Mini Split Size Calculator
BTU and tonnage for a ductless mini split, room by room.
Updated · How we check our maths
- Recommended size
- 9,000 BTU (0.75 ton)
- Calculated load
- 8,000 BTU/h
- Base load
- 20 BTU per sq ft
- Tip
- Size each indoor head to its own room; avoid oversizing, which short-cycles and dehumidifies poorly
Quick answer
Start with about 20 BTU per square foot: a 400 sq ft room needs roughly 8,000 BTU, so a 9,000 BTU (¾ ton) mini split. Add 10–20% for hot climates, sunny rooms, high ceilings or poor insulation, and about 4,000 BTU for a kitchen.
What the Mini Split Size Calculator does
Enter the room size and conditions. The calculator applies the common 20-BTU-per-square-foot rule of thumb, adjusts for ceiling height, climate, sun, insulation and kitchen heat, and picks the nearest standard mini split size (6k, 9k, 12k, 18k, 24k …).
This is a quick sizing estimate. For whole-home systems or cold-climate heat pumps, ask an installer for a Manual J load calculation.
The formula
BTU = area × 20 × (ceiling ÷ 8) × climate × sun × insulation (+4,000 for a kitchen). 12,000 BTU = 1 ton.
Worked example
550 sq ft sunny room, 9 ft ceilings, hot climate: 550 × 20 × 1.125 × 1.15 × 1.1 = 15,650 BTU → 18,000 BTU (1.5 ton).
How to use the Mini Split Size Calculator
- Measure the room's floor area.
- Enter the ceiling height.
- Choose climate, sun, insulation and whether there's a kitchen.
Frequently asked questions
What size mini split for 500 sq ft?
About 10,000 BTU before adjustments — usually a 12,000 BTU (1 ton) unit.
Is it bad to oversize a mini split?
Yes. Oversized units short-cycle, waste energy and leave rooms clammy.
Is mini split sizing the same for heating?
Use the same estimate, but in cold climates check the unit's heating capacity at low outdoor temperatures.
Results are estimates for planning and learning. Check important figures with a qualified professional. How we build and check our calculators.