How do I calculate the BTU I need for a room?

Multiply the room square footage by 20 BTU per square foot as a baseline assuming an 8-foot ceiling. Then adjust for ceiling height (add 10% per foot above 8 ft), sun exposure (10% more for sunny, 10% less for heavily shaded), kitchens (add 4,000 BTU for cooking heat), occupancy above two people (600 BTU each), climate zone (90% cool to 130% hot-dry), and insulation quality (90% well-insulated to 120% poor). This calculator performs every step automatically and rounds the result up to the nearest standard window AC size between 5,000 and 24,000 BTU.

What size window air conditioner do I need for a 12x12 bedroom?

A 12 x 12 bedroom is 144 sqft, giving a baseline of about 2,880 BTU. With an average climate, normal sun, two people, and 8-foot ceilings the recommendation is the smallest standard window unit: 5,000 BTU. Even a hot-humid climate (factor 1.2) with poor insulation only pushes the requirement to about 4,150 BTU, still well within the 5,000 BTU minimum standard size.

What size window AC do I need for a 500 square foot room?

At the baseline 20 BTU/sqft rule, 500 sqft needs 10,000 BTU. Add factors for sun, climate, and ceilings and the typical real-world requirement lands between 10,000 and 14,000 BTU, mapping to a standard 12,000 BTU or 14,000 BTU unit. Use the calculator above to nail down the exact recommendation for your specific conditions.

Is it bad if my window AC is too big?

Yes, oversizing is one of the most common mistakes. A unit larger than the load cools the air to the thermostat setpoint too quickly, shuts off, and never runs long enough to dehumidify. The result is a cold, clammy, "swamp" feeling room with high humidity and condensation on windows. Oversized units also short-cycle, which shortens compressor life and wastes electricity. Industry guidance is to size within +/- 15% of the calculated load, not to "go one size up just in case."

What happens if a window AC is too small?

An undersized unit runs continuously without ever reaching the setpoint, struggling to cool the room on hot days. The compressor runs at full duty cycle so energy use is high, the unit wears out faster, and the room stays uncomfortable. Undersized units do dehumidify well because they run constantly, but the loss in comfort and equipment life outweighs that benefit. Our calculator rounds up to the next standard size, eliminating this risk.

What is the difference between EER and SEER?

EER (Energy Efficiency Ratio) is the cooling output in BTU/h divided by the power draw in watts at a single steady-state test condition (95°F outdoor / 80°F indoor). SEER (Seasonal Energy Efficiency Ratio) averages efficiency across a typical cooling season including part-load operation. Window AC units are rated in EER (or the newer CEER, which adds standby power). Central systems and mini-splits use SEER and SEER2. Higher numbers mean more cooling per watt of electricity. An EER of 10 is baseline, 11+ qualifies as Energy Star for most window units.

How much electricity does a window AC use?

Divide the BTU rating by the EER to get watts. A 10,000 BTU window unit at EER 10 draws 1,000 watts = 1 kW. At a US average of $0.16 per kWh, running it for 8 hours costs about $1.28. Over a 30-day month at 8 hours/day that is roughly $38. The calculator above estimates both EER 10 (older or basic units) and EER 12 (Energy Star units) so you can compare ongoing operating cost when choosing between models.

Window AC vs portable AC - which is better?

Window units are 20-40% more efficient than equivalent-BTU portable units because they vent both hot air and condensate out the window directly, while portables exhaust through a hose and lose some cooling capacity to the room. Window units also tend to be cheaper to buy and quieter. Portables make sense only when window installation is impossible (slider windows, casement, HOA restrictions). For most rooms, a window unit at the size this calculator recommends will outperform a portable at a higher BTU rating.

Window AC vs mini split - when to choose which?

Window AC is the cheapest path to cooling one room (typically $200-700 installed). Mini-splits cost $2,000-5,000 per zone but are dramatically more efficient (SEER 18-30+), much quieter, reversible for heat, and do not block a window. Choose a window unit for renters, short-term cooling, or budget installs. Choose a mini-split if you own the home, plan to stay, want year-round comfort, or already need a major electrical upgrade.

Does ceiling height really affect BTU?

Yes, because BTU per square foot is a shortcut for BTU per cubic foot of air volume. Standard residential ceilings are 8 feet, so the BTU/sqft figures assume 8 ft of vertical air. Every additional foot adds 12.5% to the volume the AC must cool, and we round to 10% per foot to also account for stratification (hot air rising). A 10-foot vaulted ceiling room therefore needs about 20% more BTU than the same floor area with 8-foot ceilings.

