Mini-Split Sizing Calculator
Size mini-splits on site, in 60 seconds. Quote accurately, avoid callbacks, close more deals.
ZIP not found — pick your climate region:
Count walls that touch outside air or an unconditioned space (garage, attic, below-grade).
- 0 — interior room, no outside walls
- 1 — one exterior wall
- 2 — corner room
- 3 — three outside walls
- 4 — detached structure
The direction the largest exterior wall faces — not the front door.
- Room with one exterior wall → that direction
- Corner room → the wall with more window area
- 3-4 exterior walls → the wall with the greatest combined window + door area
- Excellent — post-2018 code or deep retrofit. R-19+ walls, R-38+ attic, spray foam.
- Good — 2010-era. R-15 walls, R-30 attic, double-pane low-E windows.
- Average — standard-code compliant. R-13 walls, R-30 attic, double-pane windows.
- Poor — pre-1990 unimproved. R-11 or less, single-pane windows, visibly drafty.
- Bad — no wall insulation. Pre-1950 construction; cold spots, daylight around outlets.
Measure the glass (including skylights), not the rough opening.
- Standard double-hung: ~3 × 5 ft = 15 sqft
- Large picture window: ~4 × 6 ft = 24 sqft
- Sliding patio door: ~6 × 6.5 ft = 39 sqft
Include sliding glass doors and French doors here — not in Door Area.
Solid doors only. Sliding glass doors and French doors go in Window Area — they behave as windows for heat transfer.
Room floor area: 144 sq ft
Volume: 1,152 cu ft
Who's this for?
Your mini-split size is ready
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Your Mini-Split Size
Estimate only. Match to the closest standard size — oversizing leads to short-cycling and humidity problems.
How to Use This Calculator
1. Enter the ZIP of the job site. Auto-detects climate region — one of the top drivers of cooling load.
2. Pick the room type. Bedroom, living, kitchen, bathroom, office, dining, garage, basement, sunroom, or addition. This sets internal gains automatically (kitchens get 4,000+ BTU extra for cooking; bedrooms and bathrooms far less).
3. Measure the room. Inside wall to inside wall. Length × Width × Ceiling height. We derive floor area and volume automatically.
4. Count exposed walls. An interior room with 1 wall touching outside air behaves very differently from a 3-walled corner. This factor changes the load by up to 40%.
5. Pick the primary orientation. Which direction the main exterior wall faces. South and west take more solar load in summer.
6. Measure window and door area. Actual glass/door area in sqft — a standard door is 20 sqft. Glass is the dominant envelope heat transfer path.
7. Click Calculate. Enter your email. We'll send the cooling BTU, closest mini-split size (9k/12k/18k/24k/36k), and suggested CFM.
How the Calculation Works
We use a 25 BTU/sqft room baseline (higher than whole-house because individual rooms don't benefit from shared thermal mass and ductwork losses), then apply multipliers for each driver.
base = length × width × 25 × (ceiling / 8)
envelope = base × exposedWallFactor × orientationFactor × insulationFactor
gains = windowArea × 870 + doorArea × 50 + roomTypeInternal
coolingBTU = (envelope + gains) × climateFactor
heatingBTU = coolingBTU × heatingFactor(region)
closest = nearest([9000, 12000, 18000, 24000, 36000])
CFM = coolingBTU / 30
Exposed wall factor: 0.85 (interior) → 1.25 (4-wall detached). Orientation factor: N 0.95, E 1.00, W 1.10, S 1.15. These follow standard Manual J adjustment patterns for room-level calculations.
When to Use This vs Whole-House
Use this tool when: sizing an individual mini-split head for a single room, sizing a zoned addition, troubleshooting a room that's too hot or cold, or designing room-level ductwork.
Use the whole-house Load Calculator when: sizing a central system, a single outdoor heat pump unit, or quoting a full-house replacement. Room-level accuracy doesn't matter as much at that scale.
For permit work: neither of our tools is ACCA-accredited. Use ACCA-accredited Manual J software instead.
Frequently Asked Questions
What size mini-split do I need for a 12x14 bedroom?
How is this different from whole-house sizing?
What BTU sizes do mini-splits come in?
What does CFM mean in the results?
Does orientation really matter?
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