable air conditioner BTU calculation, but check the unit's SACC rating (seasonally adjusted cooling capacity) on the label. It is usually lower than the headline BTU figure, so size up if the listing only shows the older rating.
Wall and mini-split units. For a wall air conditioner or ductless mini-split, size each indoor unit to the room it serves rather than to the whole house.
Central air. A central air calculator has to account for the whole home, ductwork and climate. As a rough rule, one ton (12,000 BTU/h) covers around 500 to 600 square feet in many homes, but a professional Manual J load calculation is the accurate way to size central air.
EER, watts and running cost
EER (energy efficiency ratio) equals BTU/h divided by the watts of electricity the unit draws. A 12,000 BTU/h air conditioner drawing 1,000 W has an EER of 12. To use this as an EER air conditioner calculator, divide the BTU/h result above by the EER printed on the unit's label to estimate its electrical draw. The "approx. watts" figure in this tool is the cooling capacity converted to watts (1 BTU/h = 0.293 W), not the electricity the AC will use.
BTU guide for homes and commercial spaces
This home BTU calculator suits bedrooms, living rooms, apartments and small offices. Commercial spaces such as restaurants, server rooms, salons and storefronts usually carry higher loads from occupancy, lighting, equipment and door traffic, so treat a commercial BTU calculator result as a starting estimate and ask an HVAC contractor for a commercial load calculation before buying equipment.
Whether you call it an aircon, airco, aircond or A/C, the math is the same: match the BTU/h of the unit to the heat load of the space.
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Frequently asked questions
How many BTU do I need for my room?
Multiply the room area by 22, 31 or 45 BTU per square foot depending on insulation (excellent, average or poor), add about 500 BTU/h for each person and each electronic device, then apply a sun exposure factor. This BTU calculator does all of that instantly from your room size.
How do I estimate BTU from square footage?
Start with room area and adjust for insulation, sun exposure, people, and electronics. For average insulation the starting point is about 31 BTU/h per square foot. This calculator automates those steps.
What size air conditioner do I need for a 300 sq ft room?
With average insulation, medium sun, two people and one electronic device, a 300 sq ft room needs about 11,900 BTU/h, or roughly 1 ton. A 12,000 BTU/h air conditioner is the nearest standard size.
What does “tons” mean in HVAC?
Cooling tons are another way to express capacity: 1 ton = 12,000 BTU/h. The calculator converts BTU/h to tons automatically.
Should I round to a standard AC size?
Yes. The calculator rounds up to the next 100 BTU/h, but real units come in standard steps such as 5,000, 8,000, 10,000, 12,000, 18,000 and 24,000 BTU/h. Choose the nearest size at or slightly above your result.
Is oversizing an air conditioner bad?
Yes. An oversized unit can short-cycle and remove less humidity, while an undersized one may run constantly. Proper sizing improves comfort and efficiency.
How do I size a window or portable air conditioner?
Enter your room dimensions, then pick a window or portable unit rated at or just above the BTU/h result. For portable units, check the SACC rating on the label, which is usually lower than the headline BTU number.
What is EER and how does it relate to BTU?
EER (energy efficiency ratio) is BTU/h divided by watts of electricity used. A 12,000 BTU/h unit drawing 1,000 W has an EER of 12. A higher EER means lower running costs for the same cooling capacity.
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Terms
This tool provides informational estimates only and is not professional engineering advice. Real HVAC loads depend on many variables (ceiling height, windows, infiltration, climate zone, equipment efficiency, etc.).
By using this site, you agree the operator is not responsible for decisions made from these estimates. Always verify final sizing with a licensed HVAC professional.
These terms may change over time without notice.