Tropical climate sailboat example

Marine Air Conditioning Capacity Calculator

Marine A/C System Size Calculator

Please confirm that you have read and understand this notice before calculating.

How to Use

Important

This tool provides a general estimate only based on industry rules of thumb for typical marine cabin spaces. Actual BTU requirements vary depending on insulation, windows, heat-producing equipment, airflow, occupancy, and operating conditions. Final system sizing should be verified by a qualified marine HVAC technician.

The customer is solely responsible for selecting the appropriate system size for their vessel and comfort preferences. We do not guarantee performance, satisfaction, or suitability based on calculator results and assume no responsibility for system selection decisions made using this tool.

Step 1: Measure the enclosed space only. Measure the cabin/enclosed area you want to cool—do not include cockpits, open decks, engine rooms, or the boat’s overall length/beam.
Step 2: Use average dimensions. Measure the longest and shortest points and calculate the average. Example: width is 7 ft at the sole and 9 ft at the ceiling → (7 + 9) ÷ 2 = 8 ft.
Step 3: Choose location, climate, and sun exposure. If your boat is used in multiple areas, select the warmest climate and highest sun exposure it typically experiences.
Standard Industry Ballpark Figures
You can use these common rules of thumb to estimate marine A/C capacity. (Shown as BTU per cubic foot, with a rough BTU per square foot equivalent.)
  • 10–12 BTU/ft³ (~60 BTU/ft²)
    Best for cabins used mostly at night with minimal sunlight (typical below-deck sleeping areas).
  • 14 BTU/ft³ (~90 BTU/ft²)
    A standard baseline for most cabin areas with some windows and shading.
  • 16–19 BTU/ft³ (~120 BTU/ft²)
    For sunny spaces like pilothouses/helms with large windows and direct sun exposure.
Location to Cool (Examples)
Example of a below-deck cabin space

Below Deck

Enclosed cabins below the main deck (staterooms/berths). Typically lower sun exposure and often used after sunset. These spaces usually need fewer BTU per cubic foot than areas with large windows and direct sun.

Example of a mid-deck cabin space

Mid Deck

Typical main-cabin living areas with moderate sunlight and mixed use. This is a good “standard cabin” category for many boats.

Example of an above-deck pilothouse or helm area with large windows

Above Deck / Pilothouse

Helm/pilothouse/bridge areas with large windows and direct sun exposure. These spaces often have the highest heat gain and may require higher BTU per cubic foot.

Climate (How to Choose)
Temperate coastal climate example

Temperate

Mild conditions with less extreme heat. Choose this when A/C demand is lower and evenings typically cool down. Mild summers, lower humidity. Typical highs: ~70–85°F A/C used occasionally

Moderate warm climate example

Moderate

Regular warm-weather use where A/C is commonly needed. Choose this for consistently warm regions that are not extreme year-round. Warm summers, moderate humidity. Typical highs: ~80–95°F | A/C used regularly

Tropical hot and humid climate example

Tropical

Hot, humid environments with intense sun exposure where A/C may run continuously. Choose this for South Florida/Caribbean/Gulf-style conditions. Typical highs: 90°F+

  • If you cruise in multiple regions: select the warmest typical conditions.
  • Still unsure: choose the more demanding option or consult a qualified marine HVAC technician.
Why Oversizing Is Not Always Better
If the marine air conditioner is substantially oversized, the following issues may occur:
  1. Unnecessary energy use: power is wasted without improving comfort.
  2. Higher equipment cost: the unit may cost more than required.
  3. Higher installation cost: larger units often require larger ductwork, a bigger pump, and heavier-gauge power wiring.
  4. More frequent compressor cycling: short on/off cycling can increase wear and reduce compressor life expectancy.
  5. Poor humidity removal: the unit may not run long enough to remove moisture properly, and humidity may remain high (or even feel worse).
  6. Less comfort: rapid temperature swings can be disruptive—especially while sleeping—and may cause discomfort.