Home Window AC Size Calculator
Window AC Size Calculator — BTU by Room Size
Find the right window air conditioner with our most detailed calculator. 12 inputs including window fit check, voltage validation, energy cost estimate, and noise guidance.
Updated for 2026 room-AC sizing, CEER guidance, and current window AC models.
On this page: Calculator · BTU chart · Voltage guide · BTU & CEER ratings · Noise levels · Installation · Alternatives · FAQ
Need a portable instead? Try the portable AC calculator. For whole-home cooling, use the AC size calculator or mini split calculator.
Quick sizing rule
20 BTU per square foot for average conditions. A 150 sq ft bedroom → 5,000 BTU. A 350 sq ft living room → 8,000 BTU. A 550 sq ft open plan → 12,000 BTU. Adjust up for sunny rooms, kitchens, or poor insulation. Use the calculator below for your exact room.
This calculator sizes for cooling only. Window ACs do not provide heating. For a deep-dive room analysis, use the BTU calculator. For heating needs, try the space heater calculator.
Window AC BTU chart by room size
Standard window AC sizes and the rooms they fit. Based on average insulation, moderate sun, Zone 4. Adjust up 10–20% for hot climates, kitchens, or poor insulation.
| Room (sq ft) | BTU needed | Unit size | Voltage | Typical rooms |
|---|---|---|---|---|
| 100–200 | 5,000 | 5,000 BTU | 115V | Small bedroom, home office |
| 200–350 | 6,000–8,000 | 8,000 BTU | 115V | Bedroom, large office, studio |
| 350–450 | 8,000–10,000 | 10,000 BTU | 115V | Master bedroom, small living room |
| 450–600 | 10,000–12,000 | 12,000 BTU | 115V or 230V | Living room, open kitchen |
| 600–800 | 12,000–15,000 | 15,000 BTU | 230V | Large living room, open plan |
| 800+ | 15,000+ | Consider mini split, central AC, or get HVAC quotes | 230V | Very large rooms, or split into 2 units |
115V vs 230V window air conditioners
Your available electrical circuit determines the maximum BTU you can install without calling an electrician. Getting this wrong is one of the most common window AC buying mistakes.
| Circuit type | Outlet looks like | Max BTU (typical) | Common locations |
|---|---|---|---|
| 115V / 15A | Standard 3-prong | Up to ~12,000 BTU | Every room (most common US outlet) |
| 115V / 20A | 3-prong with one T-slot | Up to ~14,000 BTU | Kitchens, bathrooms, some bedrooms |
| 230V / 20A | Large 3-prong or 4-prong | 15,000–25,000 BTU | Laundry room, dedicated AC outlet |
If you need more than 12,000 BTU but only have standard 115V outlets, consider a portable AC (some 14,000 BTU models run on 115V), a ductless mini split (which has its own dedicated circuit installed during setup — shop mini splits), or running two smaller window units on separate circuits.
Window AC ratings: cooling capacity and CEER
Window air conditioners do not use DOE SACC. SACC is a federal rating for portable air conditioners. For window/room ACs, DOE testing reports cooling capacity in BTU/h, and federal efficiency standards are expressed using CEER (Combined Energy Efficiency Ratio).
| Window AC specification | What it tells you | How to use it |
|---|---|---|
| Cooling capacity (BTU/h) | How much cooling the room AC is rated to deliver | Match this to your calculated room requirement |
| CEER (Btu/Wh) | Combined energy-efficiency ratio under the DOE room-AC test procedure | When comparing similar sizes, a higher CEER means better efficiency |
| Annual energy use | Estimated electricity consumption under standardized assumptions | Useful for comparing operating cost between models |
| Dimensions and voltage | Physical opening and electrical requirements | Verify against your actual window and circuit before buying |
If a product page lists SACC beside a larger ASHRAE/marketing BTU figure, you are generally looking at a portable AC specification rather than the rating system used for a window room air conditioner.
References: DOE room air conditioner guidance · ENERGY STAR room air conditioners · portable AC ASHRAE vs SACC explainer.
Window AC noise levels
All window ACs make some noise. The compressor cycling on and off is the loudest part. Here's how common noise levels compare:
| Decibels (dB) | Comparable to | Suitable for |
|---|---|---|
| 40–44 dB | Library, quiet whisper | Bedroom, nursery, studio |
| 44–50 dB | Quiet office, light rain | Home office, living room |
| 50–55 dB | Normal conversation | Living room, kitchen, den |
| 55–60 dB | Dishwasher, running faucet | Garage, workshop, utility room |
Larger units (12,000+ BTU) are generally louder than small units. Features that reduce perceived noise: variable-speed compressors, sleep mode (drops to lowest fan + raises temp 1°F/hour), and inverter technology. Expect to pay $30–$80 more for a "quiet" labeled unit.
For the quietest possible cooling, a ductless mini split runs at 19–32 dB indoors — significantly quieter than any window unit because the compressor sits outside.
