Room AC BTU Calculator
The calculator uses the ENERGY STAR baseline of 20 BTU per square foot (for standard 8-foot ceilings), then applies official adjustment factors from the DOE and ENERGY STAR for sun exposure, ceiling height, occupancy, and room type. It outputs your recommended BTU, the closest standard AC size, estimated wattage, amperage, and approximate annual running cost.
Air Conditioner BTU Chart by Room Size
This is the chart you came for. It's based on the official ENERGY STAR room air conditioner sizing guidelines and assumes standard 8-foot ceilings with average sun exposure.
| Room Size (sq ft) | Recommended BTU | Common AC Sizes That Fit |
|---|
| 100–150 sq ft | 5,000 BTU | 5,000 BTU window unit |
| 150–250 sq ft | 6,000 BTU | 6,000 BTU window unit |
| 250–300 sq ft | 7,000 BTU | 8,000 BTU window unit |
| 300–350 sq ft | 8,000 BTU | 8,000 BTU window unit |
| 350–400 sq ft | 9,000 BTU | 10,000 BTU window unit |
| 400–450 sq ft | 10,000 BTU | 10,000 BTU window unit |
| 450–550 sq ft | 12,000 BTU | 12,000 BTU window unit |
| 550–700 sq ft | 14,000 BTU | 14,000 BTU window/through-wall |
| 700–1,000 sq ft | 18,000 BTU | 18,000 BTU or mini-split system |
| 1,000–1,200 sq ft | 21,000 BTU | Central AC system |
Source: ENERGY STAR Room Air Conditioner Sizing Chart (energystar.gov)
Note: This chart uses ASHRAE BTU ratings — the standard for window and through-wall units. If you're shopping for a portable AC, you need the SACC BTU rating instead, which we cover in the window AC vs portable AC section below. For a deep dive into these two standards, check out our BTU SACC vs ASHRAE explainer.
The standard rule of thumb — endorsed by both the DOE and ENERGY STAR — is simple:
BTU Needed = Room Square Footage × 20 BTU/sq ft
That's your starting point for rooms with 8-foot ceilings. A 200 sq ft room needs 4,000 BTU as a baseline; a 500 sq ft room needs 10,000 BTU.
But 20 BTU/sq ft is just the baseline. Real rooms almost always need adjustments for sun, ceiling height, occupancy, and room type — which is why ENERGY STAR publishes correction factors. We break those down in the adjustment factors section below.
BTU for Specific Room Types
Not every room is the same. A kitchen throws off more heat than a bedroom, and a sunroom absorbs more solar energy than a shaded north-facing office. Here's how to size for the most common room types.
How Many BTU for a 150 Sq Ft Bedroom?
A standard bedroom of 150 sq ft needs a 5,000 BTU air conditioner. This is the smallest common window AC size, and it's a perfect match for bedrooms up to 150 sq ft with standard 8-foot ceilings.
If the bedroom gets direct afternoon sun, bump your estimate to about 5,500 BTU — in practice, stick with a 5,000 BTU unit and make sure it has a solid EER or CEER rating. You only add 600 BTU per person above two occupants, so a couple sleeping in this room won't push past 5,000 BTU.
How Many BTU for a 300 Sq Ft Living Room?
A 300 sq ft living room needs 7,000–8,000 BTU. In practice, you'll buy an 8,000 BTU window AC, since 7,000 BTU models are uncommon on the market.
One important caveat: if the living room opens into a dining area or hallway with no door separating them, measure the entire connected space. An open 300 sq ft living room connected to a 100 sq ft dining nook is really 400 sq ft — and that bumps you up to 10,000 BTU.
How Many BTU for a 400 Sq Ft Master Suite?
A 400 sq ft master suite (bedroom plus ensuite bathroom) needs 9,000–10,000 BTU. If you have vaulted 10-foot ceilings, add 25% — now you're looking at roughly 12,000–12,500 BTU.
In most cases, a 10,000 BTU window AC handles this comfortably. That's enough cooling power to keep even a south-facing master suite cool. Once you've sized your unit, check our guide on the ideal AC temperature for comfort and savings.
How Many BTU for a 500 Sq Ft Open Plan Room?
Open floor plans are tricky because cool air disperses across the entire connected area. A 500 sq ft open plan space needs a minimum of 12,000 BTU.
If the space includes a kitchen, ENERGY STAR recommends adding 4,000 BTU on top. That brings your total to 16,000 BTU for a 500 sq ft kitchen/living combo — at that point, you're looking at a large window unit or, more practically, a ductless mini-split system.
How Many BTU to Cool a Garage?
