Marine Tank Capacity Calculator

Because a cylindrical tank's volume is computed from its circular cross-section (using Tank Width as the diameter) times its length -- there is no separate height dimension for a cylinder, since the diameter already defines both its horizontal and vertical extent. Tank Height only matters for rectangular and oval/elliptical tank shapes, where the cross-section genuinely has independent width and height dimensions.

Because different fluids have meaningfully different densities -- fresh water at about 8.34 lb/gallon is roughly 32% heavier than gasoline at about 6.3 lb/gallon for the exact same volume -- and weight, not volume, is what affects a vessel's trim, stability, and how it sits in the water. Two tanks holding the same number of gallons can add very different amounts of weight depending on what they contain.

Because liquids expand with temperature, and a completely full rigid tank leaves no room for that thermal expansion -- filling to around 90% leaves headroom to prevent the tank from overflowing through its vent or, in an enclosed system, building dangerous pressure as the fluid warms. This is standard marine tankage practice, not a hard physical limit the calculator enforces on your input.

Treat it as a rough planning figure only -- it applies one fixed 5 gallons-per-hour consumption rate regardless of your vessel's actual size, engine, or operating speed, none of which the calculator asks about. For real voyage planning, replace this assumption with your specific engine's actual fuel burn rate at your intended cruising speed. It shows "N/A" for Fresh Water and Black Water because gallons-per-hour fuel consumption doesn't apply to either -- Fresh Water gets its own Water Range figure (days at 50 gallons/day) instead, and Black Water has no comparable consumption-based range at all.

Yes -- Fill Percentage scales the tank's total volume down to a Usable Capacity in direct proportion, and Fluid Weight is that usable capacity multiplied by the selected fluid's fixed density, so raising fill percentage always increases both usable capacity and fluid weight together, holding tank dimensions and fluid type fixed.