What Size Air Conditioner Do I Need? (BTU Chart)
Oversized and undersized AC units both fail. Learn how to size an air conditioner using the 20 BTU per square foot rule, a room-by-room BTU chart, and adjustments for sun, kitchens, and ceilings.
Buying an air conditioner that is too small leaves rooms warm and sticky on the hottest afternoons; buying one that is too large is just as bad, because an oversized unit cools the air quickly, shuts off, and never runs long enough to pull humidity out of the room. The result is a clammy, cold space that cycles on and off constantly, wearing out the compressor and wasting electricity. Getting the size right is the single most important decision when picking an AC, and it comes down to one number: BTUs. This guide explains exactly what size air conditioner you need, how to read a BTU chart, and how to adjust for sun, ceilings, kitchens, and crowded rooms.
What a BTU actually measures
BTU stands for British Thermal Unit. One BTU is the amount of heat needed to raise the temperature of one pound of water by one degree Fahrenheit. For air conditioners, the BTU rating describes how much heat the unit can remove from a room in one hour. A 5,000 BTU window unit removes 5,000 BTUs of heat per hour; a 12,000 BTU unit removes more than twice that. The bigger the number, the more cooling power, and the larger the space it can handle.
You will sometimes see cooling capacity expressed in "tons" instead of BTUs, especially for central air and mini-split systems. One ton of cooling equals 12,000 BTU per hour. So a 2-ton system is 24,000 BTU, and a 3-ton system is 36,000 BTU. The term is a leftover from the days when buildings were cooled with blocks of ice, and one ton represented the cooling effect of a ton of ice melting over 24 hours.
The simple rule: 20 BTU per square foot
The U.S. Department of Energy's baseline guideline is roughly 20 BTU per square foot of living space. That figure assumes an average room with 8-foot ceilings, reasonable insulation, and moderate sun exposure. To get a starting estimate, measure the length and width of the room, multiply them to get the area, then multiply by 20.
For example, a 12 ft by 16 ft bedroom is 192 square feet. Multiply by 20 and you get 3,840 BTU, which rounds comfortably to a 4,000 to 5,000 BTU window unit. If you would rather skip the arithmetic and get a sized recommendation with adjustments built in, use our BTU calculator to enter your room dimensions and conditions and get an instant answer.
BTU chart by room size
This chart maps common room sizes to the recommended BTU range under average conditions. Treat it as a baseline; the adjustments in the next section can push you up or down a notch.
| Room area (sq ft) | Recommended BTU | Typical example |
|---|---|---|
| 100 – 150 | 5,000 | Small bedroom, home office |
| 150 – 250 | 6,000 | Bedroom, small den |
| 250 – 300 | 7,000 | Large bedroom |
| 300 – 350 | 8,000 | Studio apartment |
| 350 – 400 | 9,000 | Living room |
| 400 – 450 | 10,000 | Large living room |
| 450 – 550 | 12,000 | Open-plan living/dining |
| 550 – 700 | 14,000 | Large open space |
| 700 – 1,000 | 18,000 – 24,000 | Small apartment / multiple rooms |
| 1,000 – 1,400 | 24,000 – 30,000 | Multi-room zone, small home |
If your square footage lands between two rows, round to the higher BTU only if the room has heat-adding factors. Otherwise, the lower figure usually runs more efficiently and dehumidifies better.
How many BTU do you really need? Adjustments that matter
The flat 20-BTU-per-square-foot figure is a starting point, not a final answer. Real rooms gain and lose heat for many reasons, and the Energy Star program publishes well-known adjustments you should apply:
- Heavy sun: If the room is very sunny, increase capacity by about 10 percent. South- and west-facing rooms with large windows soak up afternoon heat.
- Heavy shade: If the room is heavily shaded, reduce capacity by about 10 percent.
- Extra occupants: The standard assumes two people. Add 600 BTU for each additional person who is regularly in the room, since the human body radiates heat.
- Kitchens: If the unit cools a kitchen, add 4,000 BTU. Ovens, stovetops, and refrigerators throw off serious heat.
- High ceilings: The 20-per-square-foot rule assumes 8-foot ceilings. If yours are taller, the room has more air volume, so scale up proportionally (a 10-foot ceiling adds roughly 25 percent more volume).
- Top floor or poor insulation: Rooms directly under a hot roof, or older homes with thin insulation, may need 10 to 20 percent more capacity.
These adjustments stack. A sunny top-floor kitchen with four people present could need far more than the chart suggests. Rather than juggling all of these by hand, the AC sizing calculator lets you toggle each condition and see how the recommended BTU shifts.
