Mini Split Sizing Calculator

Worksheet · Method

How the Mini Split Sizing Calculator Works

From ZIP code to BTU load in four steps, using ACCA Manual J 8th Edition methodology.

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 same factors an ACCA Manual J 8th Edition load calculation applies — outdoor design temperature for your ZIP, ceiling height and volume, glass and doors, air leakage, ventilation, and internal gains. Everything runs in your browser: no account, no email, nothing to download.

  1. Pick Basic or Pro. Basic asks for the essentials and fills in Manual J 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, the load breakdown behind it, 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 36,000 BTU three-zone system can comfortably serve rooms that add up to more than 36,000 BTU on paper.

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 Manual J has default leakage rates for each era.

Basic mode fills in Manual J 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 only verify one thing, verify ceiling height — it's the easiest input to misjudge from the floor and one of the most impactful on the result.

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