What Size Mini-Split Do I Need for One Room?
Updated March 4, 2026
Just want the number? Use the AC and mini-split sizing calculator.
For a single room, size a mini-split at roughly 20 BTU per hour of cooling per square foot of floor space. A 400 square foot bedroom comes in around 8,000 BTU, which rounds to a common 9,000 BTU head. Mini-split heads are sold in fixed sizes, so the real job is matching your room to the closest size and then nudging up or down for sun, kitchen heat, and how many people use the space.
Here is the table, the standard head sizes, and the adjustments that move you between them.
The quick by-room table
Using the 20 BTU per square foot rule, common room sizes land roughly here:
- 250 sq ft (small bedroom or office): about 5,000 BTU, so a 6,000 BTU head
- 400 sq ft (large bedroom or living room): about 8,000 BTU, so a 9,000 BTU head
- 600 sq ft (open living and dining area): about 12,000 BTU, so a 12,000 BTU head
These are starting points for a reasonably insulated room in a moderate climate. A sunny, leaky room pushes the number up. A shaded, well sealed one pulls it down.
Common head sizes
Single-zone mini-split indoor units come in a handful of standard capacities. The four you will see most often:
- 9,000 BTU: small to medium bedrooms, home offices, rooms up to about 450 sq ft
- 12,000 BTU (one ton): larger bedrooms and modest living rooms, up to about 600 sq ft
- 18,000 BTU (1.5 tons): big open living areas or two combined rooms, up to about 900 sq ft
- 24,000 BTU (two tons): great rooms, finished basements, or a large open floor plan up to about 1,200 sq ft
One ton equals 12,000 BTU per hour, which is why the sizes step up in even chunks. If your room sits between two heads, do not automatically reach for the bigger one. Read the adjustments first.
What moves the number
Two rooms of the same floor area can need different heads. The factors that matter most:
- Sun exposure. A room with large west or south facing windows takes on a lot of afternoon heat. Heavy glass and direct sun can add 10 percent or more to the load. A shaded north facing room needs less.
- Kitchen heat. If the head serves a kitchen or an open plan that includes the stove, add capacity. An oven and range dump a lot of heat into the space while they run.
- Occupancy. People give off heat, roughly 230 BTU per hour each at rest, so a room that regularly holds several people carries more load than a quiet office.
- Ceiling height and insulation. You cool volume, not floor area, so a vaulted ceiling raises the load. A drafty, poorly insulated room pushes toward the high end.
The U.S. Department of Energy publishes guidance on sizing room air conditioners that walks through these same factors, and the math carries over to a ductless head.
Why oversizing a single-zone mini-split backfires
It is tempting to round up to the next head to feel safe on the hottest day. With a single-zone mini-split, that choice tends to make the room worse, not better.
An oversized head cools the air fast, hits the thermostat target, and shuts off, a pattern called short cycling. That creates two real problems in one room.
First, the unit never runs long enough to pull moisture out of the air. Dehumidifying happens only while the coil is running, so a short cycle leaves the room cool but damp and clammy. The Environmental Protection Agency notes that indoor humidity above about 60 percent encourages mold and dust mites, which is exactly the problem you are trying to solve.
Second, the constant stopping and starting wears the compressor and uses more energy than a right sized head that settles into long, steady cycles. Modern inverter mini-splits are built to run at low, variable speed for hours, which is also where they sip the least power. An oversized unit cannot reach that efficient sweet spot, so you pay more for a clammier room. Right sizing, not maximum size, is the goal.
Get your number
Plug the room dimensions, climate, sun exposure, and insulation into the AC and mini-split sizing calculator and it returns a BTU estimate with the math shown, which you can match to the nearest head. If you are cooling several rooms or weighing a multi-zone system, the whole-home heat load calculator totals the load across the house in one pass.
For whole-house central cooling instead of a single head, what size air conditioner do I need covers the full-house BTU-per-square-foot math and the central versus mini-split comparison.
For a tricky room or a multi-head install, ask your contractor for a Manual-J load calculation before you buy, and use the estimate here to check that a quote is not oversized. Run your own numbers in the calculator below.