Marine Air Conditioning Sizing and BTU Guide
Use boat length, cabin count, glazing and cruising climate to estimate marine air conditioning capacity in BTU per hour. Start with the calculator or browse the sizing tables below. These estimates help prepare an installer brief; they do not replace a vessel-specific heat-load assessment.
Marine Air Conditioner BTU Requirements by Boat Size
Every page below is a slice of this. Cooling required rises with the cube of nothing in particular — it rises with accommodation volume, which rises faster than length — and the three climates pull apart as the boat grows. At 28 ft the spread between Hobart and Darwin is 2,500 BTU; at 50 ft it is 8,500, which is a whole extra unit.
Computed for a light hull with ordinary glass, at a typical cabin count for each length. The same figures are in the table on every length page.
Marine Air Conditioning by Boat Length
What a hull of this length needs. Figures shown for the subtropics.
Marine Air Conditioner BTU Capacity Options
What a unit of this size will actually cool, and what it draws.
- 5,000 BTUabout 22 ft
- 7,000 BTUabout 26.5 ft
- 9,000 BTUabout 30 ft
- 12,000 BTUabout 34.5 ft
- 16,000 BTUabout 40 ft
- 24,000 BTUabout 49.5 ft
Marine Air Conditioning by Cabin Count
Cabin count does not change the capacity. It changes the architecture.
- One cabinSelf-contained
- Two cabinsSelf-contained
- Three cabinsSplit
- Four or more cabinsChilled water
Marine Air Conditioning by Climate
The factor that separates the same boat in Darwin from the same boat in Hobart.
- Tropics18,500 BTU at 40 ft
- Subtropics16,000 BTU at 40 ft
- Temperate13,000 BTU at 40 ft
Sizing Calculator Assumptions and Limitations
The sizer works from accommodation volume — length gives a waterline beam and a floor area, floor area gives a volume, and volume times a climate factor gives a capacity. Then a dark hull adds about ten per cent and a lot of glass another ten. That is the whole model, and it is deliberately simple, because a more elaborate one would imply a precision none of the inputs support.
What it cannot know is your boat. It does not know that the locker under your settee has the water tank in it, that the run to the forward cabin passes through a structural bulkhead, or that your marina's pedestals are 10 A and already loaded. Those are the things that decide an install, and they are why every page here ends by suggesting you ask rather than by suggesting you buy.