How to Use This Provisioning Calculator
Enter your passage details on the left and the results update instantly on the right. Start with the Passage section — you can type the number of days directly or let it auto-calculate from a distance and average speed. Then work through Water, Fuel, and Food. All outputs respond immediately to any change.
- Water: choose a comfort preset or set a custom daily litre figure per person. The calculator subtracts any watermaker production and tells you how many jerry cans you need above your tank capacity.
- Fuel: enter your engine HP and how much of the passage you expect to motor. The formula used is the well-established 1 litre per hour per 10 HP of applied load — multiply HP × load% × 0.1 to get L/hr.
- Food: set meal counts and cost-per-meal per person. The totals include your chosen buffer (30% is the offshore standard) so you can see raw totals and buffered totals side by side.
The Formulas Explained
Water Formula
Total water needed = (daily rate L/person/day × crew × days) × (1 + margin/100) minus any watermaker production (output L/hr × hrs/day × days). The result is then compared against your tank capacity to show extra jerry cans required. Standard offshore jerry cans hold 20 L (5 US gal).
Fuel Formula
Fuel consumption rate (L/hr) = engine HP × (load% / 100) × 0.1. This derives from the verified rule of thumb: a diesel at cruising load burns 1 L/hr per 10 HP in use, documented by eoceanic.com, Boating Magazine, and multiple sailboat owners' forum threads. Motoring hours = total passage hours × motoring%. Total fuel = L/hr × motoring hours × (1 + margin/100).
Food Formula
Total meal costs = crew × days × [(breakfasts/day × breakfast cost) + (lunches/day × lunch cost) + (dinners/day × dinner cost) + (snacks/day × snack cost)]. A buffer percentage is added to give the final provisioning budget. The actual days provisioned = passage days × (1 + buffer/100) so you can see how many extra days of food you carry.
When & Why to Use This Tool
Use this before any passage of more than a few days — Atlantic crossings, Pacific passages, island-hopping voyages, or any offshore leg where you cannot rely on reprovisioning. The three-panel approach (water, fuel, food) replaces the multiple spreadsheets most cruisers maintain separately. It is particularly useful when crew size changes close to departure, when deciding whether to install a watermaker, or when comparing motoring 30% vs 50% of a doldrums passage.
Common Mistakes Offshore Sailors Make
- Provisioning for the expected days, not the possible days. A 14-day Atlantic crossing can become 20+ days in light air. Always build in the buffer.
- Forgetting cooking water. The 2 L/person/day figure is sometimes cited as drinking only — this tool lets you set your own realistic total including cooking and hygiene.
- Underestimating motoring. Doldrums passages, calms in the Med, and windless approaches to anchorages all add up. If in doubt, set motoring higher, not lower.
- Not checking if fuel + reserve fits the tank. This tool flags if you need deck jerries and calculates how many, preventing the surprise of needing to carry more fuel than your tank holds.
- Single point of water supply. Even with a watermaker, carry bottled reserve for emergencies. The tool shows you the gap between production and total need.
Frequently Asked Questions
How much water do I need per person for an offshore passage?
The widely cited offshore minimum is 2 litres per person per day (drinking only), with a 50% safety margin added — as published by SAIL Magazine and NauticEd. For comfortable cruising that includes navy-style showers and normal cooking, 4–6 litres per person per day is more realistic. In hot tropical conditions, increase your daily figure by 15–20%. Always add your safety margin on top.
How do I estimate diesel fuel for a sailing passage?
The established rule of thumb for sailboat auxiliary diesels is 1 litre per hour for every 10 HP at cruising load. A 40 HP engine at 60% load (24 effective HP) burns about 2.4 L/hr. Multiply by your expected motoring hours and add a 20–25% reserve. This figure is well-documented across multiple sailing engineering sources and confirmed by cruisers who have measured actual consumption over hundreds of passage hours.
How much food should I provision for offshore?
Plan three meals per day per person and add a 30% buffer over your expected passage length. This buffer covers seasick days when little is eaten (those meals still need to exist later), heavy-weather days when cooking is hard, and the very common scenario of passages running longer than expected. For passages over 21 days, some experienced bluewater sailors recommend a 50% buffer.
Do I need jerry cans if my tank holds enough fuel?
Only if your calculated fuel requirement (including the safety reserve) exceeds your tank capacity. This calculator shows the gap and converts it to 20 L jerry can equivalents. If the number is zero or negative, your tank is sufficient. If positive, you must carry that many extra cans on deck — a common sight on offshore boats in the trade-wind belts.
Does a watermaker eliminate the need to carry tank water?
No. A watermaker reduces the volume you need to carry, but you should always keep full tanks as backup. Watermakers fail, foul, or cannot run in heavy weather when the boat is heeled. The calculator subtracts watermaker production from your total need, but you should treat the tank as an emergency reserve and fill it regardless.
How much propane/LPG does a passage use?
A typical offshore boat cooking one hot meal and making drinks per day uses roughly 150–250 grams of LPG per day. The default of 0.20 kg/day is a reasonable starting point for a modest passage galley. A 4.5 kg cylinder lasts approximately 22 days at that rate. If you cook two hot meals or have a larger crew, increase the daily figure accordingly.
What is the standard safety margin for offshore provisioning?
The bluewater cruising community consistently recommends a 50% buffer on water, a 30% buffer on food, and a 20–25% buffer on fuel. These figures come from decades of passage-making experience. The 50% water margin accounts for the reality that passages regularly take half again as long as planned, and dehydration at sea is a serious safety risk.