How to use this aquarium volume calculator
Choose the geometry that actually matches the tank. Measure inside the walls, not across the outside trim. Enter the internal tank height and water height from the bare inside bottom, including the height occupied by substrate. Enter estimated solid displacement below that waterline in the selected water unit. Changing a unit converts the existing entries rather than reinterpreting them. Read capacity, waterline volume, and estimated net water as three different results.
How the calculation works
A tank's advertised size is not necessarily its current water content. Wall thickness, a gap below the rim, internal equipment, rocks, and substrate all change the amount of water present. This calculation separates geometric capacity to the top from the volume enclosed below the current waterline, then subtracts the displacement you supply. Capacity to the top is a geometric reference, not an instruction to fill that high.
For a rectangular aquarium, multiply internal length by internal width by vertical water height. Dimensions in centimeters produce cubic centimeters; dividing by one thousand gives liters. A cube uses the same rule. For a vertical cylinder with a flat base, the cross-sectional area is pi times the squared internal radius. The diameter field is divided by two before calculating that area.
The triangular option models a prism with a right-triangular horizontal base. Enter the two perpendicular base sides, not a diagonal and a side. Its cross-sectional area is half their product. A curved bow-front corner tank is not this shape, and a spherical bowl is not a cylinder. For unsupported geometry, obtain a manufacturer's internal capacity or measure a safe fill using known water quantities instead.
Displacement means the water excluded by submerged solids. A loose bag of gravel includes air spaces before filling, many of which later hold water. Subtracting the entire loose bulk volume can therefore understate actual water. Use a measured or separately justified displacement estimate. Do not subtract headspace a second time: entering the waterline already accounts for the empty space above it.
The exact unit relationships used here are one US liquid gallon equals 3.785411784 liters and one imperial gallon equals 4.54609 liters. An inch is 2.54 centimeters. The word gallon alone is ambiguous, so all results identify the convention. Conversion preserves a quantity; it cannot improve an uncertain measurement or turn rounded tank dimensions into an exact fill volume.
Formulas
Rectangular waterline volume
Other supported shapes
Estimated net water
Worked examples
A 90 cm rectangular tank
A 90 × 45 × 45 cm internal tank holds 182.25 L geometrically to the top. At a 40 cm water height, the geometric volume is 162 L. Subtracting 12 L of actual solid displacement leaves an estimated 150 L, about 39.626 US gallons or 32.995 imperial gallons.
A cylindrical tank
A flat-based cylinder with internal diameter 40 cm and water height 50 cm encloses π × 20² × 50 / 1000, or about 62.832 L before displacement. Doubling diameter with the same height quadruples volume because diameter enters the formula squared.
Common uses
Establish a common water-volume reference
Use a consistent net-water estimate when comparing maintenance quantities and hydraulic ratios. Recheck it after major changes to waterline or hardscape rather than carrying an old estimate into every later calculation.
Compare tank geometry
Compare internal water capacity across supported shapes while remembering that equal volumes can have different swimming length, floor area, and maintenance needs. Shape conversion does not establish equivalent animal housing.
Accuracy and limitations
Dimensions assume straight vertical walls and the selected cross-section throughout the entered height. Curved fronts, rounded bases, tapered vessels and irregular shapes require a different model. Outputs retain full calculation precision internally and display up to three decimal places.
Displacement is user supplied, not inferred from substrate bulk liters. Errors in dimensions or displacement can dominate rounding. This tool does not determine stocking, medication dosage, salinity or a safe fill line.
Sources and methodology
References support unit conversions, relevant material or energy calculations, and husbandry context where listed. NumUtility's geometric and arithmetic models are explained above; they do not establish safe stocking, maintenance requirements, or structural capacity.
Frequently asked questions
Does this calculate how many fish I can keep?
No. Water volume alone cannot determine stocking. Adult size, swimming space, behavior, species compatibility, water quality, filtration and maintenance also matter.
Should I include sump water?
The geometry covers one tank only. For connected-system water volume, add separately measured operating sump and plumbing water without double-counting the display tank.
Can displacement exceed waterline volume?
No. The calculator rejects that input. Check units and whether loose substrate volume was mistaken for actual solid displacement.
Are my aquarium measurements saved?
No. These calculations run locally in your browser. Entered measurements are not sent to NumUtility, and no account or saved aquarium profile is required.