What is the difference between a 5,000 and 6,000 BTU window AC?

About 50–100 sq ft of coverage. If your calculation lands between two standard sizes, round up to the next size — especially if the room gets afternoon sun or has high ceilings. The energy cost difference between adjacent BTU sizes is small.

Window AC Size Calculator: BTU by Room Size (2026)

Q: Can one window AC cool multiple rooms?

Only if the rooms are fully open to each other (open-plan layout). For rooms separated by walls or doors, size each space independently. A single unit trying to cool connected rooms must be sized for the combined square footage.

Q: When should I use a mini-split instead of a window AC?

Consider a ductless mini-split when you need more than 14,000–18,000 BTU, have casement windows that won't fit a standard window unit, want a permanent quiet installation, or are cooling a space without any operable windows (like a finished basement or garage).

How to Size a Window Air Conditioner

The most common mistake people make when buying a window AC is going too big. An oversized unit cools the air fast but shuts off before it can remove humidity — leaving the room cold and clammy. The right size is the one that runs in longer cycles, dehumidifying as it cools. This calculator gives you the Energy Star-recommended BTU for your specific room.

The BTU Formula

BTU (British Thermal Unit) is the measure of an air conditioner's cooling power. The baseline rule from Energy Star and ACCA (Air Conditioning Contractors of America) is 20 BTU per square foot of floor area, with four key adjustments:

Sun exposure: A heavily shaded room needs 10% fewer BTUs. A room with a south-facing wall or large unshaded windows needs 10% more.
Kitchen heat load: Cooking appliances, stovetops, and ovens add significant heat. Add 4,000 BTU for any kitchen or combined kitchen space.
Occupancy: Each person adds roughly 600 BTU/hr of body heat. Add 600 BTU for every person regularly in the room beyond the first two.
Ceiling height: Standard 8-foot ceilings are the baseline. Rooms with 9 or 10-foot ceilings have proportionally more air volume — multiply by ceiling height ÷ 8.
Climate: In hot, humid climates (Florida, Gulf Coast, Arizona) where outdoor temperatures regularly exceed 95°F, add 10% to account for the greater heat infiltration.

Standard BTU Size Chart

Window AC units come in standard BTU increments. Once you calculate your needed BTU, round up to the next available size:

Room Size (sq ft)	Recommended BTU	Typical Use Case
Up to 150 sq ft	5,000 BTU	Small bedroom, office
150–250 sq ft	6,000 BTU	Master bedroom, medium bedroom
250–350 sq ft	8,000 BTU	Large bedroom, studio apartment
350–450 sq ft	10,000 BTU	Living room, open studio
450–550 sq ft	12,000 BTU	Large living room, open-plan space
550–700 sq ft	14,000 BTU	Great room, large combined space
700–1,000 sq ft	18,000 BTU	Large open plan, small apartment
1,000+ sq ft	24,000 BTU or mini-split	Large apartment or consider multi-zone

Why Oversizing Is Worse Than You Think

A 12,000 BTU unit in a 200 sq ft room doesn't cool faster — it short-cycles. The compressor runs for a few minutes, drops the air temperature to set point, then shuts off. Because the cycle is short, the evaporator coil never gets cold enough to pull significant moisture from the air. You end up with a room that's at 72°F but feels like 78°F because the relative humidity is high. Properly-sized ACs run for 15–20 minute cycles, dehumidifying continuously. Match the unit to the room.

Window Type and Installation Considerations

Most window ACs require a double-hung window (slides up). Casement windows (swing out on a hinge) require a special casement-style unit or a portable AC. Before buying, measure your window's width and height — units typically require a minimum opening of 13–15 inches high and come with accordion side panels to fill the remaining width (up to about 36 inches per side panel; wider openings need an additional extension kit).

Check the outlet: most units under 14,000 BTU run on standard 115V/15A household outlets. Units 14,000 BTU and above often require a 220–240V dedicated circuit. Confirm the outlet type in your window before ordering.

Room Length (feet)

Room Width (feet)

Ceiling Height

Sun Exposure

People Regularly in Room Add 600 BTU per person over 2

Climate

This space includes a kitchen or cooking area (+4,000 BTU)

Recommended AC Size

Enter room dimensions above to see results

Pro Tips

Round up, not down — if your BTU lands between two standard sizes, go to the larger one
Check your outlet — units 14,000+ BTU often need a 220V dedicated circuit; confirm before ordering
Measure your window opening before buying — width, height, and whether it's double-hung or casement
Look for an Energy Star label — a certified 6,000 BTU unit uses 10–15% less electricity than a standard model
Clean or replace the filter monthly during cooling season — a clogged filter reduces efficiency by 5–15%
If you need to cool more than two rooms, consider a portable AC or mini-split system instead

Window AC Units by BTU Range

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5,000 BTU Window Air Conditioners — For rooms up to 150 sq ft. Look for quiet operation (under 50 dB) and a mechanical or electronic thermostat. Frigidaire and LG dominate this tier.
6,000 BTU Window Air Conditioners — The sweet spot for average bedrooms (150–250 sq ft). Most 6K units fit standard double-hung windows and plug into a 115V outlet.
8,000 BTU Window Air Conditioners — Best for large bedrooms and studio apartments (250–350 sq ft). Check for dehumidifier mode — useful in humid climates even when full cooling isn't needed.
10,000 BTU Window Air Conditioners — For living rooms and larger open spaces (350–450 sq ft). Many 10K units include a remote control, 24-hour timer, and sleep mode.
12,000 BTU Window Air Conditioners — Covers 450–550 sq ft. At this size, look for units with a slide-out chassis for easier filter cleaning and installation. Midea, LG, and Frigidaire all make reliable 12K units.
Ductless Mini-Split AC Systems — For spaces needing 14,000+ BTU, no suitable window, or if you want permanent quiet installation. Mini-splits require professional electrical and refrigerant work but are significantly more efficient long-term.

Frequently Asked Questions

Q: How many BTUs do I need per square foot?
A: The standard rule is 20 BTU per square foot of floor area. A 200 sq ft bedroom needs about 4,000 BTU base, rounded up to a 6,000 BTU unit. Adjust upward 10% for sunny rooms, add 4,000 BTU for kitchens, and 600 BTU per occupant beyond the second person.

Q: What happens if I buy a window AC that's too large?
A: An oversized AC short-cycles — it cools the air quickly then shuts off before running long enough to dehumidify. The result is a room that feels cold and clammy. Bigger is not better with air conditioners — match the unit to the room.

Q: What's the difference between a 5,000 and 6,000 BTU window AC?
A: About 50–100 sq ft of effective coverage. If your BTU calculation lands between two standard sizes, round up to the next — especially if the room gets afternoon sun or has high ceilings. The energy cost difference between adjacent sizes is small.

Q: Can one window AC cool multiple rooms?
A: Only if the rooms are fully open to each other. For spaces separated by walls or doors, size each independently and add the square footages for the total load if the air flows freely between them.

Q: When should I use a mini-split instead of a window AC?
A: Consider a ductless mini-split when you need 14,000+ BTU, have casement windows that won't fit a standard unit, want a permanently quiet installation, or are cooling a space without operable windows (garage, finished basement). Mini-splits require professional installation but are more efficient long-term.