This site is owned and operated by Zone Air.
Worksheet · Questions
The questions every DIY sizing job runs into, answered with load-calculation logic, not just rules of thumb.
Sizing FAQ
A common starting point is about 20 BTU per square foot, which is roughly what ENERGY STAR's room air conditioner chart works out to, so a 400 sq ft room starts around 9,000–12,000 BTU and a 600 sq ft room around 12,000–18,000 BTU. Ceiling height, the age of the house, windows and your local climate all move that number, which is why the calculator asks for your ZIP, ceiling height and windows (plus year built and window direction in Pro mode) instead of using a flat multiplier. Run it for a figure based on your actual room, then shop 9,000 BTU, 12,000 BTU, or 18,000 BTU systems in the matching size class.
It's a simplified, Manual J-style estimate built on the same kinds of inputs a full load calculation uses. It is not ACCA-approved Manual J software, so treat it as a planning number — fine for DIY installs, room additions, and single-room jobs. For a whole-home replacement, a heavily glazed great room, or mixed construction, a professional Manual J is still worth paying for.
Square footage gets you a starting point, but it can miss by a lot in either direction. Two 500 sq ft rooms can land a full size class apart: an older room with a lot of glass can need far more than a newer, tighter room with one small window. Insulation, climate zone, ceiling height, and sun exposure all bend the answer, which is what the calculator accounts for.
For an inverter-driven mini split — like every Zone Air system this calculator matches — pick the size closest to your calculated load. Inverter systems ramp up and down instead of just switching on and off: Zone Air lists its 12,000 BTU unit's cooling range as 2,500–12,500 BTU/h, so it follows the load as conditions change through the day. With a single-speed air conditioner there's less room for error, so stay close to the calculated load rather than jumping up a size.
It short-cycles. An oversized head drives the room to setpoint, shuts off, and restarts a few minutes later, never running long enough to pull humidity out of the air — so the room reads the right temperature but feels clammy. ENERGY STAR notes that short cycling increases wear and tear and can shorten the equipment's life. Oversizing is an easy mistake to make in DIY sizing.
Start at the room level. Each indoor head serves one room or one connected open area, so the load calculation is per zone rather than whole-house. For a multi-zone project, run each room, then size the outdoor unit to the simultaneous peak rather than the raw sum — bedrooms and living rooms don't always peak in the same hour. The calculator's Whole Home scope gives you every room's load and the straight total in one pass; it doesn't discount for rooms peaking at different times, so read the total as the conservative number.
Basic mode fills in typical default values for everything it does not ask about (a 1990 build, double-pane low-E windows and so on), so you still get a usable number from a ZIP code and a room size. Switch to Pro when you can supply real values — year built, window type, shading, duct location — and the estimate tightens. If you can only add one real value, make it the year built: in this calculator it moves the result far more than a foot or two of ceiling height.
For a single room or a straightforward addition, this calculator gives you a planning number without paying for an in-home Manual J. For whole-home replacements, complex multi-zone layouts, or homes with vaulted ceilings, large banks of glass, or mixed construction, pay for the professional calculation. Paired with the DIY installation guide, the calculator covers most homeowner jobs.