Why bigger is not better
It is tempting to "future-proof" by buying the largest unit you can afford. Resist that urge. An oversized air conditioner cools the room to the thermostat setpoint so fast that it shuts off within a few minutes. Because dehumidification happens gradually as air passes over the cold coil, those short cycles never run long enough to wring much moisture out of the air. You end up with a room that feels cold and damp at the same time.
Short cycling also stresses the compressor, the most expensive component in the system. Each start draws a surge of current and adds wear. A correctly sized unit runs in longer, steadier cycles, holds humidity down, uses less electricity over the season, and lasts longer. When in doubt, size for the room as it is, not for a worst-case heat wave that lasts a few hours a year.
Window units vs. portable vs. mini-split
BTU sizing principles apply to every type, but the formats differ in efficiency and placement:
- Window units are the cheapest per BTU and vent directly outside, so almost all their rated cooling reaches the room. They suit single rooms and apartments.
- Portable units sit on the floor and vent through a hose to a window. They are convenient but less efficient; the exhaust hose radiates heat back into the room, so you may want to size up by 1,000 to 2,000 BTU compared to a window unit.
- Ductless mini-splits are the most efficient option for whole rooms or zones, run very quietly, and often include heating. They cost more upfront and usually require professional installation, but their inverter compressors modulate output and avoid the on/off cycling of cheaper units.
Step-by-step: sizing your room
Here is the full process from tape measure to purchase:
- 1. Measure the floor. Multiply length by width in feet to get square footage. For odd shapes, split the room into rectangles and add the areas together.
- 2. Apply the baseline. Multiply the area by 20 BTU.
- 3. Adjust for conditions. Add or subtract for sun, shade, extra people, kitchen heat, ceiling height, and insulation.
- 4. Round to an available size. AC units come in standard increments (5,000, 6,000, 8,000, 10,000, 12,000, and so on). Pick the nearest standard size to your adjusted number.
- 5. Check the electrical requirement. Units up to about 12,000 BTU usually run on a standard 115-volt outlet. Larger units may need a dedicated 230-volt circuit.
Run those numbers through the BTU per square foot calculator for a clean recommendation before you shop.
Common sizing mistakes
A few errors come up again and again. Ignoring ceiling height is the most frequent; two rooms with identical floor area but different ceilings hold very different volumes of air. Forgetting kitchen heat is another, leading to a unit that struggles every time the stove is on. People also overlook that an open-plan layout cools as one large space, not as separate rooms, so a single small unit cannot handle a combined kitchen, dining, and living area. Finally, many buyers forget about doors and openings; if the conditioned room connects to unconditioned space through a wide opening, you are effectively cooling both.
Understanding SEER, EER, and energy efficiency
BTU tells you the cooling capacity of an air conditioner, but two units with identical BTU ratings can cost wildly different amounts to run. That difference is captured by efficiency ratings. In the US, the headline number is SEER (Seasonal Energy Efficiency Ratio), which measures cooling output over a typical season divided by the electricity used. A higher SEER means lower running costs. Older units might sit at SEER 10, while modern high-efficiency models reach SEER 20 or beyond.
EER (Energy Efficiency Ratio) is a related figure measured at a single high-temperature operating point, which makes it useful for sizing window and portable units that run hard in peak heat. In the UK and Europe, you will more often see the energy rating label (A to G) and a figure called SCOP or SEER expressed differently, alongside the annual energy consumption in kilowatt-hours. Whatever the label, the principle is the same: pick the smallest unit that adequately cools your space, then choose the highest efficiency you can afford.
| SEER rating | Relative efficiency | Typical use |
|---|---|---|
| 10-13 | Older / minimum standard | Legacy units due for replacement |
| 14-16 | Standard modern efficiency | Most new mid-range installs |
| 17-20 | High efficiency | Hot climates, heavy use |
| 21+ | Premium / variable-speed | Lowest running cost, long-term savings |
Estimating running costs by room size
Once you know the BTU and efficiency, you can estimate what an air conditioner will actually cost to run, which is often the deciding factor. The rough method is to convert BTU to watts (divide BTU by the EER to get watts), multiply by the hours of use, then multiply by your electricity price per kilowatt-hour. A 10,000 BTU unit with an EER of 10 draws about 1,000 watts, or 1 kWh per hour of running.
