Water Provisioning Calculator
Adequate fresh water provisioning is essential for safety on offshore passages. This calculator computes the total water requirement for your crew, passage duration, and daily usage rate, with an optional safety margin. Water needs include drinking, cooking, and minimal hygiene. The standard offshore provisioning guideline is a minimum of 1 gallon per person per day for survival rations, with 2-3 gallons for comfort. Always add a safety buffer of at least 50% for extended offshore passages in case of delays or equipment issues.
Water provisioning formula
Minimum water = Crew * Days * Daily per person
Total with margin = Minimum water * (1 + Margin / 100)
Total in liters = Total gallons * 3.7854
Jerry cans = ceiling(Total gallons / 5)
1 US gallon = 3.7854 liters. Standard offshore emergency minimum: 1 gallon per person per day. Comfortable offshore: 2-3 gallons per person per day.
Water management at sea
- Use saltwater for washing dishes and clothing; rinse with fresh water to remove salt.
- Collect rainwater when possible using your awning or boom tent to supplement stored water.
- A watermaker (reverse osmosis) produces potable water from seawater but requires electrical power.
- Keep an emergency water ration separate from the main supply in case of contamination or tank failure.
Water provisioning: frequently asked questions
How much water should I provision per person per day?
A commonly used figure for passage provisioning is 1-2 liters (0.26-0.53 gallons) per person per day for drinking, plus 1-2 liters for cooking, plus 1-2 liters for personal hygiene, giving a total of 3-6 liters (0.8-1.6 gallons) per person per day at sea in minimal-use conditions. Hot weather or active sailing increases this.
Should I add a safety margin to my water calculation?
Always provision for 150% of the calculated amount on offshore passages. Passages take longer than planned due to weather. Equipment failure may prevent making watermaker water. A watermaker can fail. Additional water is cheap insurance and takes relatively little space.
How does a watermaker affect provisioning?
A working watermaker dramatically reduces the water you need to carry. A typical small watermaker produces 8-20 gallons per hour when running. Even so, carry enough tank water for several days in case the watermaker fails, and factor in the power consumption required to run it.
What is a practical daily water allowance offshore?
Offshore cruisers on water budget typically allow 0.5-1 gallon per person per day for drinking and cooking, with sponge baths. With a watermaker, 2-3 gallons per person per day is more comfortable. Showering uses 10-20 gallons per session and should be rare offshore.
What type of water containers are best for offshore passages?
Jerry cans (5-10 gallon) are the most practical portable water containers. Avoid relying solely on fixed tanks; distributed jerry cans ensure you retain water even if a tank is contaminated or damaged. HDPE (high-density polyethylene) containers are food-safe and resistant to UV and salt water.
Official sources
- US Coast Guard Boating Safety: USCG Boating Safety Division.
- National Geospatial-Intelligence Agency: Bowditch American Practical Navigator.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.