Watermaker Output Calculator

Enter your watermaker's rated output, run hours, crew size, and passage length to calculate daily water production, demand coverage, and a full day-by-day tank schedule.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter watermaker output
Input the rated output of your watermaker in Gallons Per Hour (GPH). This specification is typically found in the unit's manual or on its label.
2
Specify daily run hours
Indicate how many hours per day you plan to operate the watermaker. This is crucial for determining daily production and managing power consumption.
3
Input days underway
Enter the total length of your offshore passage or trip in days. This helps calculate total water demand and production over the duration.
4
Provide crew size
Input the total number of crew members aboard. This directly influences the total daily water demand for the vessel.
5
Estimate water per person
Enter the average gallons of water consumed per person per day. A typical offshore allowance for drinking and cooking is 1–2 gallons per person per day.
6
Enter tank capacity
Input the total capacity of your vessel's fresh water tanks in gallons. This is used to track tank levels and assess refill rates.
7
Review your water budget
The calculator will display your daily output, demand, coverage ratio, and a day-by-day water budget, providing a comprehensive overview for your voyage.

Example Calculation

A crew of three plans a 14-day offshore passage with a 4 GPH watermaker, running it 4 hours per day, and a 60-gallon tank.

Watermaker Output (GPH)

Run Hours Per Day (h/day)

Days Underway (days)

Crew Size (people)

Water Per Person (gal/person/day)

1.5

Tank Capacity (gal)

Results

16 gal/day

Tips

Pre-Fill Tanks Before Departure

Always depart with full water tanks. The watermaker should be used to top off tanks, not to generate a large initial supply, reducing wear on the unit and ensuring a buffer in case of mechanical issues.

Implement Water-Saving Habits

Even with a watermaker, practice water conservation. Short showers, minimal dishwashing water, and efficient cooking habits can significantly reduce daily demand, extending the time between watermaker runs and conserving energy.

Factor in Maintenance and Filter Changes

Watermakers require regular maintenance and filter changes. Factor these into your passage planning and budget. Clogged filters or worn membranes can drastically reduce output and efficiency, potentially creating a water deficit.

Charting Your Course: Mastering Water Self-Sufficiency with a Watermaker

For sailors, cruisers, and long-distance travelers, a watermaker is the ultimate enabler of self-sufficiency, transforming endless oceans into a source of fresh, potable water. The Watermaker Output Calculator provides a comprehensive water budget for any offshore passage, meticulously detailing daily output, demand, coverage ratio, and potential surplus or deficit. By factoring in watermaker specifications, crew size, and tank capacity, this tool empowers mariners to plan with confidence, ensuring a continuous supply of fresh water throughout their journey.

Ensuring Water Self-Sufficiency on Offshore Voyages

The critical role of watermakers for long-distance cruising cannot be overstated, as they enable unparalleled self-sufficiency and safety by providing a continuous source of fresh water. This technology dramatically reduces the need to carry large, heavy water tankage, which can negatively impact a vessel's performance and fuel efficiency, or to make frequent, time-consuming port calls for resupply. For a typical crew of three on an offshore passage, average daily consumption for drinking and cooking alone might be 1-2 gallons per person per day. With a watermaker, vessels can generate this essential supply on demand, mitigating the significant risk of running out of water far from shore, which is a primary safety concern for any extended voyage.

Plotting Your Vessel's Daily Water Balance

Calculating your vessel's water budget involves understanding both the production capacity of your watermaker and the daily consumption of your crew. This balance determines your self-sufficiency.

Daily Water Output (gal/day) = Watermaker Output (GPH) × Run Hours Per Day (h/day)
Daily Demand (gal/day) = Crew Size (people) × Water Per Person (gal/person/day)
Coverage Ratio = Daily Water Output / Daily Demand
Passage Surplus / Deficit = (Daily Water Output - Daily Demand) × Days Underway

The Watermaker Output and Run Hours Per Day establish your production. This is compared to Daily Demand (derived from Crew Size and Water Per Person) to determine the crucial Coverage Ratio. Finally, the Passage Surplus / Deficit provides a long-term view of your water independence.

💡 Beyond water, proper weight distribution is crucial for boat stability and performance. Our Boat Weight Distribution Calculator can help you optimize your vessel's balance.

Budgeting Water for a Two-Week Offshore Passage

Consider a crew of three undertaking a 14-day offshore passage. Their watermaker produces 4 GPH, which they plan to run for 4 hours daily. Each person is allotted 1.5 gallons of water per day for drinking and cooking, and the vessel has a 60-gallon fresh water tank.

Watermaker Output: 4 GPH
Run Hours Per Day: 4 h/day
Days Underway: 14 days
Crew Size: 3 people
Water Per Person: 1.5 gal/person/day
Tank Capacity: 60 gal

First, calculate the daily water output: Daily Water Output = 4 GPH × 4 h/day = 16 gal/day

Next, determine the crew's daily water demand: Daily Demand = 3 people × 1.5 gal/person/day = 4.5 gal/day

This shows a Daily Water Output of 16 gal/day, providing a substantial surplus over the 4.5 gal/day demand.

💡 For long voyages, understanding a boat's stability characteristics is paramount for safety. Our Capsize Screening Formula Calculator provides an important metric for assessing seaworthiness.

Water Quality Standards for Potable Water at Sea

While watermakers are highly effective at removing salt and impurities, the water they produce, though fresh, does not automatically meet all potable water standards without further consideration. The World Health Organization (WHO) provides comprehensive guidelines for drinking water quality, which often serve as a benchmark for onboard consumption. These guidelines address microbial safety, chemical contaminants, and aesthetic considerations. For watermakers, it's crucial to ensure that the source water is not heavily contaminated with oil, chemicals, or excessive particulates, as these can foul membranes and compromise output quality. Many experienced mariners employ post-treatment methods such as UV sterilization to eliminate any potential bacteria or viruses that might pass through the membrane, or activated carbon filters to improve taste and remove residual odors. Regular testing of the produced water for salinity and other parameters is also a best practice to ensure it consistently meets safety and palatability standards for the crew.

Frequently Asked Questions

What is a watermaker on a boat?

A watermaker, also known as a desalinator or reverse osmosis system, is a device on a boat that converts saltwater into potable freshwater. It uses a semi-permeable membrane to remove salt and impurities, providing a reliable source of drinking water for extended voyages, reducing reliance on shore-based resupply.

How much water does a typical cruising boat need per day?

A typical cruising boat with a crew of 2-4 people generally needs between 5 to 10 gallons of water per person per day for all uses, including drinking, cooking, and basic hygiene. For drinking and cooking alone, a more conservative estimate of 1-2 gallons per person per day is common.

What is the 'coverage ratio' for a watermaker?

The coverage ratio for a watermaker indicates how well its daily output meets the vessel's daily water demand. A ratio of 1.0 means output exactly matches demand, while a ratio above 1.0 indicates a surplus, allowing tanks to be refilled. A ratio below 1.0 signals a deficit, requiring careful rationing or alternative water sources.

Can I drink water directly from a watermaker?

Water produced by a properly functioning watermaker is typically safe to drink. However, many cruisers add post-treatment, such as UV sterilization or activated carbon filtration, to ensure potability and improve taste, especially for long-term storage or in areas with questionable source water quality.