Window AC Size Calculator

A window AC size calculator returns the BTU capacity a room needs, using ENERGY STAR's sizing table. It starts from the room's area and adjusts for sun exposure, occupants, and kitchen use. A 300 sq ft room with typical sun and two regular occupants needs 8,000 BTU. A very sunny 300 sq ft kitchen with four occupants needs 14,000 BTU.

Last reviewed September 2026Method & sources Report an error

Recommended BTU Capacity

8,000 BTU

Breakdown
Recommended BTU Capacity	8,000 BTU
Recommended Capacity (Tons)	0.667 tons
Base BTU (area only, before adjustments)	8,000 BTU

How to Calculate Window AC Size

The base BTU figure comes directly from ENERGY STAR's published room air conditioner sizing table, which maps room area to recommended capacity. That table is calculated for an 8 ft ceiling. ENERGY STAR's own guidance then adjusts the base figure. Reduce capacity by 10% for a heavily shaded room, and increase it by 10% for a very sunny room. Add 600 BTU per occupant beyond two. Add 4,000 BTU if the room is a kitchen, to account for extra heat from cooking appliances.

Example: 300 sq ft room, typical sun exposure, 2 regular occupants

A 300 sq ft room, typical sun exposure, 2 regular occupants, not a kitchen: 1. Base BTU (300 sq ft falls in the "300 up to 350 sq ft" bracket): 8,000 BTU 2. No exposure adjustment (typical) 3. No occupancy adjustment (2 occupants is the baseline) 4. No kitchen adjustment Recommended capacity: 8,000 BTU. The same room, very sunny, 4 regular occupants, and a kitchen instead: 8,000 × 1.10 (sunny) = 8,800, plus 2 × 600 (two extra occupants) = 10,000, plus 4,000 (kitchen) = 14,000 BTU. That is 1.75 times the base figure once every adjustment applies.

Window AC Size: Compare Scenarios

Scenario	Recommended BTU Capacity	Recommended Capacity (Tons)	Base BTU (area only, before adjustments)
Small room (150 sq ft)	6,000 BTU	0.500 tons	6,000 BTU
Medium room (300 sq ft)	8,000 BTU	0.667 tons	8,000 BTU
Large sunny kitchen (1,000 sq ft, Very Sunny, 5 occupants)	28,900 BTU	2.408 tons	21,000 BTU

What Is Window AC Size?

A window AC size calculator converts a room's area, sun exposure, occupancy, and kitchen status into a recommended BTU capacity. It follows ENERGY STAR's published consumer sizing guidance. It's a reference for window and portable units. Whole-house central air needs a full Manual J calculation.

Window AC Size: Pros and Cons

Pros

Directly sourced from ENERGY STAR's own published consumer sizing table and adjustment guidance, not an invented or approximated rule of thumb
Accounts for the specific factors ENERGY STAR itself flags as meaningfully changing the right size: shade or sun exposure, occupancy, and kitchen use
Reports the base, area-only BTU alongside the fully adjusted figure, so you can see exactly how much each factor changes the recommendation

Cons

This table is calculated for an 8 ft ceiling. A taller ceiling means more room volume to cool than the area-based table accounts for
Doesn't cover ductless mini-splits or central air sizing, which follow different sizing methodologies. For central air, use a proper Manual J calculation
An oversized unit cools too quickly without adequately removing humidity, leaving a room cold but clammy. Bigger isn't automatically better, even within this guidance

Window AC Size: What a Pro Checks

Size up if your ceiling runs well above 8 ft. ENERGY STAR's table is calculated for an 8 ft ceiling, and a taller room has more volume to cool than the area-based figure accounts for.
Don't assume bigger is always better. An oversized unit cools the room quickly but shuts off before it can adequately remove humidity, leaving the space cold but clammy.
Apply ENERGY STAR's own adjustments rather than the base area figure alone. Add 10% for a very sunny room, subtract 10% for heavy shade, add 600 BTU per occupant beyond two, and add 4,000 BTU flat for a kitchen.

Frequently Asked Questions

What size air conditioner do I need for a 300 square foot room?

About 8,000 BTU per ENERGY STAR's sizing table, before any adjustments for sun exposure, occupancy, or kitchen use. A very sunny 300 sq ft kitchen with several regular occupants could need considerably more. In a fuller example, that comes to 14,000 BTU.

Should I size up if the room gets a lot of sun?

Yes. ENERGY STAR specifically recommends increasing capacity by 10% for a very sunny room, since direct sunlight adds real heat load beyond what the base area-only figure accounts for. Conversely, a heavily shaded room can use 10% less capacity than the base figure.

Does the number of people in a room affect AC sizing?

Yes. Each person in a room adds body heat, and ENERGY STAR's guidance adds 600 BTU for each regular occupant beyond the first two. A room regularly used by a larger group needs more cooling capacity than the base area figure alone suggests.

Why does a kitchen need a bigger air conditioner?

Cooking appliances, such as stovetops and ovens, generate significant extra heat beyond what a typical room produces. That's why ENERGY STAR's guidance adds a flat 4,000 BTU for a kitchen on top of the area-based base figure.

What happens if I buy an air conditioner that's too big?

An oversized unit cools the room to the target temperature quickly, then shuts off before it has run long enough to adequately remove humidity. The room feels cold and clammy. It also cycles on and off more frequently, which wastes energy and adds wear.

How do I convert BTU to tons for an air conditioner?

Divide BTU/hr by 12,000. A 1.5-ton unit is rated at 18,000 BTU/hr. Window and portable room units are usually labeled in BTU, while central and split-system equipment is more commonly labeled in tons. This conversion comes up often when comparing the two.

Sources

ENERGY STAR Room Air Conditioners (BTU sizing table and adjustment guidance for shade, sun, occupancy, and kitchen use) (opens in a new tab)

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