Garages are the hardest spaces to cool. They have minimal insulation, large sun-exposed doors, and often no finished ceiling to retain conditioned air.
As a rule of thumb, multiply garage square footage by 30–40 BTU per sq ft instead of the standard 20. A 400 sq ft two-car garage needs 12,000–16,000 BTU at minimum.
An insulated garage with a sealed door can get by at the lower end. An uninsulated garage with west-facing exposure needs the upper end — or a dedicated mini-split.
What Size Room Will a 5,000 / 8,000 / 10,000 / 12,000 / 14,000 BTU AC Cool?
Here's the reverse lookup. If you already know the BTU size you're shopping for, this table tells you the maximum room size it can handle.
| AC Size (BTU) | Cools Rooms Up To | Best For |
|---|
| 5,000 BTU | 100–150 sq ft | Small bedroom, home office |
| 6,000 BTU | 150–250 sq ft | Standard bedroom, nursery |
| 8,000 BTU | 250–350 sq ft | Large bedroom, small living room |
| 10,000 BTU | 350–450 sq ft | Master bedroom, medium living room |
| 12,000 BTU | 450–550 sq ft | Large living room, open plan area |
| 14,000 BTU | 550–700 sq ft | Very large room, combined spaces |
| 18,000 BTU | 700–1,000 sq ft | Loft, large open plan, small apartment |
Source: ENERGY STAR sizing guidelines (energystar.gov)
What Size Room Will a 5,000 BTU Air Conditioner Cool?
A 5,000 BTU window AC cools rooms between 100 and 150 sq ft — roughly a 10×12 or 10×15 bedroom. This is the smallest and most affordable window AC size.
Most 5,000 BTU models run on about 450–550 watts and draw only 3.5–5 amps, so they'll work on any standard 15-amp household outlet without issues.
Is 5,000 BTU enough for a bedroom? Yes — as long as the bedroom is under 150 sq ft with standard ceilings. For larger bedrooms, step up to 6,000 or 8,000 BTU.
What Size Room Will an 8,000 BTU Air Conditioner Cool?
An 8,000 BTU air conditioner handles rooms between 250 and 350 sq ft. This covers most standard living rooms and larger bedrooms.
This is the most popular window AC size in the US — the sweet spot of cooling power, energy efficiency, and price. An 8,000 BTU unit typically uses 660–900 watts and draws 5.5–7.5 amps at 120V, running comfortably on a 15-amp dedicated circuit.
What Size Room Will a 10,000 BTU Air Conditioner Cool?
A 10,000 BTU air conditioner cools rooms between 350 and 450 sq ft. This size handles master bedrooms, large living rooms, and studio apartments.
Expect about 900–1,200 watts of power consumption and 7.5–10 amps draw at 120V. At this size, make sure the circuit is dedicated — nothing else on the same breaker — and a 20-amp breaker gives you extra headroom.
What Size Room Will a 12,000 BTU Air Conditioner Cool?
A 12,000 BTU air conditioner — also known as 1 ton of cooling — handles rooms between 450 and 550 sq ft. This is the largest common window AC size that still fits a standard 120V outlet.
Expect 1,060–1,500 watts and 8.8–12.5 amps at 120V. Most 12,000 BTU units need a dedicated 15-amp or 20-amp circuit.
After sizing, you can learn how to install a window AC properly to avoid common problems like leaking water or freezing up.
What Size Room Will a 14,000 BTU Air Conditioner Cool?
A 14,000 BTU air conditioner cools rooms between 550 and 700 sq ft. This is the upper end of room AC units — many 14,000 BTU models are through-the-wall units or large window units.
At this capacity, you're looking at 1,170–1,560 watts and 9.8–13 amps. Some 14,000+ BTU units require 230V power, so check the electrical requirements before buying.
How Many Watts Does an Air Conditioner Use?
Knowing your AC's wattage tells you two things: what size circuit you need, and how much it costs to run. The formula is straightforward:
Watts = BTU ÷ EER
The Energy Efficiency Ratio (EER) of most window ACs ranges from 9 to 12. A higher EER means fewer watts for the same cooling output — and lower electricity bills.
