Mini split sizing calculator
This mini split sizing calculator finds the BTU per hour and the standard head size for each room you add.
Your rooms
How each room is figured
We run the same heat flow math as the whole-house tools, one room at a time. Each room is treated as a small one story house.
- The room gets its floor area and the ceiling height you pick.
- All its windows face the side you choose. A room with no outside wall has no windows.
- Wall heat counts only for the outside walls you list. The engine figures four walls, so we scale it by the number of outside walls out of four.
- The ceiling and floor count in full. There are no ducts, doors or kitchen heat.
- Rooms of 300 ft2 or more have two people. Smaller rooms have one.
Standard head sizes
Heads come in these sizes, in BTU/h: 6,000, 9,000, 12,000, 15,000, 18,000, 24,000, 30,000, 36,000. We pick the smallest one that covers the load, unless a smaller size is within 10 percent of it.
When to be careful
Kitchens, sunrooms and rooms with a lot of glass need extra care. Rooms with doors left open share air. A very tight, well insulated room can need less than a small head. Over sizing a ductless head makes it cycle often and hurts comfort in humid weather.
Get the full version in the app
The web page gives a fast estimate. The iPhone app does the whole job.
- Every window, wall and duct, not just six choices
- Load split by room with CFM and duct sizes
- A PDF report with your logo
- Works with no signal. Your jobs stay on your phone.
Questions people ask
What size mini split do I need for one room?
Add the room above with its floor area, windows and outside walls. The page shows the cooling and heating BTU/h and the standard head size that covers it. Sun and outside walls matter as much as floor area.
How are the standard head sizes chosen?
Heads come in sizes like 6,000, 9,000, 12,000, 18,000 and 24,000 BTU/h. We pick the smallest one that covers the room load. If a smaller size falls short by 10 percent or less, we take the smaller one.
Should the outdoor unit match the sum of all the heads?
Not always. Rooms rarely peak at the same time. The maker sets which outdoor units can serve which heads. Check their tables before you buy.
Why does an open room need more than its floor area suggests?
Open plans share air with the next room. A large room with big windows and no shade can need a bigger head. Treat the open area as one room, and add a head for the room it joins if needed.
More free tools
An estimate in the style of ACCA Manual J. It does not replace a certified room by room design. Manual J is a trademark of ACCA. HVAC Load is not affiliated with ACCA.