Methodology: How This Calculator Sizes a Mini Split
Every rule in the calculator is either (A) quoted from a public US-government source, or (C) a clearly-labeled industry rule of thumb. Nothing is invented, and the line between A and C is drawn explicitly below.
This is an estimate, not a Manual J load calculation. ACCA Manual J — the standard residential load calculation — accounts for windows, orientation, infiltration, ducts and local design temperatures. This tool is for shortlisting equipment quickly; confirm the final size with a qualified installer before buying.
1. Baseline: the ENERGY STAR chart (grade A)
The baseline comes verbatim from the ENERGY STAR room air conditioner sizing chart (“Area To Be Cooled → Capacity Needed, BTUs/hour”):
| Area (sq ft) | BTU/h |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
Note the chart is non-linear: ~33 BTU/sq ft at 150 sq ft but ~15 BTU/sq ft at 2,000 sq ft. That's why flat "20 BTU per sq ft" rules mis-size at both ends.
2. Adjustments quoted from ENERGY STAR (grade A)
- "If the room is heavily shaded, reduce capacity by 10 percent."
- "If the room is very sunny, increase capacity by 10 percent."
- "If more than two people regularly occupy the room, add 600 BTUs for each additional person."
- "If the unit is used in a kitchen, increase capacity by 4,000 BTUs."
3. Rules of thumb we added (grade C — disclosed)
- Ceiling height: the chart is conventionally applied to ~8 ft ceilings; we scale capacity with air volume (9 ft = +12.5%, 10 ft = +25%, 12 ft = +50%). Editorial.
- Insulation: good −5%, average 0, poor/leaky +15%. Editorial; real envelope losses need Manual J.
- Climate zone (cooling): zones 1–3 get +15/+10/+5% for longer, hotter cooling seasons; zones 4–8 get no cooling increase. Zone definitions and the county lookup are DOE/IECC (map, lookup); the multiplier values are ours.
- Heating-primary in zones 5+: heat pump output falls with outdoor temperature, so if the unit is the primary heat source we size up one class and flag cold-climate-rated models (−13°F or lower). Editorial; the correct tool is a Manual J heating load at your design temperature.
4. Size classes and model matching
The adjusted BTU figure is rounded up to the nearest standard single-zone class (9,000 / 12,000 / 18,000 / 24,000 / 36,000). Matching models come from data/models.json: 14 units across MrCool, Pioneer, Senville, Gree, each verified against a live manufacturer or authorized-dealer page on 2026-06-10, with the source URL stored per model. Models whose spec sheets could not be verified were omitted entirely (that's why some brands you may expect are absent).
5. Honest limitations
- Cooling-first: the baseline chart is a cooling chart. Heating dominance in cold zones is handled by a conservative nudge, not a load calc.
- Whole-home multi-zone systems, ducted units and rooms over 2,500 sq ft are out of scope.
- Manufacturer coverage claims in the model tables are their numbers and often more generous than the ENERGY STAR chart — we show both so you can see the gap.
Sources
- ENERGY STAR room air conditioner sizing chart — energystar.gov
- DOE/IECC climate zone map — basc.pnnl.gov
- DOE Building America climate guidance (county lookup) — energy.gov
- Per-model spec sources — linked in every table row, also in data/models.json.