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.
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.
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.