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Window AC BTU Calculator: What Size Window Air Conditioner for Your Room

Enter your room dimensions, climate, and details below to find the right window air conditioner BTU. Window units are sold by BTU in standard sizes from 5,000 to 24,000 BTU, so we match your room's cooling load to the nearest available unit.

Window AC BTU Calculator

Feet
InchesSwitch between feet and inches measurement units
This form calculates the optimal air conditioner size for your room based on dimensions, climate, and various factors.

Room Dimensions

Room Dimensions Input Fields
ft
ft
ft

Location & Room Type

Location and Room Type Selection

Room Characteristics

Room Characteristics

Environmental Factors

Environmental Factors

Heat-Generating Appliances

Heat-Generating Appliances

Recommended Window AC Size

Room Area:144.00 sq ft
Room Volume:1440.00 cubic ft
Room Cooling Load:6,667 BTU

Recommended Window AC Size

8,000 BTU

8,000 BTU

What This Means

Based on your room's dimensions and conditions, we recommend a 8,000 BTU window air conditioner. This size will cool your room while maintaining proper humidity levels. The calculation is based on your local climate conditions and applies a combined adjustment factor of 1.79x.

This calculator uses the same room cooling-load engine as our main AC tonnage calculator, then matches it to standard window air conditioner sizes. Inspired by ASHRAE standards and ACCA Manual J.

What Size Window AC Do I Need?

To find the right window AC size, calculate your room's cooling load (room square footage × 20–25 BTU, adjusted for climate, ceiling height, sun, and occupancy) and then round up to the nearest standard window unit: 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 15,000, 18,000, or 24,000 BTU. A typical 250 sq ft bedroom needs about a 10,000–12,000 BTU window AC.

Window units realistically top out around 24,000 BTU (about 2 tons - see how many tons any BTU rating equals). If your room needs more than that, a split system or ductless mini split is the better choice. Use the calculator above for a number tailored to your climate zone.

Window AC BTU by Room Size (Quick Reference)General window unit sizing at 20–25 BTU per sq ft. Enter your climate zone above for an exact recommendation.
Room Size (sq ft)Window AC Size (BTU)Tonnage (approx.)
100 – 150 sq ft5,000 – 6,000 BTU0.5 Ton
150 – 250 sq ft8,000 – 10,000 BTU0.75 – 1 Ton
250 – 350 sq ft12,000 – 14,000 BTU1 – 1.25 Tons
350 – 550 sq ft15,000 – 24,000 BTU1.25 – 2 Tons

How Window AC Sizing Works

A window air conditioner cools a single room with no duct losses, so the sizing math is the same room cooling-load calculation used by our main air conditioner tonnage calculator. We start with your room's square footage, apply a baseline of about 22 BTU per square foot, then adjust for ceiling height, climate zone, sun exposure, insulation, occupants, and appliances.

The difference is the final step. Window units are sold in fixed BTU sizes, so we round your calculated load up to the nearest available capacity and report the result in BTU (the number printed on the box) rather than tonnage.

Want the full formula? See our methodology page. For a whole-house ducted system, use the split AC tonnage calculator, or for a ductless single-zone unit try the mini split size calculator.

Frequently Asked Questions

What size window AC do I need for my room?

Multiply your room's square footage by 20-25 BTU (use the higher end for hot climates, sunny rooms, or kitchens), then round up to the nearest standard window unit size: 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 15,000, 18,000, or 24,000 BTU. A typical 250 sq ft bedroom lands at 10,000-12,000 BTU. The calculator above does this automatically with your exact climate zone and room details.

How many BTUs do I need for a 12x12 room?

A 12x12 room is 144 square feet, which works out to roughly 3,200-3,600 BTU of cooling load. Since the smallest window units are 5,000 BTU, a 5,000 BTU window AC is the right choice - it's already comfortably above the load, so never size up further for a room this small unless it's a kitchen or gets intense afternoon sun.

What size window air conditioner do I need for 500 square feet?

A 500 sq ft space needs roughly 11,000-12,500 BTU of cooling in a moderate climate, so a 12,000 BTU window unit is the typical answer. In hot climates like Texas, Florida, or Arizona, or with high ceilings and sun exposure, step up to 14,000-15,000 BTU. Spaces this large also have to be fairly open for a single window unit to distribute air evenly.

Do window air conditioners have SEER ratings?

No. Window units use CEER (Combined Energy Efficiency Ratio) instead of the SEER rating used for central and split systems. CEER also accounts for standby power. A CEER of 11-12 is average for modern units, and Energy Star certified window ACs are roughly 10-15% more efficient than the federal minimum. The higher the CEER, the less electricity the unit uses for the same cooling.

Can a window AC be too big for a room?

Yes. An oversized window AC cools the air so fast that it shuts off before removing humidity, a problem called short cycling. The room ends up cold but clammy, and the constant on-off cycling wastes electricity and wears out the compressor. Match the BTU rating to your room's actual load rather than buying the biggest unit that fits the window.

How much does it cost to run a window air conditioner?

A 10,000 BTU window unit draws roughly 900 watts while running. At the US average of about $0.17/kWh, running it 8 hours a day costs around $1.20 per day, or $37 per month. A small 5,000 BTU unit costs about half that. Higher-CEER and Energy Star models trim 10-15% off these numbers.

What tonnage is a window air conditioner?

Window ACs span roughly 0.4 to 2 tons. Divide the BTU rating by 12,000 to get tons: a 5,000 BTU unit is about 0.4 tons, a 12,000 BTU unit is exactly 1 ton, and the largest 24,000 BTU window units are 2 tons. Anything above 2 tons is split-system or central AC territory.