Do I need to convert BTU to kW for UK or EU installation?

Yes. UK and EU air conditioner specifications use kilowatts (kW) of cooling capacity. The conversion is 1 BTU/h = 0.000293 kW, or equivalently 1 kW = 3,412 BTU/h. Common British units include 3.5 kW (~12,000 BTU), 5 kW (~17,000 BTU), and 7 kW (~24,000 BTU). The calculator above reports both BTU/h and kW. For metric room dimensions, switch the input toggle to meters or square meters.

What is a "Manual J" load calculation?

Manual J is the ACCA (Air Conditioning Contractors of America) standard procedure for calculating residential heating and cooling loads. A full Manual J considers wall U-values, glazing area and SHGC, infiltration, internal loads, ductwork, and orientation. The short-form method used here is the DOE-recommended quick-sizing approach for single-room window units where a full Manual J would be overkill. For whole-house systems or new construction, a full Manual J by a licensed HVAC contractor is strongly recommended.

Window AC Size Calculator

Find the right window air conditioner size in BTU (or kW) for any room. DOE-style Manual J short-form sizing with adjustments for ceiling height, sun, kitchens, occupancy, climate zone, and insulation. Recommends the nearest standard unit and estimates running electricity cost.

Method

Manual J Short-Form

9 Standard

BTU Sizes

5 Climate

Zones (US/Global)

Live

$/hr & $/mo

Quick Room Presets

Room Inputs

Input Mode

Units

Length (ft)

Width (ft)

Ceiling Height (ft)

Standard is 8 ft. Adds 10% per foot above 8 to account for extra air volume.

Sun Exposure

Kitchen?

People

Climate Zone

Hot, humid summers with sustained 85-95°F (29-35°C).

Examples: Washington DC, St Louis, Kansas City, mid-Atlantic, eastern China, northern India

Insulation Quality

kWh Rate (USD)

Runtime (hr/day)

Recommended Unit

Excellent match

6,000

BTU/h (1.76 kW)

Required Load

5544 BTU/h

1.62 kW

Room Area

252 sqft

23.4 sqm

Room Visualizer

Floor Area252.0 sqft (23.41 sqm)

Ceiling Height8 ft (2.44 m)

Air Volume2016 cu ft

BTU per sqft22.0 BTU/sqft

Calculation Breakdown

Base (sqft x 20)5040 BTU

Ceiling x 1.005040 BTU

Sun x 1.005040 BTU

+ Kitchen (0)5040 BTU

+ Extra people (0)5040 BTU

Climate x 1.105544 BTU

Insulation x 1.005544 BTU

EER 10 (Basic Unit)

$0.096

per hour at $0.16/kWh

Monthly: $23.04 (8 h/day)

Draw: 0.600 kW

EER 12 (Energy Star)

$0.080

per hour at $0.16/kWh

Monthly: $19.20 (8 h/day)

Draw: 0.500 kW

Understanding BTU Sizing for Window Air Conditioners

Sizing a window air conditioner is one of the most consequential decisions a renter or homeowner makes about indoor comfort, and it is also one of the most misunderstood. The unit of cooling capacity is the BTU per hour (British Thermal Unit per hour), where one BTU is the energy needed to raise the temperature of one pound of water by one degree Fahrenheit. A 10,000 BTU/h window AC therefore removes 10,000 BTU of heat from a room every hour. In the UK, EU, Australia, and most of the metric world the same capacity is expressed in kilowatts: 1 kW equals approximately 3,412 BTU/h, so a 10,000 BTU unit is roughly 2.93 kW of cooling.

The classic 20 BTU per square foot rule of thumb is the starting point used by the US Department of Energy and the Energy Star program for residential single-room sizing. It assumes a standard 8-foot ceiling, moderate climate, average insulation, two occupants, and no special heat loads. Real rooms vary, so this calculator layers in the same correction factors that a Manual J short-form sizing worksheet uses: ceiling height (each foot above 8 ft adds about 10% to the air volume the AC must cool), sun exposure (a south- or west-facing room with large windows gains an extra 10%, a deeply shaded north room loses 10%), kitchen heat (range, oven, refrigerator, and cooking add about 4,000 BTU of latent and sensible load), and occupancy (each person above two contributes about 600 BTU of body heat and respiration moisture).

