Boat Freshwater Tank Duration Calculator
Tank and daily use
Freshwater result
Enter tank, reserve, crew, and daily-use assumptions to calculate.
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Freshwater endurance formula
The calculator treats tank level and reserve as percentages of total volume. It subtracts the protected reserve before estimating endurance.
Available water = capacity × (current level − reserve level) ÷ 100
Daily use = people × use per person + shared use
Duration = available water ÷ daily use
All values are converted internally to litres, so switching among US gallons, litres, and Imperial gallons does not change the physical result.
Trip water and starting level
For an optional trip duration, required water includes the entered daily demand for the whole trip plus the protected reserve.
Trip requirement = daily use × trip days + reserve volume
Trip margin = current water − trip requirement
If required starting level exceeds 100%, this tank cannot carry the modeled trip supply and reserve by itself. Reduce demand, shorten the interval between dependable refills, or make a suitable independent plan.
How to use this calculator
- Choose the unit and enter the freshwater tank's verified capacity. If the stated capacity includes inaccessible water below the pickup, use a lower usable capacity.
- Enter the current volume percentage and a reserve percentage below it. A calibrated sight gauge, flow meter, or measured fill history is stronger evidence than an uncalibrated sender.
- Count everyone aboard and estimate per-person daily use from your own measured or defensible planning data. Add shared demands separately.
- Optionally enter the planned number of days to see the supply margin, shortfall, and required starting level.
- Recheck the plan for weather delays, leaks, contamination, refill reliability, extra crew, medical needs, heat, and independently stored drinking water.
Example: 100 US gallons for four people
A 100 US gal tank at 100% with a protected 20% reserve has 80 US gal available before reserve. Four people using 3 US gal each per day plus 2 US gal of shared daily use consume 14 US gal per day.
The estimated duration is 80 ÷ 14 = 5.71 days. A five-day trip uses 70 US gal, so it finishes with 30 US gal aboard: the 20 US gal reserve plus a 10 US gal margin. This arithmetic example is not a recommended allowance or a guarantee.
Frequently asked questions
How is boat freshwater tank duration calculated?
The calculator subtracts the selected reserve volume from the current water volume, then divides the remainder by crew-dependent plus shared daily use. The result assumes the entered average daily use continues unchanged.
What should I include in daily freshwater use?
Include every outlet and activity supplied by the tank: drinking, cooking, dishwashing, handwashing, showers, cleaning, toilet flushing where applicable, pets, and equipment use. Avoid counting the same demand in both per-person and shared use.
How much freshwater reserve should I keep?
No single percentage fits every vessel or voyage. Consider passage length, crew needs, climate, medical needs, refill dependability, tank and gauge uncertainty, leak or contamination risk, independently stored water, and applicable operating rules. This calculator lets you enter the reserve rather than prescribing one.
Why can actual tank duration be shorter?
Irregular tanks can make gauge percentages nonlinear, and heel or trim can move water away from a sender or pickup. Leaks, guests, hot conditions, illness, cleaning, showers, delayed arrival, and a missed refill can all increase demand or reduce usable supply.
Does this include watermaker production or rain collection?
No. The estimate intentionally counts only water already in the tank. Treat watermaker output, rain capture, and future refill access separately because production rate, water quality, weather, power, maintenance, and access can change.
Are my freshwater planning inputs uploaded or tracked?
No. Calculations run locally in the browser, and this tool does not send, store, or attach input values to analytics events.
Limits and water-safety disclaimer
- This is a constant-average-use model. It does not forecast weather delays, variable daily use, guests, illness, leaks, tank contamination, pump failure, inaccessible residual water, or refill closures.
- A gauge percentage is assumed to equal volume percentage. That can be inaccurate in irregular or baffled tanks and can change with heel and trim.
- The protected reserve is a user-entered planning boundary. The calculator does not determine legal carriage requirements, potable-water quality, storage life, treatment, plumbing safety, or the correct contingency for a voyage.
- US liquid gallons and Imperial gallons are different units: 1 US gal is 3.785411784 L; 1 Imp gal is 4.54609 L.
Safety disclaimer: Water shortage or contamination can become an emergency. Do not rely on this estimate, one tank, an uncertain refill, or unverified watermaker output as the only source of safe drinking water. Carry suitable independent backup, monitor actual use and condition, and follow authoritative guidance for your vessel, jurisdiction, route, and crew.
Methodology and sources
Last reviewed: August 2, 2026. The calculation is unit conversion and arithmetic; voyage and water-safety context was checked against current government guidance:
- UK Maritime and Coastguard Agency: pleasure-vessel voyage planning, including crew, stores, weather, and vessel limitations
- UK Maritime and Coastguard Agency: fresh-water quantity and quality considerations aboard covered vessels
- Ready.gov: basic emergency supply checklist and water-planning context (PDF)
These sources provide planning context, not a universal recreational-boat allowance. Enter a consumption rate and reserve appropriate to the actual voyage and applicable requirements.