Window Air Conditioner Size Calculator
Find the right BTU capacity for your window air conditioner by entering your room details below.
List of the Best Window AC:
How to Size a Window AC (BTU, EER, and Window Fit)
Window air conditioners are sized primarily by room area, but three things matter more than most shopping guides admit: the ratio of BTU to actual heat load (too small = never cools, too large = short cycles and leaves rooms clammy), the unit's EER (efficiency, not size), and whether it physically fits your window. This guide gives the sizing formula, worked examples, the EER context, and the practical window-compatibility rules that decide whether a given unit will work at all.
Window AC Sizing Formula (Rule of Thumb)
Cooling BTUs ≈ Area (sq ft) × 20 × adjustments
The 20 BTU/sq ft base applies to standard 8-ft ceilings, moderate sun, average insulation, and two occupants. Adjustments for sun, insulation, occupancy, and equipment change the result by 5–25%. The final BTUs should be matched to a standard window AC size — units typically come in 5,000, 6,000, 8,000, 10,000, 12,000, 15,000, 18,000, and 24,000 BTU/hr. Round up to the nearest size, then confirm whether the next size down would also work — oversizing is a bigger problem than undersizing for window ACs.
Worked Example
250 sq ft bedroom, 8-ft ceiling, high sun, average insulation, 2 occupants, one TV:
Base: 250 × 20 = 5,000 BTU High sun: 5,000 × 1.20 = 6,000 BTU Average insulation: 6,000 × 1.10 = 6,600 BTU Occupancy: 0 (only 2 people) TV: 6,600 + 1,000 = 7,600 BTU → Choose an 8,000 BTU window AC
A 10,000 BTU unit would also work but would short-cycle in moderate weather. The 8,000 BTU unit runs closer to a continuous cycle on hot days and dehumidifies better — which is the practical difference between a correct size and an oversized one.
Window AC BTU Sizing Chart
| Room area (sq ft) | Base BTU | Poor insulation (+20%) | High sun (+20%) | Common units |
|---|---|---|---|---|
| 100–150 | 5,000 | 6,000 | 6,000 | 5k–6k BTU |
| 150–250 | 6,000 | 7,200 | 7,200 | 6k–8k BTU |
| 250–350 | 8,000 | 9,600 | 9,600 | 8k–10k BTU |
| 350–450 | 10,000 | 12,000 | 12,000 | 10k–12k BTU |
| 450–550 | 12,000 | 14,400 | 14,400 | 12k–15k BTU |
| 550–700 | 14,000 | 16,800 | 16,800 | 14k–18k BTU |
| 700–1,000 | 18,000 | 21,600 | 21,600 | 18k–24k BTU |
| 1,000–1,200 | 24,000 | 28,800 | 28,800 | 24k BTU (largest single-unit) |
Values are rules of thumb for typical conditions. Add extra BTU for additional occupants (~600 BTU/person beyond two) and heat-generating equipment (1,000 BTU for a computer or TV; 3,000 BTU for an in-room kitchen). For rooms above ~1,200 sq ft, a single window AC struggles — consider two units, a mini split, or central AC. See the portable AC size calculator for the portable-AC alternative, or the mini split sizing calculator for permanent options.
Adjustment Multipliers
Sunlight Exposure
- Low (north / shaded): baseline
- Moderate (east / west): +10%
- High (south / direct sun): +20%
- Large unshaded glass: +20–25%
Insulation
- Good: baseline
- Average: +10%
- Poor / drafty: +20%
- Uninsulated attic room: +30%
Occupants
- Each person above two: +600 BTU (ASHRAE 55 baseline)
- High-activity rooms: +800–1,200 BTU per person
- Children / sleeping occupants: baseline
Equipment
- Each computer or monitor: +1,000 BTU
- TV / entertainment: +1,000 BTU
- Kitchen appliances (in-room): +3,000 BTU
- Heavy lighting: +1,000 BTU per kW
Why Oversizing Is Worse Than Undersizing
Both mistakes hurt, but for window ACs, the consequences differ in how you feel them:
- Undersized: The unit runs continuously on hot days, never quite reaching setpoint, uses more electricity, and wears out faster. It also tends to over-dehumidify relative to cooling, but the room still feels warm.
- Oversized: The unit cools the room quickly, then shuts off before removing humidity. The room feels cold and clammy — 65 °F but muggy. The compressor short-cycles, wearing out faster. Overall energy use may be similar or slightly higher, but comfort is clearly worse.