Climate matters enormously. The same 250 square-foot bedroom in Seattle, Chicago, and Phoenix has very different cooling demand because outdoor design temperature drives the temperature differential the AC must overcome. We use a five-zone scale derived from the ASHRAE/IECC climate zones, with factors from 0.9 (cool/marine) through 1.3 (hot-dry desert). Insulation quality multiplies the whole result by 0.9 for tight modern construction, 1.0 for typical, and 1.2 for old uninsulated walls. After all factors are applied, we round up to the nearest standard window AC capacity from the nine sizes commercially available between 5,000 and 24,000 BTU.

Sizing in window AC is not "more is better." A unit larger than the calculated load cools the air past the thermostat setpoint too fast, shuts off, and never runs long enough to wring moisture out of the air. The result is the dreaded "cold and clammy" room with damp surfaces and a swampy feel. Oversized units also short-cycle, which wears the compressor out early and uses more electricity per BTU delivered. Undersized units run constantly without catching up. The correct size - the one returned by this calculator - hits the setpoint in 15-30 minutes, then cycles a few times per hour, dehumidifying as it goes. Pair that with weatherstripping the window seal, drawing curtains during peak sun, and cleaning the filter monthly, and a properly sized window unit will outperform a much larger oversized one in comfort, electricity bill, and lifespan.

BTU Reference Table by Room Size

Baseline 20 BTU/sqft at 8-ft ceilings, average climate, normal sun, two occupants. Adjust up or down using the factors above for your specific conditions. Recommended unit is the next standard size at or above the load.

Room Size (sqft)	Sqm (metric)	Base BTU/h	Recommended Unit	kW (metric)
100 sqft	9.3 sqm	2,000	5,000 BTU	1.47 kW
150 sqft	13.9 sqm	3,000	5,000 BTU	1.47 kW
200 sqft	18.6 sqm	4,000	5,000 BTU	1.47 kW
250 sqft	23.2 sqm	5,000	5,000 BTU	1.47 kW
300 sqft	27.9 sqm	6,000	6,000 BTU	1.76 kW
400 sqft	37.2 sqm	8,000	8,000 BTU	2.34 kW
500 sqft	46.5 sqm	10,000	10,000 BTU	2.93 kW
600 sqft	55.7 sqm	12,000	12,000 BTU	3.52 kW
700 sqft	65.0 sqm	14,000	14,000 BTU	4.10 kW
800 sqft	74.3 sqm	16,000	18,000 BTU	5.28 kW
1000 sqft	92.9 sqm	20,000	24,000 BTU	7.03 kW
1200 sqft	111.5 sqm	24,000	24,000 BTU	7.03 kW

US and Global Climate Zone Reference

The five climate zones below align with simplified IECC/ASHRAE residential climate zones. Pick the zone that best matches your region; the BTU calculation scales by the listed factor.

Zone 1 - Cool / Marine

x0.9

Mild summers, rarely above 80°F (27°C).

Examples: Seattle, San Francisco, Portland, coastal Pacific Northwest, UK, Ireland

Zone 2 - Mixed-Cool

Warm summers, occasional 90°F (32°C) days.

Examples: Chicago, Boston, Denver, New York metro, much of central Europe

Zone 3 - Mixed-Humid

x1.1

Hot, humid summers with sustained 85-95°F (29-35°C).

Examples: Washington DC, St Louis, Kansas City, mid-Atlantic, eastern China, northern India

Zone 4 - Hot-Humid

x1.2

Long hot season, high humidity, 90°F+ (32°C+) most days.

Examples: Atlanta, Houston, Orlando, New Orleans, southeast US, Gulf Coast, Southeast Asia

Zone 5 - Hot-Dry / Desert

x1.3

Extreme heat, 100°F+ (38°C+) common, low humidity.

Examples: Phoenix, Las Vegas, Tucson, Riyadh, Dubai, interior Australia, Sahel

How to Size a Window AC in 5 Steps

1
Measure the room
Use a tape measure to find the length and width of the room you want to cool. Multiply for the floor area, or enter dimensions directly into the calculator. Use the metric toggle if you measure in meters.
2
Enter ceiling height and room conditions
Standard ceilings are 8 ft; vaulted or loft ceilings need more BTU. Mark whether the room is heavily sun-exposed (south/west-facing with big windows) or heavily shaded (north-facing with trees), and toggle the kitchen flag if the room contains a range or oven.
3
Set occupancy and climate
Enter the typical number of regular occupants - each person above two adds 600 BTU. Pick the climate zone that matches your region from Cool/Marine (zone 1) through Hot-Dry Desert (zone 5).
4
Choose insulation quality
Well-insulated (new construction, foam-sealed) subtracts 10%; average leaves the load unchanged; poor (old or uninsulated) adds 20%. When in doubt, pick average.
5
Read the recommended unit and electricity cost
The calculator outputs the required BTU/h, the nearest standard unit (5,000-24,000 BTU), the kW equivalent for metric specs, and estimated electricity cost at EER 10 and EER 12. Compare against any unit you are considering at the retailer.

