A sunroom is the worst-case room for the rule of thumb, because two of the multipliers stack against it: it is 'very sunny' by definition (+10%) and its walls are mostly single- or double-pane glass with little or no insulation, which earns the 'poor insulation' factor (+15%). Together that is a 26.5% penalty before you account for the fact that many sunrooms have vaulted glass roofs and ceiling heights of 10 to 12 feet. The table below applies both penalties.
Realistically, even that understates a west-facing glass room on a July afternoon. Solar gain through 100 square feet of unshaded glass can exceed 20,000 BTU/hr at peak. No reasonable air conditioner is sized to fight that, and the honest answer is to shade the glass first: exterior awnings, solar screens, or reflective film cut the load by half or more and cost far less than the next two sizes of AC. Then size the equipment for the shaded room.
Sunrooms also tend to be additions with poor connection to the home's ductwork, which is why they are the most common single-zone mini-split installation in the country. A 12,000 BTU mini-split with a heat pump handles a 250 to 300 sq ft sunroom in summer and, in moderate climates, keeps it usable in winter too.
Cooling BTU by sunroom size
Baseline: very sunny sun, poor insulation, moderate climate, 8 ft ceiling, 2 people.
| Floor area | Best case | Baseline | Worst case | Unit to buy |
|---|---|---|---|---|
| 150 sq ft | 2,400 | 3,800 | 3,800 | 5,000 BTU |
| 200 sq ft | 3,200 | 5,100 | 5,100 | 6,000 BTU |
| 250 sq ft | 4,100 | 6,300 | 6,300 | 8,000 BTU |
| 300 sq ft | 4,900 | 7,600 | 7,600 | 8,000 BTU |
| 400 sq ft | 6,500 | 10,100 | 10,100 | 12,000 BTU |
| 500 sq ft | 8,100 | 12,600 | 12,600 | 14,000 BTU |
Best case = good insulation and heavy shade; worst case = poor insulation and full sun.
Sunroom-specific advice
Shade before you size
Solar screens or exterior shades on the sunny glass reduce heat gain 60–80%. Size the AC for the room after shading, and you will typically drop one full unit class.
Use the actual ceiling height
Vaulted sunroom ceilings of 10–12 ft add 25–50% to the load. Measure to the peak and average it with the wall height for the calculator.
A heat pump mini-split extends the season
The same unit that cools the sunroom in summer heats it in spring and fall. In cold climates check the unit's capacity at 17°F before relying on it as the only heat source.
Common questions
How many BTU for a 300 sq ft sunroom?
Roughly 7,600 BTU with the sunny and poor-insulation penalties at 8 ft ceilings, which points to an 8,000 BTU unit; with a 10 ft vaulted ceiling the load rises to about 9,500 BTU and a 10,000 or 12,000 BTU unit fits better.
Can a window AC cool a sunroom?
For sunrooms under about 250 sq ft with some shading, yes. Larger or fully glazed rooms are better served by a 12,000–18,000 BTU mini-split, which also avoids cutting a hole in the glass wall.