| AC Size (BTU) | Low Efficiency (EER 9) | Average (EER 10.2) | High Efficiency (EER 12) | ENERGY STAR Typical |
|---|
| 5,000 BTU | 556 W | 490 W | 417 W | ~450 W |
| 6,000 BTU | 667 W | 588 W | 500 W | ~540 W |
| 8,000 BTU | 889 W | 784 W | 667 W | ~720 W |
| 10,000 BTU | 1,111 W | 980 W | 833 W | ~900 W |
| 12,000 BTU | 1,333 W | 1,176 W | 1,000 W | ~1,080 W |
| 14,000 BTU | 1,556 W | 1,373 W | 1,167 W | ~1,260 W |
| 18,000 BTU | 2,000 W | 1,765 W | 1,500 W | ~1,620 W |
Sources: Calculated from BTU ÷ EER formula; ENERGY STAR certified product database
What does this mean for your electricity bill? At the national average rate of about $0.16 per kWh (per the U.S. Energy Information Administration), here's what each AC size costs to operate:
| AC Size (BTU) | ENERGY STAR Watts | kWh per 8-Hr Day | Monthly Cost (30 days) | Annual Cost (~1,000 hrs) |
|---|
| 5,000 BTU | ~450 W | 3.6 kWh | $17/mo | $72/yr |
| 8,000 BTU | ~720 W | 5.8 kWh | $28/mo | $115/yr |
| 10,000 BTU | ~900 W | 7.2 kWh | $35/mo | $144/yr |
| 12,000 BTU | ~1,080 W | 8.6 kWh | $41/mo | $173/yr |
| 14,000 BTU | ~1,260 W | 10.1 kWh | $48/mo | $202/yr |
| 18,000 BTU | ~1,620 W | 13.0 kWh | $62/mo | $259/yr |
Based on $0.16/kWh (EIA national average) and 8 hours of daily use during cooling season (~125 days)
For a full breakdown with your local electricity rate, see our cost to run an air conditioner calculator.
How Many Amps Does a Window Air Conditioner Use?
This matters for one critical reason: you need to know if your electrical circuit can safely handle your AC. Most household outlets in the US run on 15-amp or 20-amp breakers at 120V.
The National Electrical Code (NEC) requires that a continuously-running appliance like an AC not exceed 80% of the circuit's rated amperage. That means a 15-amp breaker safely supports up to 12 amps, and a 20-amp breaker handles up to 16 amps.
| AC Size (BTU) | Typical Running Amps | Max Amps (Low EER) | Min. Circuit Breaker | Plug Type |
|---|
| 5,000 BTU | 3.5–5.0 A | 5.0 A | 15 A | Standard 3-prong (NEMA 5-15P) |
| 6,000 BTU | 4.2–5.5 A | 6.0 A | 15 A | Standard 3-prong |
| 8,000 BTU | 5.5–7.5 A | 8.0 A | 15 A | Standard 3-prong |
| 10,000 BTU | 7.5–10.0 A | 10.5 A | 15 A (dedicated) | Standard 3-prong |
| 12,000 BTU | 8.8–12.5 A | 12.5 A | 15–20 A (dedicated) | Standard or NEMA 6-15P |
| 14,000 BTU | 9.8–13.0 A | 14.5 A | 20 A (dedicated) | NEMA 6-15P or 6-20P |
| 18,000 BTU (220V) | 7.5–9.0 A | 10.0 A | 15–20 A (240V) | NEMA 6-20P |
| 24,000 BTU (220V) | 8.3–11.1 A | 12.5 A | 20–30 A (240V) | NEMA 6-20P or 6-30P |
Sources: Manufacturer spec sheets; NEC circuit sizing guidelines; ENERGY STAR product database
Key takeaways for circuit sizing:
- 5,000–8,000 BTU units plug into any standard outlet on a 15-amp circuit. No special wiring needed.
- 10,000–12,000 BTU units need a dedicated 15-amp or 20-amp circuit (no other appliances on the same breaker).
- 14,000+ BTU units often require a 20-amp dedicated circuit or a 240V outlet. Always check the spec sheet.
If your AC keeps tripping the breaker or stops blowing cold air, an oversized amperage draw on an insufficient circuit is one of the most common causes.
Window AC vs Portable AC: BTU Sizing Differences
This is where most people get tripped up. A 12,000 BTU window AC and a 12,000 BTU portable AC do not deliver the same cooling — even if the number on the box looks identical.
The reason comes down to how BTU is measured. Window ACs are tested using the ASHRAE standard, which closely reflects real-world performance. Portable ACs also list an ASHRAE number, but since 2017 the Department of Energy (DOE) requires them to additionally display a SACC (Seasonally Adjusted Cooling Capacity) rating.