Installation tips
Window types that work
Standard window ACs are designed for double-hung windows (the lower sash slides up). Most units fit windows 23–36 inches wide using adjustable side panels. Sliding (horizontal) windows require a different mounting kit or a specially designed vertical unit. Casement (crank-out) windows are generally not compatible — consider a portable AC instead.
Sealing and weatherstripping
A poorly sealed window AC can waste 10–20% of its cooling capacity through air gaps. Most units include foam strips, but aftermarket weatherstripping and window insulation panels improve the seal significantly. Fill the accordion side panels with foam board for an even better seal.
Support brackets
Units over 50 lbs should have a support bracket bolted to the exterior wall — don't rely on the window sash alone. This also reduces vibration noise. Brackets cost $15–$30 and take 20 minutes to install.
Drip management
Window ACs produce condensate that should drip outside, away from the wall. Tilt the unit slightly backward (1/4 inch per foot is standard) so water drains to the exterior. Do not block the drain holes.
Reduce your room's cooling load
Small improvements can let you buy a smaller, cheaper, quieter window AC:
Window film / reflective insulation ($10–30) — blocks solar heat through south/west windows. Weatherstripping ($5–15) — seal gaps that let hot air in. Outlet gaskets ($5) — stop air infiltration on exterior walls. Thermal leak detector ($25–40) — find exactly where heat enters your room.
Dropping from an 8,000 to a 5,000 BTU unit saves $50–$150 on the unit and uses 30% less electricity. A ceiling fan makes a room feel 3–4°F cooler, often letting you set the AC 2–3 degrees higher.
Add smart scheduling to any window AC with a smart AC controller ($25–60) — auto shut-off when you leave can save 10–20%.
When to choose something other than a window AC
| Situation | Better option | Why |
|---|---|---|
| Casement / crank-out windows | Portable AC | Doesn't require a window sash opening |
| Need heating too | Mini split or heat pump | Heat pump provides year-round heating + cooling |
| HOA or building won't allow | Portable AC | No visible exterior unit |
| Room over 800 sq ft | Mini split or central AC | Window ACs max out at ~15,000 BTU |
| Bedroom — noise is critical | Mini split | Compressor is outside (19–32 dB indoors) |
| Renter, need portability | Portable AC | Rolls between rooms, no installation |
Frequently asked questions
What size window AC do I need for a 300 sq ft room?
A 300 sq ft room with average insulation typically needs 7,000–8,000 BTU. An 8,000 BTU window AC is the standard recommendation. For sunny or poorly insulated rooms, consider a 10,000 BTU unit. For shaded rooms with good insulation, a 5,000–6,000 BTU unit may suffice. Use the calculator above for your exact conditions.
Do I need a 115V or 230V window AC?
Units up to about 12,000–14,000 BTU typically run on standard 115V household outlets. Above that, most require a dedicated 230V circuit. A 230V unit physically cannot plug into a standard outlet without an electrician installing a new circuit ($150–$400). The calculator checks your voltage selection against the recommended BTU and warns if there's a mismatch.
Do window air conditioners use SACC?
No. DOE SACC (Seasonally Adjusted Cooling Capacity) applies to portable air conditioners, not window room ACs. For a window AC, compare the unit's cooling capacity in BTU/h and its CEER efficiency rating, then verify window dimensions and voltage. See the BTU and CEER explanation above or the portable AC SACC guide if you are comparing a portable unit.
How much electricity does a window AC use?
A 5,000 BTU unit uses roughly 450–500 watts. An 8,000 BTU unit uses 660–750 watts. A 12,000 BTU unit uses 1,000–1,200 watts. Running 8 hours per day at $0.16/kWh, an 8,000 BTU unit costs roughly $25–$35 per month. Energy Star units use 10–15% less. The calculator above estimates your monthly cost based on climate zone usage patterns.
How loud is a window AC?
Typical range is 50–60 dB on high fan speed. "Quiet" models run at 42–48 dB. For bedrooms, look for sleep mode (drops to lowest fan speed and auto-adjusts temperature). See the noise level guide for comparisons. For the quietest cooling possible, a mini split moves the compressor outside and runs at 19–32 dB indoors.
Will a window AC fit my window?
Most window ACs need a double-hung window at least 23 inches wide and 13 inches tall. Units over 12,000 BTU may need 26–28+ inches of width and 16+ inches of height. The calculator above checks your window dimensions against typical unit sizes and warns if the fit is tight. For casement or sliding windows, try the portable AC calculator.
Can I cool two rooms with one window AC?
A window AC is most effective in the room where it's installed. It can partially cool an adjacent room through an open doorway, but expect a 30–50% temperature drop-off. For two rooms, a better approach is two smaller units, or a multi-zone mini split with an indoor head in each room. Use the BTU calculator to size each room separately.
Smart window AC controllers
Most window ACs use basic controls, but a smart AC controller or smart plug can add scheduling, temperature triggers, and app control. A smart AC controller works with any AC that has a remote, using infrared signals. Save 10–20% on electricity by auto-shutting off when you leave. Estimate savings with the energy cost calculator.