For most rooms, the correct size is the smaller standard size that can still handle the calculated load. Rounding up one size is safe; rounding up two is not.
EER and DOE Efficiency Standards
EER (Energy Efficiency Ratio) is the window-AC efficiency metric: BTU/hr of cooling output divided by watts of electrical input at a specific test condition. Higher is better. U.S. DOE room AC standards have tightened over time — current minimum EER for most room ACs is around 11.0 (varies by capacity class), and ENERGY STAR certified units exceed that. SEER (seasonal efficiency) applies mainly to central systems; window ACs are rated by EER (and, historically, CEER for combined efficiency).
When comparing units of the same capacity, a higher EER saves electricity every hour the unit runs. A 12,000 BTU unit at EER 11 draws about 1,090 W; the same 12,000 BTU unit at EER 13 draws about 920 W — a ~15% electricity savings over the season. For operating-cost estimates, see the power consumption calculator.
Window Fit and Installation
Window Types
- Double-hung: easiest fit, most common
- Slider: fits with side panels reversed
- Casement: requires a special bracket or a different AC type
- Awning: same issue as casement
- Bay / picture windows: not recommended
Physical Fit
- Measure opening width and height
- Verify the unit's min/max window width
- Check weight vs. window frame strength
- Confirm the unit will sit level
Electrical
- Window ACs typically 115 V (up to ~12k BTU) or 230 V (larger)
- Most 115 V units need a dedicated 15–20 A circuit
- Avoid extension cords
- See the circuit breaker calculator for sizing
Sealing
- Use supplied side panels and foam seals
- Weatherstrip gaps — leaks negate efficiency
- Angle slightly downward for condensate drainage
- Support the outdoor side with a bracket
Related Sizing Tools
- Portable AC size calculator — the portable alternative
- Mini split sizing calculator — permanent ductless
- AC size calculator — general room-level BTU estimate
- Power consumption calculator — operating cost
- Ceiling fan size calculator — complementary air movement
- Circuit breaker calculator — dedicated circuit sizing
Frequently Asked Questions (Window AC Sizing)
What happens if I choose a window AC that's too small?
An undersized unit will:
- Run continuously without reaching setpoint on hot days
- Use more electricity over the season (constant full load)
- Wear faster — compressor never cycles off
- Deliver uneven cooling, especially in long or multi-room spaces
Why is oversized worse than undersized for a window AC?
Oversized units cool quickly but cycle off before removing humidity, leaving the room cold and clammy. Compressor short-cycling wears the unit out faster. Undersized units at least attempt to keep up; oversized ones fail in a subtler, more uncomfortable way.
Can a window AC cool multiple rooms?
Possible but usually inefficient. The unit must be sized for the total area served, and cooling effectiveness drops with distance and with open doorways. For multiple rooms, two smaller units or a mini split are usually better. See the mini split sizing calculator.
How do I know if my window can support an AC?
Check these factors:
- Window dimensions (width and height) versus the unit's min/max
- Window type — double-hung and slider are easiest; casement/awning need adapters or a different AC type
- Structural support for the outdoor half (bracket or floor support)
- Proximity to a dedicated electrical outlet
- Whether the window sill will let the unit sit level with a slight backward angle for drainage
Should I size up or down if between BTU ratings?
Round down to the smaller standard size if the room is shaded and well-insulated, and round up if it is sunny, poorly insulated, or has large glass. In general, prefer the smaller unit — the risk of short cycling and clammy humidity from oversizing is real, while a correctly sized unit at the top of its range still dehumidifies well.
What EER should I look for?
Current U.S. DOE minimums are around EER 11.0 for most room ACs. ENERGY STAR units exceed that. Higher EER saves electricity every hour of operation — a jump from EER 11 to EER 13 reduces power draw by about 15%. For operating-cost context, see the power consumption calculator.
Is this calculator a substitute for a professional assessment?
No. It provides a reliable planning estimate. For final equipment selection, verify window fit and electrical circuit capacity, and for anything larger than a single-room installation, consult a licensed HVAC contractor. Related tools: portable AC calculator, AC size calculator, circuit breaker calculator.
Sources and references
- AHAM — Room Air Conditioner performance standards
- U.S. DOE — Room Air Conditioners Energy Conservation Standards
- ENERGY STAR — Room Air Conditioners Key Product Criteria
- ASHRAE — Standard 55: Thermal Environmental Conditions for Human Occupancy
- AHRI — unitary equipment rating standards