If electricity costs 28 pence per kWh in the UK or around 16 US cents per kWh on average in the States, running that unit for eight hours a day for a month works out to noticeably different bills on each side of the Atlantic. The table below gives ballpark monthly figures assuming eight hours of daily use, though real consumption depends on how often the compressor cycles on and off rather than running continuously.
| Unit size | Approx. power draw | UK est. monthly | US est. monthly |
|---|---|---|---|
| 5,000 BTU | ~0.5 kW | ~£34 | ~$19 |
| 8,000 BTU | ~0.8 kW | ~£54 | ~$31 |
| 12,000 BTU | ~1.2 kW | ~£81 | ~$46 |
| 18,000 BTU | ~1.8 kW | ~£121 | ~$69 |
| 24,000 BTU | ~2.4 kW | ~£161 | ~$92 |
These are estimates for steady running; an efficient inverter unit that modulates its output rather than switching fully on and off can cut these figures substantially. To refine your own numbers, work out your room's BTU need first with a BTU calculator, then apply your local electricity tariff.
Cooling capacity for specific room types
The standard 20 BTU per square foot rule is a starting point, but certain rooms break it. Kitchens generate substantial heat from cooking and appliances, so add roughly 4,000 BTU if you are cooling one. Rooms that are routinely occupied by several people, such as a home office with visitors or a living room during gatherings, need around 600 extra BTU per additional person beyond two.
- Kitchens: add about 4,000 BTU for cooking heat.
- Sun-facing rooms: increase capacity by around 10% for heavy afternoon sun, decrease by 10% for heavily shaded rooms.
- High ceilings: the 20 BTU rule assumes 8-foot ceilings; taller rooms hold more air volume and need proportionally more cooling.
- Server rooms or rooms with electronics: heat-generating equipment can require dedicated capacity well above the floor-area estimate.
- Conservatories: large glass areas gain heat fast and may need significantly more BTU than their footprint suggests.
Climate zones and humidity considerations
Where you live changes the maths. An air conditioner sized perfectly for a mild maritime UK summer would struggle in the humid heat of the US Gulf Coast. Humidity matters because part of an air conditioner's job is to remove moisture, and that dehumidification draws on the same capacity that cools the air. In very humid climates, an oversized unit is especially counterproductive: it cools the room quickly, switches off before it has wrung out the moisture, and leaves you with a cold but clammy, uncomfortable space.
This is why "right-sizing" beats "oversizing" in damp conditions. A correctly sized unit runs for longer, steadier cycles that pull more humidity from the air. In dry desert climates the opposite is true, and evaporative coolers can sometimes do the job for a fraction of the energy. UK users dealing with mild but damp summers often find a unit with a strong dehumidify mode more valuable than raw cooling power.
Installation and placement that protect performance
Even a perfectly sized unit underperforms if it is poorly placed. Window units should ideally sit on a shaded side of the building, because direct sun on the unit's exterior forces it to work harder to dump heat. Portable units rely on an exhaust hose to vent hot air, and a long, kinked, or uninsulated hose dramatically reduces efficiency. Keep the hose as short and straight as possible.
- Clearance: leave space around the unit so it can draw and expel air freely; furniture pushed against it chokes airflow.
- Sealing: seal gaps around window installations so the cool air you paid for does not leak out and warm air does not leak in.
- Mini-split outdoor units: mount them where heat can dissipate, away from enclosed corners and direct afternoon sun.
- Thermostat position: avoid placing the unit or sensor near heat sources like lamps or electronics, which trick it into overcooling.
Maintenance that keeps capacity at rated levels
An air conditioner that delivered its full BTU rating when new can quietly lose a chunk of that capacity within a couple of seasons if it is neglected. The single most important task is cleaning or replacing the air filter. A clogged filter restricts airflow, makes the unit run longer, raises bills, and in extreme cases causes the coils to ice up and shut the system down. Check filters monthly during heavy use and clean them at least every few weeks.
Beyond filters, keep the condenser coils clean, because a layer of dust acts as insulation and traps heat inside the unit. Make sure the condensate drain is clear so collected moisture can escape rather than backing up. For split systems, a professional service every year or two checks the refrigerant charge; a unit low on refrigerant loses cooling capacity and runs inefficiently without obviously failing. A few minutes of routine care preserves the cooling power you sized for and extends the life of the equipment by years.
When to choose multiple smaller units
For larger homes or open-plan layouts, a single large unit is not always the best answer. Cooling capacity does not travel well around corners or up stairs, so one big BTU number in the living room may leave bedrooms stubbornly warm. In these cases, multiple smaller units or a multi-zone mini-split system, where one outdoor compressor feeds several indoor heads, often delivers more even comfort and lets you cool only the rooms you are using.
