Mini Split Square Footage Calculator
Calculate mini split BTU requirements from room square footage. A general rule is 20-30 BTU per square foot, but adjustments apply for ceiling height, sun exposure, insulation, and climate zone.
Mini Split Sizing Calculator
BTU per square foot rules
Standard rule: 20-30 BTU per square foot for cooling. A 400 sq ft bedroom needs 8,000-12,000 BTU - so a 9,000 BTU mini split is the typical pick.
Common mini split sizes: 6,000 BTU (covers 100-250 sq ft), 9,000 BTU (250-400), 12,000 BTU (400-550), 18,000 BTU (550-800), 24,000 BTU (800-1,200).
Adjustments to the rule
Add 10% for kitchen (heat from cooking). Add 10% for very sunny rooms. Add 10% for ceilings over 8 ft. Subtract 10% for shaded rooms or rooms with above-average insulation. Climate zones 1-2 (hot south) need higher BTU; zones 6-7 (cold north) need a heating-focused unit.
For a more accurate sizing, use Manual J load calculation (the industry standard). Most HVAC contractors offer this as part of a free quote.
BTU sizing — the rough rule
20 BTU per sq ft for cooling in moderate US climates.
25-30 BTU per sq ft in hot, humid climates (Southern US, Texas, Florida).
16-18 BTU per sq ft in cool, dry climates (Pacific Northwest).
Worked example: 300 sq ft bedroom in Atlanta. 300 × 25 = 7,500 BTU. Round up to standard size: 9,000 BTU (1 ton).
Common mini-split sizes by coverage: 9,000 BTU handles 300-450 sq ft. 12,000 BTU handles 450-600 sq ft. 18,000 BTU: 600-900 sq ft. 24,000 BTU: 900-1,200 sq ft. 36,000 BTU: 1,500-1,800 sq ft.
Why the 20-BTU rule isn't enough
Ceiling height: standard 8-ft assumed. 9-10 ft adds 10-15% to load. Vaulted/cathedral: 20-30%.
Insulation quality: poor insulation can add 30-50% to cooling/heating load.
Windows: lots of west-facing glass in hot climates adds 30-50%.
Occupancy: 600 BTU per person added to cooling load.
Heat-generating equipment (kitchens, home offices with multiple monitors): can add 1,000-3,000 BTU.
Climate zone: Phoenix vs Seattle differ by 50%+ in cooling load even at the same square footage.
Always have a contractor do a Manual J load calculation for accurate sizing. Over-sized units short-cycle and don't dehumidify. Under-sized units run constantly and never maintain temperature.
Mini-split cost ranges
Single-zone (one indoor unit, one outdoor): $3,000-7,000 installed. Covers 300-1,500 sq ft depending on capacity.
Dual-zone: $5,000-10,000 installed.
Tri-zone: $7,000-13,000 installed.
Quad-zone: $9,000-16,000 installed.
DIY units (Mr Cool DIY, others): material cost $1,500-3,500 for single-zone. Saves $1,000-3,500 on labor but may void manufacturer warranties.
Per-sq-ft installed: $5-15 typical, depending on system size and ducting requirements.
Operating cost: mini-splits are 30-50% more efficient than central air, 50-70% more efficient than window units. Typical annual savings: $300-1,200 vs window units for a 1,500 sq ft home.
Pro tips
Don't oversize
An oversized mini split short-cycles, doesn't dehumidify properly, and feels clammy. 20% oversized is okay; 50% oversized is a comfort problem.
Sunny west walls add load
Afternoon sun on a large west window can add 2,000+ BTU of solar gain. Account for room exposure when sizing.
Open floor plans need one bigger unit
A combined kitchen-living-dining isn't the sum of three small units - it's one large room. Size for the total square footage as one space.
Multi-zone for multiple rooms
For 3+ rooms, a multi-zone outdoor unit with separate indoor heads costs less per zone than 3 individual systems and gives independent room control.