Mini-Split & Room AC Size (BTU)
Size a single room or ductless mini-split zone. This uses the EPA room-AC guideline of about 20 BTU per sq ft, then adjusts for sun, occupancy, a kitchen and ceiling height, and points you at the nearest standard head size.
Calculator
A 300 sq ft room needs about 6,000 BTU/h — pick the nearest standard mini-split / room-AC size (9k, 12k, 18k, 24k BTU). Right-size it: an oversized unit short-cycles and leaves the room clammy.
Whole-house rules of thumb over-size a single room, so room air conditioners and ductless mini-splits use a tighter guideline: the EPA’s 20 BTU per sq ft starting point. This tool applies that base and then the same physics-based adjustments — heavy sun, extra bodies, a kitchen and tall ceilings — to estimate the capacity a bedroom, home office, sunroom or garage conversion needs.
Mini-splits and window units come in standard sizes — 9,000, 12,000, 18,000 and 24,000 BTU (often written 9k, 12k, 18k, 24k). Once you have an estimate, pick the nearest standard head rather than rounding up two sizes: an oversized ductless head short-cycles and leaves the room humid, the same failure mode as an oversized central system.
Formula
Base load at 20 BTU/sq ft plus the standard adjustments:
BTU = area × 20\n × sun_factor (1.0 average, 1.10 heavy sun)\n × ceiling / 8 (only when ceiling > 8 ft)\n + 600 × max(0, occupants − 2)\n + kitchen_BTU (0 or 4,000)
The 20 BTU/sq ft figure is the labeled EPA room-AC guideline. See Sources.
Worked example
A 300 sq ft bedroom, average sun, two occupants, no kitchen, 8 ft ceiling:
base = 300 × 20 = 6,000 BTU/h\nsun × 1.0 = 6,000\npeople (2, no add) = 6,000 BTU/h
That points at a 9,000 BTU (9k) mini-split head — the smallest common size, which comfortably covers a 6,000 BTU load with margin for defrost and off-design days. A sunny 450 sq ft living room, by contrast, lands near 9,900 BTU and is a clean match for a 12k head.
Picking a standard head
Round to a real size, not up two. Standard heads are 9k / 12k / 18k / 24k BTU. Choose the nearest one at or just above your estimate. Jumping to the next size “to be safe” brings back the short-cycling and clammy-room problem that ductless systems are otherwise very good at avoiding, thanks to their inverter-driven modulation.
Refrigerant line-sets are a pro job. Mini-splits are refrigerant systems; charging and line-set work require EPA Section 608 certification. This tool estimates capacity only.
Reference table
| Standard head | Typical area | Common use |
|---|---|---|
| 9,000 BTU (9k) | 150–350 sq ft | bedroom, small office |
| 12,000 BTU (12k) | 350–550 sq ft | living room, large bedroom |
| 18,000 BTU (18k) | 550–850 sq ft | open living / kitchen |
| 24,000 BTU (24k) | 850–1,200 sq ft | great room, small home |
Typical ranges at 20 BTU/sq ft before adjustments — use your calculated BTU to pick the head.
Frequently asked questions
What size mini-split do I need for a 300 sq ft room?
About 6,000 BTU/h at the 20 BTU/sq ft guideline, which maps to the smallest standard 9,000 BTU head. Add for heavy sun, a kitchen or extra people.
Why is the room rule 20 BTU/sq ft, not the whole-house band?
Whole-house bands (20–35 BTU/sq ft) fold in distribution and diversity across many rooms. A single room uses the tighter EPA room-AC guideline of about 20 BTU/sq ft.
What are the standard mini-split sizes?
9,000, 12,000, 18,000 and 24,000 BTU (9k / 12k / 18k / 24k). Pick the nearest one at or just above your estimate rather than oversizing.
Can one outdoor unit run several heads?
Yes — a multi-zone condenser drives several indoor heads. Size each head to its room with this tool, then have an installer confirm the combined outdoor capacity and line-set layout.