Marine Air Conditioner Sizing Guide: How Many BTUs Do You Need?
Picking the right marine air conditioner comes down to one number: BTUs. Too small and the unit runs nonstop without ever cooling the cabin; too large and it short-cycles, wastes power, and leaves the air clammy. This guide shows you how to size a marine AC correctly the first time.
The quick formula
Marine air conditioners are sized by the volume of the space they cool, not just floor area. Start by measuring the cabin:
Cabin volume (ft³) = length × width × average height
BTU needed = volume × climate factor
- 14 BTU/ft³ — shaded, well-insulated cabin in a mild climate
- 16 BTU/ft³ — typical boat, some sun exposure
- 18–20 BTU/ft³ — hot, humid climates like South Florida, large windows or lots of sun
Because Florida boaters run their AC hard, we recommend sizing at the higher end (16–20 BTU/ft³) so the unit holds temperature on the worst afternoons.
Worked example
A mid-cabin measuring 10 ft × 8 ft with an average 6.5 ft headroom = 520 ft³. In a Florida climate at 18 BTU/ft³, that’s about 9,360 BTU — so a 10,000 BTU unit is the right call. Round up to the next standard size, never down.
Common marine AC sizes
| Unit size | Typical space |
|---|---|
| 6,000–8,000 BTU | V-berth, small cabin or cuddy |
| 10,000–12,000 BTU | Mid-size cabin / salon |
| 16,000 BTU | Large salon or open cabin |
| 24,000 BTU+ | Whole-boat or multi-zone (chilled water) |
Self-contained vs chilled-water
Self-contained units put the compressor, evaporator and blower in one box — ideal for single cabins up to ~16,000 BTU. Chilled-water systems use a central chiller feeding multiple air handlers, and make sense for larger yachts cooling several zones at once.
Don’t forget airflow and pump sizing
A correctly sized unit still underperforms if ducting is too long or kinked, the seawater pump is undersized, or the strainer is clogged. Match the circulation pump’s GPH to the unit and keep duct runs short and smooth.
Shop & install
Browse our full range of marine air conditioners — including self-contained and CTM chiller systems. Not sure which size fits your boat? Our team in Pompano Beach, FL sizes and professionally installs marine AC — get a quote and we’ll spec it for your exact cabin.
Marine AC sizing FAQ
What size marine air conditioner do I need for my cabin?
Measure the cabin volume (length x width x height in feet) and multiply by 16 to 20 BTU per cubic foot for South Florida conditions. A typical mid-cabin lands between 10,000 and 16,000 BTU. When the math falls between sizes, round up in our climate.
Can a marine air conditioner be too big?
Yes. An oversized unit cools the air before it dehumidifies it, so it short-cycles and leaves the cabin cold but clammy. Size from the volume formula rather than buying the biggest unit that fits.
Should I choose self-contained or chilled-water?
Self-contained units serve one cabin and suit most boats under about 45 feet. Chilled-water systems serve multiple cabins from one central plant and belong on larger yachts. For a single-cabin Florida boat, self-contained is almost always the answer.
Do I need a professional to install a marine air conditioner?
Usually yes. A marine AC install involves a through-hull seawater intake, a raw-water pump, condensate drainage, ducting and dedicated wiring. The seawater plumbing is the part that should not be improvised. At Marine Overhauling in Pompano Beach we sell the unit and handle the complete installation, which also saves you up to 15% on the package.
Already have a CTM unit that is underperforming instead of undersized? Before replacing it, see our CTM marine A/C troubleshooting guide to match the symptom to the replacement part, from filters and capacitors to blowers and compressors, all stocked in Pompano Beach.