The SACC accounts for the warm air a single-hose portable AC pulls back into the room through its exhaust process. The SACC rating is always 25% to 45% lower than the ASHRAE number. For a detailed breakdown of why, see our SACC vs ASHRAE BTU guide.
| Portable AC (ASHRAE BTU) | Real-World Output (SACC BTU) | Equivalent Window AC | Effective Room Size |
|---|
| 8,000 BTU ASHRAE | 5,000–6,000 SACC | ≈ 5,000–6,000 BTU window | 150–250 sq ft |
| 10,000 BTU ASHRAE | 6,000–7,000 SACC | ≈ 6,000–7,000 BTU window | 250–350 sq ft |
| 12,000 BTU ASHRAE | 7,500–8,500 SACC | ≈ 7,000–8,000 BTU window | 300–400 sq ft |
| 14,000 BTU ASHRAE | 7,700–10,000 SACC | ≈ 8,000–10,000 BTU window | 350–450 sq ft |
Sources: DOE 10 CFR Parts 429 and 430 (2017); Sylvane (sylvane.com); PickHVAC (pickhvac.com)
The bottom line: if you choose a portable AC, size using the SACC rating, not the ASHRAE number on the box. A "14,000 BTU" portable delivers roughly the same real cooling as an 8,000–10,000 BTU window unit.
No portable ACs currently carry ENERGY STAR certification due to their lower efficiency. Whenever possible, a window AC gives you more cooling per dollar and per watt. Portables make sense for renters, rooms with casement windows, or spaces where window mounting isn't feasible.
ENERGY STAR Adjustment Factors for Room AC Sizing
The base 20 BTU/sq ft rule only applies to a "standard" room — 8-foot ceilings, average sun, two or fewer occupants, no kitchen. Real rooms are rarely this simple.
Here are the official correction factors published by ENERGY STAR and the DOE:
| Condition | BTU Adjustment | Example |
|---|
| Heavy shade (north-facing, trees) | −10% | 10,000 BTU → 9,000 BTU |
| Heavy sun (south/west-facing, lots of glass) | +10% | 10,000 BTU → 11,000 BTU |
| Each additional person above 2 | +600 BTU per person | Family of 4 in living room → +1,200 BTU |
| Room includes kitchen | +4,000 BTU | 300 sq ft kitchen: 8,000 + 4,000 = 12,000 BTU |
| 9-foot ceilings (vs. 8 ft standard) | +12.5% | 10,000 BTU → 11,250 BTU |
| 10-foot ceilings | +25% | 10,000 BTU → 12,500 BTU |
| Poor insulation or old single-pane windows | +10% to +20% | 10,000 BTU → 11,000–12,000 BTU |
| Top-floor apartment (room below roof) | +10% | 10,000 BTU → 11,000 BTU |
Sources: ENERGY STAR (energystar.gov); DOE (energy.gov/energysaver/room-air-conditioners)
How to use this table: start with the BTU from the sizing chart above, then stack all applicable adjustments. If multiple percentage factors apply, add all percentages together before multiplying. We demonstrate exactly how in the worked examples below.
A properly-sized AC removes both heat and moisture from the air. An oversized unit short-cycles and leaves excess humidity behind — which makes the room feel clammy even at a cool temperature. To understand more about this relationship, check our guide on whether AC removes humidity.
Worked Examples
Example 1: Small Bedroom in a Phoenix Apartment
Room specs: 12 ft × 12 ft = 144 sq ft, 8-foot ceilings, south-facing windows (sunny), 1 occupant, no kitchen.
Base BTU = 144 sq ft × 20 BTU/sq ft = 2,880 BTU
Adjustments: +10% for sunny room = +288 BTU.
Total = 3,168 BTU. The closest standard AC size is 5,000 BTU. That leaves comfortable headroom for Phoenix heat. Estimated wattage: ~450 W. Estimated amps: ~3.8 A. Plugs into any standard outlet.
Example 2: Living Room for a Family of 4 in Chicago
Room specs: 15 ft × 20 ft = 300 sq ft, 8-foot ceilings, average sun, 4 people regularly in room, no kitchen.
Base BTU = 300 sq ft × 20 BTU/sq ft = 6,000 BTU
Adjustments: +1,200 BTU for 2 extra occupants (4 people − 2 base = 2 × 600 BTU).
Total = 7,200 BTU. Recommended AC: 8,000 BTU window unit. Estimated wattage: ~720 W. Estimated amps: ~6.5 A. Runs on a standard 15-amp circuit.
Room specs: 20 ft × 20 ft = 400 sq ft, 9-foot ceilings, west-facing with large windows (very sunny), 2 occupants, no kitchen.
Base BTU = 400 sq ft × 20 BTU/sq ft = 8,000 BTU
Adjustments: +12.5% for 9-foot ceilings = +1,000 BTU. +10% for sunny room = +800 BTU.
Total = 9,800 BTU. Recommended AC: 10,000 BTU window unit. Estimated wattage: ~900 W. Estimated amps: ~8.2 A. Use a dedicated 15-amp circuit.
