This site is owned and operated by Zone Air.

Mini Split Sizing Calculator

Worksheet · Method

How the Mini Split Sizing Calculator Works

From ZIP code to BTU load in four steps, using a Manual J-style method.

Method

From Square Footage to a Real Load Number

The calculator is a BTU load calculator for ductless mini splits. It starts from your room's square footage, then adjusts that baseline with the kinds of factors a Manual J load calculation looks at — outdoor design temperature for your ZIP, ceiling height and volume, glass and doors, air leakage, ventilation, and internal gains. The math runs in your browser: no account, nothing to download, and the email step is optional (just hit Skip).

  1. Pick Basic or Pro. Basic asks for the essentials and fills in typical defaults for the rest. Pro exposes year built, window type, shading, duct location, wind shielding, exact dimensions, and per-window detail.
  2. Choose your scope. Room(s) for one space or a few; Whole Home to size a multi-zone system floor by floor.
  3. Enter your ZIP and the room. The ZIP resolves your outdoor design temperature and climate, which is why the same 400 sq ft bedroom returns different answers in Phoenix and Portland.
  4. Read the result. You get a BTU/h load per room and the DIY systems that match — single zone for one room, multi zone for several.

For multi-room projects the calculator keeps each room separate and totals them, so you can size the outdoor condenser. Size that condenser to the simultaneous peak rather than the raw sum — bedrooms and living rooms rarely peak in the same hour, which is why a multi-zone outdoor unit is usually rated below the combined size of its heads: Zone Air's 3 zone systems run up to 36,000 BTU of indoor heads on a 27,000 BTU outdoor unit.

Read it as a planning number, not a stamped design. Rooms with wall-to-wall glass, cathedral ceilings, or unusual internal loads need a common-sense check on top of the calculator, or a professional Manual J.

What the Inputs Do

Why It Asks What It Asks

ZIP code resolves your outdoor design temperature, altitude, and latitude — the climate half of the equation. Square footage and ceiling height set the volume of air being conditioned. Windows and doors add solar gain and conduction losses; direction matters, which is why Pro mode asks which way each window faces. Year built stands in for insulation and air-tightness: a 1970s wall leaks far more than a 2010s one, and the calculator assumes a leakage rate for each era.

Basic mode fills in typical defaults for anything it doesn't ask about, so a ZIP code and a room size already produce a usable number. Every real value you supply in Pro mode tightens the estimate. 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.

Run the Calculator