Zoning also saves money. Cooling an empty guest bedroom all afternoon is wasted energy; a multi-zone setup lets you switch that head off entirely. The trade-off is higher upfront cost and installation complexity. As a rule, single-room cooling problems are best solved with a single right-sized unit, while whole-home comfort across several rooms usually justifies the move to a zoned system. Size each zone individually using the floor-area method rather than dividing one large figure across rooms, since each space has its own sun exposure, occupancy, and heat sources.
Reducing the cooling load before you buy
The cheapest BTU is the one you never have to install. Before sizing a unit, it is worth reducing how much heat enters the room in the first place, because a smaller cooling load means a smaller, cheaper, quieter unit and lower running costs for years to come. Much of a room's summer heat arrives through windows as solar gain, so addressing that is the highest-leverage step.
Closing blinds or curtains during the hottest part of the day, fitting reflective or low-emissivity window film, and using external shading such as awnings or trees on sun-facing windows can cut solar gain dramatically. Sealing draughts and improving loft or ceiling insulation keeps the cooled air where you want it and stops hot attic air from radiating down. Even switching to LED lighting helps, because old incandescent and halogen bulbs throw off meaningful heat. Tackle these first, then size your unit to the reduced load rather than the original one.
- Solar control: blinds, reflective film, and external shading on sun-facing glass.
- Insulation: loft and cavity insulation reduce heat transfer through the building fabric.
- Draught-proofing: seal gaps so cool air stays in and hot air stays out.
- Lighting and appliances: switch to LEDs and avoid running heat-heavy appliances during peak heat.
Smart controls, timers, and thermostats
Modern air conditioners increasingly come with smart controls that can shave a surprising amount off your bills without sacrificing comfort. A programmable timer that pre-cools a bedroom before bedtime and then eases off overnight uses far less energy than running flat out all evening. Smart thermostats learn your routine and avoid cooling empty rooms, and many can be controlled from a phone so you are not cooling the house all afternoon for a return you might delay.
The most efficient setting is usually the highest temperature that still feels comfortable, because every degree cooler costs noticeably more energy. Many people find that setting the target a degree or two warmer and adding a ceiling fan delivers the same comfort for less money, since moving air feels cooler on the skin. Inverter units paired with a smart thermostat are especially effective because they modulate output to hold a steady temperature rather than blasting cold air and switching off. The table below shows how target temperature interacts with running cost in broad terms.
| Setting strategy | Comfort | Relative running cost |
|---|---|---|
| Low target, run all day | Very cold | Highest |
| Moderate target + ceiling fan | Comfortable | Lower |
| Timed pre-cool then ease off | Comfortable when needed | Low |
| Smart thermostat, room-aware | Comfortable, automated | Lowest practical |
Frequently asked questions
How many BTU do I need for a 12x12 room?
A 12 by 12 foot room is 144 square feet. At 20 BTU per square foot that is 2,880 BTU, so a 5,000 BTU unit (the smallest common size) is plenty for average conditions. Bump up only if the room is very sunny, on a hot top floor, or regularly holds several people.
Is it bad to buy an air conditioner that is too big?
Yes. An oversized unit cools the air fast and shuts off before it can remove humidity, leaving the room cold and clammy. The frequent on/off cycling wastes energy and wears out the compressor. Sizing the unit correctly to the room gives steadier cooling, better dehumidification, and longer equipment life.
How do I convert BTU to tons?
Divide the BTU rating by 12,000, since one ton of cooling equals 12,000 BTU per hour. A 24,000 BTU system is 2 tons, and a 36,000 BTU system is 3 tons. Tons are most often used for central air and mini-split systems rather than window units.
Does ceiling height change the BTU I need?
It does. The standard 20-BTU-per-square-foot rule assumes 8-foot ceilings. Taller ceilings mean more air volume to cool, so scale up proportionally; a 10-foot ceiling holds about 25 percent more air and needs roughly 25 percent more capacity.
How much extra cooling does a kitchen need?
Add about 4,000 BTU when the air conditioner has to cool a kitchen. Ovens, stovetops, and refrigerators all release significant heat, so a kitchen of a given size needs more capacity than a bedroom of the same size.
Will one air conditioner cool my whole apartment?
Only if the space is genuinely open. A single unit cools the room it sits in plus any space it shares air with freely. Closed bedrooms and rooms behind hallways will not cool well from one unit. For separated rooms, you need multiple units or a multi-zone mini-split system sized for the combined load.