Mini Split Sizing Calculator
Find what size mini-split you need — enter your room size, ceiling height, climate zone, sun exposure and insulation to get the exact BTU capacity, the recommended 9k–36k size, and matching units.
Prices vary by location. Verify with local suppliers.
How the mini-split sizing works
The calculator starts from the industry rule of thumb — about 20 BTU per square foot for a standard, moderately insulated US room — and then adjusts it for the things that actually change the load: your climate zone, ceiling height, sun exposure, insulation, how many people use the room, and whether it’s a kitchen. It shows that math in the open so you can see exactly why the number is what it is, then rounds up to the next standard mini-split size.
Required BTU ≈ area × 20 × climate × ceiling × sun × insulation + 600 per extra occupant (beyond two) + 4,000 for a kitchen
Rounding up to the next standard size (9k, 12k, 18k, 24k, 36k BTU) is the safety margin — so we don’t deliberately oversize on top of it. Getting the size right matters both ways: an oversized mini-split short-cycles, cools unevenly and leaves the air clammy, while an undersized one runs flat-out and never quite keeps up. For a whole home, switch to 2–4 zones and the calculator sizes one multi-zone outdoor unit for the combined load, plus a suggested per-room head size.
BTU by room size (quick reference)
| Room size | Typical size | Tonnage |
|---|---|---|
| Up to ~450 sq ft | 9,000 BTU | ¾ ton |
| ~450–550 sq ft | 12,000 BTU | 1 ton |
| ~700–1,000 sq ft | 18,000 BTU | 1.5 ton |
| ~1,000–1,400 sq ft | 24,000 BTU | 2 ton |
| ~1,400–1,800 sq ft | 36,000 BTU | 3 ton |
These ranges assume 8-ft ceilings, average sun and insulation, and a mild-to-mixed climate — the calculator adjusts them for your actual room. Every result is a planning estimate; for a precise figure an HVAC pro runs a Manual-J load calculation. DIY, pre-charged systems (MrCool, Pioneer, Senville, Cooper&Hunter) can be self-installed; the refrigerant work on other systems needs a licensed, EPA-608-certified technician, and the dedicated circuit needs a licensed electrician.
Frequently asked questions
What size mini split do I need for a room?
Start from about 20 BTU per square foot, then adjust for your climate, ceiling height, sun and insulation. A 400 sq ft room in a mixed climate lands near 8,000 BTU, so a 9,000 BTU (¾-ton) unit fits; a 600 sq ft sunny room pushes toward 12,000–18,000 BTU. The calculator above does the full math and rounds up to the right standard size. For an exact figure an HVAC pro runs a Manual-J load calculation.
Is it better to oversize or undersize a mini split?
Neither — size it right. An oversized mini-split cools the room too fast, short-cycles, and shuts off before it removes humidity, leaving the air cold and clammy while wasting energy and wearing the compressor. An undersized one runs constantly and never reaches temperature on the hottest or coldest days. The calculator sizes to your actual load and rounds up only to the next standard size, which is the built-in safety margin.
How many BTU do I need per square foot for a mini split?
About 20 BTU per square foot is the common rule of thumb for a standard, moderately insulated room with 8-ft ceilings. It rises for hot or very cold climates, high or vaulted ceilings, strong sun, poor insulation, extra occupants, or a kitchen — and falls for well-insulated, shaded rooms. Because so many factors move it, the per-square-foot rule is only a starting point; the calculator applies each adjustment for your room.
What size mini split for a whole house?
For a whole home you typically use a multi-zone system: one outdoor condenser feeding several indoor heads, one per room or area. Add up each room’s BTU, apply a diversity factor (not every head runs at full at once), and match a 2–4 zone condenser to the total — commonly 24,000–48,000 BTU. Set the calculator to 2–4 zones to size the condenser and get a per-room head size.