Mini Split Sizing Calculator
Find what size mini split you need — enter the room size, ceiling height, sun and climate to get the cooling BTUs and the nearest standard mini-split size (9k, 12k, 18k, 24k BTU).
This is a quick rule-of-thumb estimate. For a permanent installation, a contractor’s Manual J load calculation is the accurate way to size a mini-split.
United States — national average · 2026 prices
Material only unless stated as installed. Excludes tax, permits and local labour. Verify with local suppliers — see how we calculate.
How this calculator works
Every figure above comes from the formula below, applied to the assumptions listed. Nothing is looked up from a hidden table — you can reproduce the result by hand.
Formula
base load = room area (sq ft) × 22 BTU/hr
× (ceiling height ÷ 8) × sun factor × climate factor
+ 600 BTU/hr per occupant above two + 4,000 BTU/hr if it is a kitchen
recommended head = the smallest standard mini-split capacity at or above the load
Assumptions & limits
- Identical load model to the BTU calculator — the difference is the size list it rounds to.
- Standard mini-split capacities are 9,000 · 12,000 · 18,000 · 24,000 · 36,000 · 48,000 BTU/hr, so the result names a real unit you can buy.
- The load is sized per zone. One indoor head conditions the open space it faces and will not cool rooms behind closed doors — size each zone separately and use a multi-zone outdoor unit.
- Rated capacity is a nominal figure. Inverter mini-splits modulate and their real output falls as outdoor temperature rises, so rated BTU is not guaranteed on a design day.
- A rule of thumb only. Modern inverter units tolerate slight oversizing better than older single-stage systems, but matching the load is still the goal, and a Manual J calculation is the accurate method.
Worked example
A 300 sq ft very sunny room, 9 ft ceilings, hot climate, three occupants
- Load = 300 × 22 × (9 ÷ 8) × 1.1 × 1.15 = 9,393 BTU/hr
- Occupants = +600 → 9,993 BTU/hr
- 9,993 exceeds the 9,000 unit, so the next standard size applies
- Recommended head = 12,000 BTU/hr
Result:A 12,000 BTU (one-ton) head. A 9,000 unit would be marginally undersized for this room.
Built and maintained by Christoph Ballmer, Honeygoat GmbH
Method and constants last checked against the calculator code on August 17, 2026. Independent trade review is not yet in place for this calculator — see how we handle review.
Read the site-wide calculation methodology · estimate vs. quote and safety limits
What size mini split do I need?
Mini-splits come in standard capacities — 9,000, 12,000, 18,000, 24,000 and 36,000 BTUs. To pick one, estimate the room’s cooling load (about 20–25 BTUs per square foot, adjusted for ceiling height, sun and climate) and round to the nearest size. The calculator above does this and names the unit; size each zone to the room its indoor head will serve.
Mini split size by room area
| Mini-split size | Cools about |
|---|---|
| 9,000 BTU | 350–400 sq ft |
| 12,000 BTU (1 ton) | 450–550 sq ft |
| 18,000 BTU | 700–1,000 sq ft |
| 24,000 BTU (2 ton) | 1,000–1,400 sq ft |
| 36,000 BTU (3 ton) | 1,400–1,800 sq ft |
For a whole house, use a multi-zone system and size each head separately. For window or portable units use the BTU calculator, and to understand the numbers see what a BTU is.
Head sizes and typical zones
| Head | Zone (avg. conditions) | Typical use |
|---|---|---|
| 9,000 BTU | ~350–400 sq ft | Bedroom, office |
| 12,000 BTU | ~450–550 sq ft | Master suite, large room |
| 18,000 BTU | ~700–1,000 sq ft | Open living area |
| 24,000 BTU | ~1,000–1,400 sq ft | Open-plan main floor |
| 36,000 BTU | ~1,500+ sq ft | Large open space / multi-zone outdoor unit |
Common mistakes
- One head for a whole house. A head conditions the space it faces; rooms behind closed doors stay warm. Multi-zone systems exist precisely for this — size each zone separately.
- Oversizing an inverter unit. Mini-splits earn their efficiency modulating at part load. A unit far too big cycles instead of modulating and wastes exactly the technology you paid for.
- Ignoring line-set limits. Every model has maximum refrigerant-line lengths and height differences between head and outdoor unit — check them before placing the condenser.
- DIY vacuum and charge. The refrigerant work is what makes or breaks reliability; pre-charged DIY line sets exist, but a botched vacuum shortens the compressor’s life invisibly.
As with any fixed installation: use this to shortlist, then have a contractor confirm with a Manual J calculation — window and insulation quality swing real loads far more than the rule of thumb can see. The BTU calculator covers window and portable units with the same model.
Frequently asked questions
What size mini split do I need?
Size a mini-split to the room’s cooling load — about 20–25 BTUs per square foot, adjusted for ceiling height, sun and climate — then round to the nearest standard capacity: 9,000, 12,000, 18,000, 24,000 or 36,000 BTUs. The calculator above does this and names the unit size.
How many square feet will a 12,000 BTU mini split cool?
A 12,000 BTU (one-ton) mini-split cools roughly 450–550 square feet under average conditions. A 9,000 BTU unit handles about 350–400 sq ft, an 18,000 BTU unit around 700–1,000 sq ft, and a 24,000 BTU unit about 1,000–1,400 sq ft. Sun, ceiling height and climate shift these ranges.
Can one mini split cool a whole house?
A single indoor head mainly conditions the open space it faces; it won’t evenly cool rooms behind closed doors. To cover a whole house you typically use a multi-zone system with several indoor heads on one outdoor unit. Size each zone to its own room with this calculator.
Is it better to undersize or oversize a mini split?
Aim to size it correctly. An oversized mini-split short-cycles and dehumidifies poorly, while an undersized one runs constantly and can’t keep up on the hottest days. Modern inverter mini-splits tolerate slight oversizing better than older systems, but matching the load is still the goal.