Common Use Cases

Bedroom and apartment cooling

Renters and small-home owners use this calculator to right-size a $200-500 window unit for one bedroom or studio without paying for a Manual J. Pair it with our square footage calculator if you have an irregular floor plan to measure first.

Whole-house heating cross-check

Pair the cooling BTU output of this tool with the heating BTU from our furnace BTU calculator to design year-round comfort for a single addition or in-law suite.

Mini-split / refrigerant line planning

If your sizing comes out above 18,000 BTU you are likely better off with a mini-split than a window unit. Use our refrigerant line sizing tool to plan the line set for the mini-split installation.

Whole-house ventilation cross-check

When you add cooling, you may also need to verify exhaust and supply airflow. Check the CFM calculator for bathroom and kitchen fan sizing so humidity stays under control.

Pro Tips

Do NOT oversize - "going one size up just in case" causes short cycling, poor dehumidification, and a clammy room.
Sun exposure matters more than people realize. A south-facing room with large unshaded glass can need 10-20% more BTU than a shaded equivalent.
Clean the filter monthly during cooling season. A clogged filter cuts capacity by 5-15% and wastes electricity.
Sealing the window perimeter with foam and weatherstripping does more for comfort than upsizing the BTU.
For rooms above 18,000 BTU, a ductless mini-split is usually a better investment than a giant window unit.
Energy Star units (EER 11+) pay back the price difference in 1-3 cooling seasons for medium-use rooms.
In multi-room apartments, prefer one unit per room over a single oversized unit cooling through doorways.
For metric installation, look for kW capacity on UK/EU spec sheets: 2.5 kW ~ 8,500 BTU, 3.5 kW ~ 12,000 BTU, 5 kW ~ 17,000 BTU.
Position the unit on the shadiest wall available - direct sun on the outdoor side reduces capacity.
Consider window seal quality: gaps around the unit waste cold air and let in humidity that overwhelms the dehumidification cycle.

Occupancy and Internal Loads

Every regular occupant above the assumed two adds about 600 BTU of sensible and latent load (body heat plus respiration moisture). Other internal heat sources can also bump the load:

Person at rest

~400-600 BTU/h sensible + latent

Kitchen range/oven (use)

~4,000 BTU/h average over cook period

Desktop PC + monitors

~800-1,500 BTU/h running

Large TV (50-65")

~300-500 BTU/h

Refrigerator

~600 BTU/h continuous

Incandescent lighting (100W)

~340 BTU/h per bulb

Window AC Size Calculator FAQs

Have more questions? Contact us

What Homeowners and HVAC Pros Say

4.9

Based on 3,800 reviews

“Spent a week reading conflicting advice on Reddit before finding this calculator. Plugged in my 220 sqft sunroom in Atlanta and it recommended 8,000 BTU - exactly what the HVAC guy quoted me. Bought a unit at Costco, installed it myself, and the room finally stays comfortable through Georgia summers. The oversize warning steered me away from the "12,000 BTU is safer" trap.”

Rachel Tan

Homeowner, Atlanta GA

April 2, 2026

“I manage 38 units and replace window ACs every few summers. This calculator made my standard sizing spreadsheet obsolete. Type in the floor plan dimensions, pick climate zone 2, and get the unit size plus electricity cost estimate for the tenant info packet. Saves me an hour per unit and tenants stop complaining about humidity from oversized installs.”

Marcus Reilly

Property Manager, Boston MA

April 18, 2026

“Phoenix in August is no joke. Used the climate zone 5 setting and the calculator said my 350 sqft studio needs 12,000 BTU - which matched the unit my landlord refused to upgrade. Showed her the printout and she replaced the unit. The kW output also let me cross-check the model with my electricity bill estimate. Lifesaver.”

Priya Kapoor

Apartment Renter, Phoenix AZ

April 26, 2026

“Seattle finally needs AC and I had no idea where to start. Picked climate zone 1 (Cool/Marine), entered a 14x16 master bedroom, and the calc said 6,000 BTU would do it. Bought a 6,000 BTU LG and it cools the room in 20 minutes on the hottest days. The EER vs SEER explainer also taught me what those numbers actually mean on the spec sheet.”

Daniel Hofstetter

DIY Homeowner, Seattle WA

May 5, 2026

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