Net water volume of an aquarium with stocking-density check.
Internal dimensions = external − glass on each face. Cylinder: π·r²·h. Bowfront approximated as rect × 1.12. Stocking rule of thumb: 1 cm of small fish per litre — adjust down for high-bioload species (cichlids, plecos) and up for nano dwarfs.
Aquarium volume is the foundational number for the hobby. Every rule of thumb — stocking density, filtration capacity, heater wattage, water-change schedule, dosing of fertilizers and medications — is volumetric. Yet aquarists routinely cite "a 100-litre tank" while their actual filled water volume is closer to 80 litres after subtracting glass thickness, substrate, decor, and a 90 % fill line. Ammonia treatment dosed by nameplate volume can be 25 % under-dosed; a fish-stocking density rule applied to nameplate volume can cause chronic overstocking. A dimensions-aware volume calculator that accounts for shape (rectangular, cube, cylinder, bowfront), glass thickness on each face, and fill-line percentage gives the actual water volume — the one that matters for biology and chemistry.
Internal dimensions = external dimensions − 2 × glass_thickness on each side wall, and − 1 × glass_thickness on the bottom (top is open). The calc assumes glass on bottom + 4 sides; some "rimless" tanks have thicker glass and the same formula applies.
Rectangular / cube: V_internal = (L − 2t) × (W − 2t) × (H − t). Cube is a rectangular with L = W (and the calc enforces this). Cylinder: V_internal = π × ((D − 2t)/2)² × (H − t). Bowfront: rectangular base × 1.12 — a rough empirical correction for the curved front pane increasing internal area by ~12 %.
Unit conversion: cm-input directly to cm³ → mL. in-input × 2.54 to get cm.
Filled volume = V_internal × fill_pct / 100. Most aquaria fill to ~90 % to leave headspace for top-off evaporation, drift wood, and waterless contact during top-trim work.
Outputs: - Liters (= mL / 1 000). - US gallons (= L × 0.264172). - Imperial gallons (= L × 0.219969). - Stocking-density estimate: 1 cm of small-fish body length per litre — a modern relaxed take on the older "1 inch per gallon" rule. - Water mass in kg (= L, since water density = 1 kg/L for fresh). - Total internal capacity if filled to 100 %.
Pick the shape (rectangular, cube, cylinder, bowfront). Pick the unit (cm or in). Enter the length, width, height for rectangular/cube/bowfront, or diameter + height for cylinder. Set the glass thickness (typical 0.4 cm for tanks < 60 L, 0.6 cm for 60–200 L, 1.0 cm for 200+ L). Set the water fill percentage (default 90 %). The result panel shows liters as the headline, plus US and Imperial gallons, small-fish stocking capacity in centimeters, water mass in kg, and a tank-size band gauge (nano / small / medium / large / XL).
Standard 100 cm × 40 cm × 50 cm rectangular, 0.5 cm glass, 90 % fill.
Cylinder, 40 cm diameter × 60 cm height, 0.5 cm glass, 90 % fill.
Nano cube 30×30×30 cm, 0.4 cm glass, 90 % fill.
External vs internal dimensions. Manufacturers list external dimensions. Subtracting glass + bottom + fill-percentage gives the actual water volume — typically 75–85 % of external "nominal" volume. Use the calc, not the nameplate.
Substrate, rocks, decor displace water. A 5 cm sand bed in a 100 × 40 cm footprint displaces ~20 L; a large piece of driftwood another 5–10 L. Real tank water volume can be 30 % below the calc's number for heavily aquascaped layouts. Subtract decor volume manually.
Glass thickness varies per face. Larger tanks have thicker bottom and rear panels for stress reasons. The calc uses one thickness; for precision, measure each panel and average.
Bowfront approximation is rough. The 1.12 correction is empirical for typical curvature. For exact volume, use the manufacturer-specified water capacity if available.
Stocking-density rule of thumb is just that — a rule of thumb. The "1 cm/L" or "1 inch/US gal" works for slim-bodied small-mouth fish (tetras, rasboras, danios) at moderate bioload. Cichlids, plecos, goldfish need 3–5× more space per fish. Reef-tank corals need much different metrics (gallons per coral, not body-length).
Bioload, not body length, is the real constraint. A 20 cm pleco produces 10× the waste of a 20 cm school of neon tetras. Filtration capacity (turnover rate, biomedia) matters at least as much as raw volume.
Saltwater density. The calc assumes freshwater (density 1.0 kg/L). Saltwater is ~1.025; a 200 L marine tank weighs 205 kg of water, not 200.
Acrylic vs glass. Acrylic tanks have flexible walls and slightly larger internal volume than the same external dimensions in glass. The calc is glass-aware; for acrylic, reduce thickness inputs by ~50 %.
Sumps and overflow boxes. Reef and high-end freshwater setups often add a sump (separate filter tank) of 30–50 % main tank volume. The calc handles only the main display tank.
Top-off evaporation. In an open-top tank, ~1 L/day evaporates from a 100 L tank in dry climates. Concentration of dissolved salts and minerals rises if not topped off; volume the calc gives is the initial fill.