Find the volume at your actual waterline
An aquarium’s outside dimensions and advertised capacity are not necessarily the amount of water you will put in it. This calculator starts with the inside of the tank and separates the tank’s internal height from the water height. It supports a rectangular tank or an upright circular cylinder with straight vertical walls. Use it to compare a fill level, understand the empty space above the water, or estimate the weight of the freshwater alone. It does not recommend fish numbers, filtration or medication.
Measure and calculate
Choose the shape and measurement unit. For a rectangle, enter internal length L and width W; for a cylinder, enter internal diameter D, not radius. Measure the internal height H from the inside bottom to the rim, then measure water height h from that same bottom to the intended waterline. Do not substitute the height of the glass above a table or the manufacturer’s outside dimensions.
Press Calculate to see liters first, followed by rim capacity, explicitly labelled US and Imperial gallons, water weight and fill percentage. The additional measured mass is optional: leave it blank if you do not know the combined mass of the empty tank, substrate, equipment and decorations. When provided, the last result adds that mass to the water estimate. Unknown glass weight is never invented.
Volume, weight and two examples
For a rectangular tank, V = L × W × h. For a vertical cylinder, V = π × (D/2)² × h. Lengths are converted to meters internally, then cubic meters are multiplied by 1,000 to obtain liters. Rim capacity uses H instead of h, and fill percentage is 100 × h/H. One US liquid gallon is 3.785411784 L; one Imperial gallon is 4.54609 L. The gallon labels matter because the two units are different.
Example 1: internal dimensions 60 × 30 × 40 cm with a 35 cm water height give 63 L of water, about 16.64 US gal or 13.86 Imperial gal. Rim capacity is 72 L and the tank is 87.5% filled by volume. Using the freshwater approximation of 1 kg/L gives 63 kg, about 138.89 lb. If you actually measured 12 kg of additional mass, the combined estimate is 75 kg.
Example 2: a vertical cylinder with 40 cm internal diameter, 50 cm internal height and 30 cm water height holds about 37.7 L. Rim capacity is 62.83 L and the fill level is 60%. The freshwater estimate is 37.7 kg. Using 40 cm as a radius instead would quadruple the volume, which is why the input specifically asks for diameter.
What this estimate leaves out
The volume is the geometric space below the waterline before displacement by gravel, rocks, wood or equipment. Adding their mass does not subtract their displaced volume: mass alone cannot tell us how much water an object displaces. For a decorated tank, the actual water present will be lower than this geometric figure. A measured fill volume is preferable when you need the actual amount.
Freshwater density is approximated as 1 kg/L. Temperature and dissolved substances change density, so this is not a saltwater weight calculator or a laboratory measurement. The optional combined mass is only water plus the mass you entered; it is not a floor-load, stand or glass-safety assessment. Bow-front tanks, spheres, tapered bowls and horizontal cylinders are outside the supported shapes.
Inputs, corrections and keeping results
Use positive internal dimensions no greater than 100 m. Water height may be zero for an empty tank but cannot exceed H. A zero water height produces zero water volume and weight, not an error. A blank optional mass differs from an explicitly entered zero. The supported decimal separators are a point or comma without thousands separators.
Changing units converts the current dimensions; changing shape shows the relevant dimensions and preserves other entries. Editing invalidates the confirmed result and disables Copy until the next calculation. Normal results use up to two decimals, while very small nonzero quantities use scientific notation. Copy result keeps the chosen shape, dimensions, assumptions and results as plain text in the clipboard. Clear removes numbers and results while keeping unit and shape choices. Empty fields may show pale example values after losing focus; these are not recovered personal data.
Frequently asked questions
Why does the result differ from the tank label?
A label can use outside dimensions or a nominal full capacity. Glass thickness, headspace and objects inside reduce the amount of water. Use inside dimensions and the actual waterline, and remember this calculator does not subtract solid displacement.
Is the height measured above the gravel?
No. Both H and h start at the inside bottom of the tank. Measuring above gravel would treat its irregular surface as a solid flat floor and still miss other displacement. Use measured fill volume when solids materially affect the estimate.
Can I use the total to select a stand?
No. The tool has no information about support points, loads, materials or the stand. Even with extra mass entered, omitted items and displacement affect the estimate. It provides arithmetic, not a structural approval.
Does changing the water height alter capacity?
It changes water volume and fill percentage. Rim capacity stays tied to the full internal height H. This lets you compare two waterlines without pretending the tank itself became larger or smaller.