Example 4: Open-Plan Kitchen/Living Combo in Miami
Room specs: 500 sq ft open plan, 8-foot ceilings, average sun, includes kitchen, 3 people regularly present.
Base BTU = 500 sq ft × 20 BTU/sq ft = 10,000 BTU
Adjustments: +4,000 BTU for kitchen. +600 BTU for 1 extra person above 2.
Total = 14,600 BTU. Recommended AC: 14,000 BTU window or through-wall unit — or consider a ductless mini-split for better efficiency at this load. Estimated wattage: ~1,300 W. Estimated amps: ~11.8 A. Requires a dedicated 20-amp circuit.
Example 5: Uninsulated Garage Workshop in Atlanta
Room specs: 20 ft × 20 ft = 400 sq ft, 10-foot ceilings, minimal insulation, west-facing garage door (sunny), 1 occupant.
For garages, start with 30 BTU per sq ft instead of 20 due to poor insulation and thermal gain.
Base BTU = 400 sq ft × 30 BTU/sq ft = 12,000 BTU
Adjustments: +25% for 10-foot ceilings = +3,000 BTU. +10% for sunny exposure = +1,200 BTU.
Total = 16,200 BTU. Recommended AC: 18,000 BTU window unit or a ductless mini-split. Garages are demanding cooling environments — don't undersize. For running cost estimates, see our cost to run an air conditioner calculator.
Example 6: Using a Portable AC for a Shaded 250 Sq Ft Bedroom in Seattle
Room specs: 250 sq ft, 8-foot ceilings, heavy shade (north-facing with trees), 1 occupant, single-hose portable AC.
If using a window AC:
Base BTU = 250 sq ft × 20 BTU/sq ft = 5,000 BTU. Adjustments: −10% for heavy shade = −500 BTU.
Window AC need = 4,500 BTU → buy a 5,000 BTU window unit.
But with a single-hose portable AC: Portable ACs lose 25–45% of their ASHRAE-rated capacity due to negative pressure. A "10,000 BTU ASHRAE" portable delivers roughly 6,000–7,000 SACC BTU — which covers this room with margin.
Recommended portable: A 10,000 BTU ASHRAE (~6,000 SACC) portable unit. If a window mount is an option, the 5,000 BTU window AC does the same job at about half the electricity cost.
Frequently Asked Questions
How Many BTU Do I Need for a 12×12 Room?
A 12×12 room is 144 sq ft. At the standard 20 BTU per sq ft, that's 2,880 BTU. The closest standard AC size is 5,000 BTU, which gives you plenty of headroom and will cool this room quickly.
Is 5,000 BTU Enough for a Bedroom?
Yes — if the bedroom is under 150 sq ft. A 5,000 BTU air conditioner covers 100–150 sq ft per ENERGY STAR guidelines. For bedrooms larger than 150 sq ft, step up to 6,000 or 8,000 BTU.
You can also use a properly-sized ceiling fan to circulate cool air and make a smaller AC feel more effective.
What Is the Difference Between SACC and ASHRAE BTU?
ASHRAE tests air conditioners under controlled lab conditions (80°F, 51% humidity). SACC (Seasonally Adjusted Cooling Capacity) — introduced by the DOE in 2017 — tests under real-world conditions and accounts for warm-air infiltration caused by portable AC exhaust.
SACC ratings are always 25–45% lower than ASHRAE for portable ACs. Window ACs are barely affected (less than 10% difference).
Always size portable ACs by the SACC number. Full SACC vs ASHRAE breakdown here.
The standard is 20 BTU per square foot for rooms with 8-foot ceilings, per ENERGY STAR and DOE guidelines. Add 10% for sunny rooms, subtract 10% for shaded rooms, add 4,000 BTU for kitchens, and add 600 BTU per person above 2 occupants. For ceilings above 8 feet, increase proportionally (~12.5% per additional foot of height).
How Many Watts Does a 10,000 BTU Air Conditioner Use?
A 10,000 BTU window AC typically uses 900–1,200 watts, depending on its EER rating. An ENERGY STAR-certified model averages about 900 watts.
At $0.16/kWh, that's roughly $1.15 per day running 8 hours, or about $35 per month during peak summer.
Can I Run a Window AC on a 15-Amp Circuit?
Yes, for most sizes up to 12,000 BTU. A 5,000 BTU unit draws only 3.5–5 amps, and even a 12,000 BTU unit draws 8.8–12.5 amps — within the NEC 80% rule for a 15-amp breaker (12 amps max continuous load).
The circuit should be dedicated for units above 8,000 BTU. For 14,000+ BTU units, a 20-amp or 240V dedicated circuit is typically required.