Mini Split BTU Calculator
Short answer: a mini split needs roughly 20 BTU per square foot of cooling area as a baseline, then adjustments for climate, insulation, sun exposure, ceiling height, occupancy, and kitchen heat — so a 400 sq ft room lands near 9,000 BTU and a 600 sq ft poorly-insulated garage closer to 18,000. This free calculator runs that Manual-J-lite math and tells you whether a unit you’re considering is sized right, undersized, or oversized.
Find the right mini split size for your room, or check whether a unit you're considering fits. This is a vendor-neutral Manual-J-lite estimate — every factor is shown, and we never inflate the number to sell a bigger unit.
How the calculator works
It starts from the standard cooling rule of thumb — about 20 BTU per square foot — then applies the adjustments that actually move the number:
| Factor | Effect on the estimate |
|---|---|
| Base load | square feet × 20 BTU |
| Climate zone | ×0.90 (far North) up to ×1.15 (Hot South) |
| Insulation | ×0.90 (new & sealed) to ×1.15 (older home) |
| Sun exposure | ×0.90 (shaded) to ×1.10 (heavy sun) |
| Ceiling height | × (ceiling ft ÷ 8) — taller rooms hold more air |
| People | +600 BTU for each person beyond 2 |
| Kitchen | +4,000 BTU for appliance heat |
The result is rounded up to the nearest standard mini split size: 9,000, 12,000, 18,000, 24,000, 30,000, or 36,000 BTU. Above 36,000 BTU — or rooms over about 1,500 sq ft — we recommend a multi-zone system instead of a single oversized head.
The mistake to avoid: going too big
It feels safe to round way up. It isn't. An oversized mini split blasts the room cold, hits the thermostat, and shuts off — over and over. That short-cycling means the unit never runs long enough to wring humidity out of the air, so the room ends up cold but clammy. You also wear the compressor faster and lose efficiency. In a mild climate, a unit sized exactly right (or even a notch under) is usually more comfortable than the bigger one.
Frequently asked questions
How many BTU per square foot for a mini split?
About 20 BTU per square foot as a starting point — a 500 sq ft room is roughly 10,000 BTU before adjustments — then tune for climate, insulation, sun, ceiling height, and people, and round up to a standard size.
What size mini split for a 2-car garage?
Most 2-car garages (about 400–600 sq ft) land at 12,000–18,000 BTU. Garages are often poorly insulated and sun-exposed, so set insulation to "poor" and sun to match, then run your exact footage.
Is it bad to oversize a mini split?
Yes — it short-cycles, leaving the room cold but humid, wears the compressor, and costs more to run. Right-size it.
Single-zone or multi-zone?
One room or open area up to ~1,500 sq ft / 36,000 BTU: single-zone. Multiple rooms or a higher total load: multi-zone, with one outdoor condenser feeding several indoor heads.
Related: What size mini split do I need? (full guide) · Mini split BTU chart by room size