Free tool
Mini split size calculator
A mini split size calculator: describe each room and get a planning BTU range for it, checked against your county's winter low. Nothing is sent unless you choose to.
Describe your rooms
Planning range per room, 24°F design low
- 1. Bedroom, 200 sq ftA 6,000 BTU head. Heating 3,700 to 5,100 BTU, cooling 6,000 BTU.6,000 BTU
How we worked out room 1
- Cooling: ENERGY STAR lists 6,000 BTU for 200 square feet: about 6,000 BTU.
- Heating: 200 square feet, times the 46 degree gap between 70°F inside and the 24°F design low, times 0.4 to 0.55 for the insulation you chose: 3,700 to 5,100 BTU.
A planning range, not a size to buy. We size every head from a room-by-room load calculation at the visit, which measures the walls, windows, insulation and air leaks this tool can only guess at.
How does the size calculator work?
It works out each room twice, once for summer and once for winter, and takes the larger figure. At Firlands Portland Mini Splits we use the same two checks at the visit, with measurements in place of your answers.
Cooling: the ENERGY STAR room sizing chart
ENERGY STAR's chart gives a capacity for each band of floor area, based on an 8-foot ceiling. We scale taller rooms by their volume and apply ENERGY STAR's own adjustments.
- Heavily shaded room: 10% less. Very sunny room: 10% more.
- Each person beyond two who regularly uses the room: 600 BTU more.
- A kitchen: 4,000 BTU more.
| Floor area (square feet) | Cooling capacity |
|---|---|
| 100 up to 150 | 5,000 BTU |
| 150 up to 250 | 6,000 BTU |
| 250 up to 300 | 7,000 BTU |
| 300 up to 350 | 8,000 BTU |
| 350 up to 400 | 9,000 BTU |
| 400 up to 450 | 10,000 BTU |
| 450 up to 550 | 12,000 BTU |
| 550 up to 700 | 14,000 BTU |
| 700 up to 1,000 | 18,000 BTU |
| 1,000 up to 1,200 | 21,000 BTU |
| 1,200 up to 1,400 | 23,000 BTU |
| 1,400 up to 1,500 | 24,000 BTU |
| 1,500 up to 2,000 | 30,000 BTU |
| 2,000 up to 2,500 | 34,000 BTU |
Heating: the winter design low
Heat loss grows with the gap between 70°F inside and the design low outside, the temperature only the coldest 1% of hours in a year fall below. ENERGY STAR lists it by county.
| Where the house is | Winter design low | Gap to 70°F |
|---|---|---|
| Portland, Gresham, Multnomah and Clackamas counties | 24°F | 46 degrees |
| Beaverton, Hillsboro, Tigard (Washington County) | 27°F | 43 degrees |
| Vancouver and Clark County | 28°F | 42 degrees |
We multiply the floor area, the ceiling factor and that gap by a band for the insulation you choose. The bands are our own planning assumption, kept wide and on the generous side.
| Insulation | BTU per hour, per square foot, per degree |
|---|---|
| Little or none, single-pane windows | 0.55 to 0.7 |
| Attic insulated, walls not or not sure | 0.4 to 0.55 |
| Walls and attic insulated, double-pane | 0.28 to 0.4 |
| Built or rebuilt to recent energy code | 0.2 to 0.28 |
Why is it a range and not one size?
Because square footage alone can't size a mini split. Two rooms of the same size can need different heads once you count outside walls, window area, air leaks and what's above and below them.
The tool shows the common head sizes, 6,000, 9,000, 12,000, 15,000, 18,000, 24,000 BTU, that cover each room's range. Where heating decides the size, the head's heating output at your design low matters more than its nominal size, which is why heads that heat get cold-climate models.
What does the visit add?
A room-by-room load calculation from measurements: each wall, window and ceiling, the insulation we can see, and the direction each room faces. That sets the size of every head and the outdoor unit.
For a price, the cost calculator shows our typical range by head count. The sizing guide and how many mini splits you need go deeper, and mini splits in cold weather covers the freeze.
Questions about sizing
Will the smallest head cover a small bedroom?
Usually. A 12 by 12 foot room sits on the smallest line of the ENERGY STAR chart, and even a poorly insulated one loses less heat than 6,000 BTU on a 24°F night. So the smallest common head, 6,000 BTU, covers it, and the load check at the visit confirms it.
Why does the calculator check heating as well as cooling?
Because in Portland the heating need often decides the size. Summer design is 88°F, only 13 degrees above a cool room, while the winter design low is 24°F, 46 degrees below 70. Poorly insulated rooms lose more heat in winter than they gain in summer.
Is a bigger mini split head safer, just in case?
No. An oversized head reaches temperature fast and then cycles on and off, which dries the air less in summer and wastes energy. Inverter heads slow down to match the room, so the right size is the one that meets the load on the design day, not more.
Can one mini split heat a whole floor?
Sometimes, in an open floor plan. Closed doors stop the air, so bedrooms off a hall usually need their own heads. The calculator sizes each room; the visit decides which rooms can share a head through an open doorway.
Get each room measured
Send the house details and we'll set up the visit. We add your room sizes to the request.