For a typical 300 sq ft room, plan on 7,000 to 8,000 BTU, with the final number driven by sun, ceiling height, occupants, and kitchen use.
Pick a 7,000 BTU unit for a shaded 300 sq ft bedroom, and that same unit will struggle in a west-facing kitchen with three people in it. Square footage gives you the starting point; everything else adjusts the number up. Here is the sizing math, the adjustment factors, and the mistakes that leave people with a unit that runs all day and never catches up.
What Size AC Do You Need for a 300 Sq Ft Room?
A 300 sq ft room lands at 7,000 to 8,000 BTU on the sizing charts most manufacturers publish, with 8,000 BTU the safer pick if the room gets strong afternoon sun.
Frigidaire’s room air conditioner size guide lists 7,000 BTU as the match for up to 300 sq ft. Other sizing charts group 300 to 350 sq ft with 8,000 BTU. Those two answers are not a contradiction — they reflect different assumptions about how hard the room works the unit.
Bryant’s sizing guidance runs higher still, putting a 300 to 400 sq ft room at roughly 8,000 to 10,000 BTU, and attributes the spread to climate zone, insulation, and sun exposure. TCL’s 2025 guidance sits on the lower end, grouping 150 to 300 sq ft as a medium room at about 6,000 to 7,000 BTU and 300 to 500 sq ft as a large room at 7,000 to 12,000 BTU.
Read together, those ranges point the same direction: 7,000 BTU is the floor for a typical 300 sq ft room, and 8,000 BTU is the comfortable middle.
Start With Square Footage, Then Adjust For Reality
Measuring the room is the easy part: multiply length by width to get square footage, then match that number to a capacity chart. The adjustment step is where the sizing either works or fails.
The most common shortcut is 20 BTU per square foot, which would put a 300 sq ft room at 6,000 BTU. Treat that as a floor, not a target — it assumes a well-insulated, shaded space with two occupants, and it will undersize anything harder-working than that.
Bump the capacity up when any of these apply:
- Strong sun exposure. West- and south-facing windows add meaningful load through the afternoon.
- Higher ceilings. More cubic feet means more air to cool, even at the same square footage.
- More occupants. Recent guides suggest adding about 600 BTU for each person beyond two.
- Kitchen use. One guide recommends adding 4,000 BTU for a kitchen, since cooking appliances throw off real heat.
- Hot climate. Bryant breaks sizing into climate bands of roughly 20 to 30 BTU per sq ft in cool climates, 30 to 40 in mild climates, and 40 to 60 in hot climates.
Stack a few of those together and a 300 sq ft room can legitimately need 10,000 BTU or more.
Window vs. Portable: The Capacity Numbers Are Not The Same
Portable and window air conditioners may advertise capacity using different metrics, so compare like with like before buying.
Portable units are often rated in SACC BTU, and one guide pairs 7,000 SACC BTU with rooms up to 300 sq ft for portable models. Window units typically use the standard room-size charts. A portable rated 8,000 BTU and a window unit rated 8,000 BTU will not cool a room identically.
One more check before you commit: confirm the unit matches both the room size and your installation type. Frigidaire frames its chart specifically as room air conditioner sizing, not a central-system calculation, and the same distinction matters when you compare a single-room unit against whole-home HVAC tonnage. They are different math.
Getting this wrong in either direction costs you. Undersize the unit and it runs constantly without reaching the set temperature. Oversize it and it cools in short bursts, cycling on and off before it removes the humidity that makes a room feel clammy.
If you’d rather skip the spec-hunting, this roundup of tested ACs for a 300 sq ft room matches real units to this exact size range. Bryant’s air conditioner sizing guidance lays out how climate zone and insulation shift the final number.
| Room Scenario | Suggested Capacity | Why It Shifts |
|---|---|---|
| Shaded bedroom, average ceiling | 7,000 BTU | Matches Frigidaire’s chart for up to 300 sq ft |
| Typical 300 sq ft room, mild climate | 8,000 BTU | Maps to 300–350 sq ft on common charts |
| West-facing room, strong afternoon sun | 9,000–10,000 BTU | Solar gain through glass adds real load |
| Kitchen with cooking appliances | Add about 4,000 BTU | Stoves and ovens produce direct heat |
| Three or more occupants | Add about 600 BTU each beyond two people | Bodies add heat and humidity |
| Hot climate, 300 sq ft | 9,000–12,000 BTU | Bryant’s 40–60 BTU/sq ft band for hot zones |
| Portable unit, 300 sq ft room | 7,000 SACC BTU | Portable ratings use SACC, not standard BTU |
References & Sources
- Frigidaire. “Room Air Conditioner Size Guide” Lists 7,000 BTU for rooms up to 300 sq ft and the length × width sizing method.
- TCL. “How Many BTUs Do You Need” Groups 150–300 sq ft at 6,000–7,000 BTU and 300–500 sq ft at 7,000–12,000 BTU.
- Bryant. “Air Conditioner Sizing” States a 300–400 sq ft room typically needs 8,000–10,000 BTU and breaks sizing into climate bands.
