Quick Reference
Not sure what size window unit, portable AC, or mini-split you need? This page gives you the BTU number for any room size, plus the key adjustments that can shift it by 20% or more.
Based on 20 BTU/sq ft baseline for average insulation, 9 ft ceilings, and 2 occupants. Adjust using the table below.
| Room Size | BTU / hr Needed | AC Size | Typical Use Case |
|---|---|---|---|
| 100–150 sq ft | 5,000 BTU | 5,000 BTU window unit | Small bedroom, home office |
| 150–250 sq ft | 6,000 BTU | 6,000 BTU window unit | Medium bedroom |
| 250–350 sq ft | 8,000 BTU | 8,000 BTU window unit | Large bedroom, studio |
| 350–450 sq ft | 10,000 BTU | 10,000 BTU window unit | Master bedroom, small living room |
| 450–550 sq ft | 12,000 BTU (1 ton) | 12,000 BTU / 1-ton mini-split | Small apartment, large studio |
| 550–700 sq ft | 14,000 BTU | 14,000 BTU / 1.2-ton mini-split | One-bedroom apartment |
| 700–1,000 sq ft | 18,000 BTU (1.5 ton) | 18,000 BTU / 1.5-ton mini-split | Small 2-bed home or apartment |
| 1,000–1,200 sq ft | 21,000 BTU | 2-ton central AC | Small home |
| 1,200–1,500 sq ft | 24,000 BTU (2 ton) | 2-ton central AC | Typical 2-bed home |
| 1,500–2,000 sq ft | 30,000 BTU (2.5 ton) | 2.5-ton central AC | 3-bed home |
| 2,000–2,500 sq ft | 36,000 BTU (3 ton) | 3-ton central AC | Larger 3-bed home |
| 2,500–3,000 sq ft | 42,000 BTU (3.5 ton) | 3.5-ton central AC | 4-bed home |
| 3,000–3,500 sq ft | 48,000 BTU (4 ton) | 4-ton central AC | Large 4-bed home |
| 3,500–4,000 sq ft | 60,000 BTU (5 ton) | 5-ton central AC | Large home / light commercial |
The chart above is a starting point. These factors can push your real requirement up or down significantly.
| Factor | Adjustment | Why |
|---|---|---|
| Very sunny / south-facing windows | +10% | Solar gain adds significant heat |
| Well-shaded or north-facing room | −10% | Less direct solar gain |
| Poor insulation (pre-1980 home) | +20–28% | More heat transfer through envelope |
| Excellent insulation (2015+ build) | −15–20% | Tight envelope reduces load |
| Ceiling height 10–12 ft | +5–15% | More air volume to condition |
| Kitchen present in space | +1,200 BTU | Appliance and cooking heat gains |
| Each extra occupant (beyond 2) | +600 BTU | Human body heat output ~250 BTU/hr at rest |
| Hot climate (Zone 5–7) | +10–20% | Higher outdoor design temperature |
| Cool climate (Zone 1–2) | −10–15% | Lower outdoor design temperature |
The instinct to round up — grabbing a 10,000 BTU unit for a room that only needs 6,000 — backfires. An oversized AC blasts a room to temperature in a few minutes, then shuts off before it's run long enough to pull real humidity out of the air. The result is a room that reads "cold" on the thermostat but still feels clammy, plus a compressor that short-cycles on and off all day, which wears it out faster than steady, longer runs would. Sizing to the chart above — not above it — gets you both comfort and a longer equipment life.
Once you know your BTU number, the format decision comes down to the room itself. Window units deliver the most cooling per dollar and are the default choice if you have a compatible window and don't mind the look. Portable ACs trade some efficiency (the compressor lives inside the room it's cooling, adding its own heat back into the mix) for flexibility — no window bracket, easy to move between rooms. Mini-splits cost more upfront but run quieter, don't block a window, and are the better long-term pick for a room you'll condition year-round, like a converted garage or an addition without existing ductwork.
Take a 400 sq ft bedroom with south-facing windows in a moderate climate. The chart puts a 350–450 sq ft room at 10,000 BTU. Add 10% for the sun exposure (+1,000 BTU) and 600 BTU for a third regular occupant, and you land at roughly 11,600 BTU — round up to the nearest common size, a 12,000 BTU (1-ton) unit. That's the same math the quick calculator above runs automatically, but walking through it once makes it obvious why two rooms of identical square footage can need different equipment.
Our full HVAC load calculator accounts for every room, climate zone, insulation, and duct losses.