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 | In stock |
|---|---|---|
| 6,000–8,000 BTU | V-berth, small cabin or cuddy | CTM CT-6 (6,000 BTU, 115V) |
| 10,000–12,000 BTU | Mid-size cabin / salon | CTM CT-10 (10,000 BTU) or CT-12 (12,000 BTU), 115V |
| 16,000 BTU | Large salon or open cabin | CTM CT-16 (115V) or CT-16H (230V) |
| 24,000 BTU+ | Whole-boat or multi-zone (chilled water) | CTM CT-25H (25,000 BTU, 230V) |
Every CT size above is also available in 230V and in NMEA 2000 versions. See all CTM units and parts.
Dometic and Velair units in the same sizes
The table above lists the CTM units we keep on the shelf, which is what most owners here end up fitting because the universal control board works with the thermostat already on the boat and the registered warranty runs three years. If your boat is already Dometic throughout, or you want a variable speed compressor, we stock those sizes too.
| Unit size | Dometic GT | Dometic Turbo GTX | Velair (variable speed) |
|---|---|---|---|
| 6,000 BTU | Dometic GT6 (115V) | Turbo GTX 6,000 (115V or 230V) | Not offered at this size |
| 8,000 BTU | Not offered at this size | Turbo GTX 8,000 (115V or 230V) | Not offered at this size |
| 10,000 BTU | Dometic GT10 (115V) | Turbo GTX 10,000 (115V or 230V) | Velair i10VSD 115V or 230V |
| 12,000 BTU | Not offered at this size | Turbo GTX 12,000 (115V or 230V) | Not offered at this size |
| 16,000 BTU | Dometic GT16 115V or 230V | Turbo GTX 16,000 (115V or 230V) | Velair i16VSD 115V or 230V |
The Turbo GTX runs titanium coils and R-454B refrigerant. See all Dometic units we stock.
Fixed speed or variable speed?
A fixed speed compressor runs at full output until the cabin reaches the set point, then shuts off and starts again when it drifts. That is simple, cheaper to buy and cheaper to repair. A variable speed unit, like the Velair i10VSD and i16VSD, ramps the compressor up and down instead of cycling, so the cabin holds a steadier temperature, the unit is quieter overnight and it draws less current once the space is cool. If you run on a generator or on a limited shore cord, that lower running draw is usually the deciding factor. If budget and simple parts availability matter more, fixed speed is still the safer buy.
Sizing does not change between the two. Work out the BTU rating from the table at the top of this page first, then choose the compressor type and the voltage your boat already has. If you are not sure whether your boat is 115V or 230V, send us a photo of the panel and we will tell you.
CTM marine A/C parts in stock in Pompano Beach
Blowers, capacitors, controls and complete 6,000 to 16,000 BTU self contained units, ready to ship or pick up locally. Free US shipping on orders over $199.
Shop Marine A/C Parts All CTM Products
Need help matching the part? Call +1 (786) 277-2596.
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 a self-contained or chilled-water marine AC system?
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.