Your tank's gallons & liters — plus usable dosing volume, weight & water changes
Measurement system (your dimension fields)
Output unit
Use internal glass-to-glass measurements, not the box spec.
This calculator gives you your tank's true volume in gallons and liters for every common shape — rectangular/cube, bow-front, cylinder, half-cylinder (corner), hexagon, or a custom tank where you just type the gallons. More importantly, it separates the gross to-the-brim capacity (the number on the box) from the net dosing volume you actually treat against, after accounting for your real waterline and the water displaced by substrate, rocks, driftwood and equipment. It also estimates the full water weight and your 25% / 50% water-change amounts. Everything is computed privately in your browser, free.
Save as link: the link encodes your shape, units and dimensions so the calculator reopens pre-filled. It stays in your browser, but the values are visible in the URL — don't post it publicly if you'd rather keep them private.
The size printed on the box is the to-the-brim capacity, and it's often based on external dimensions that include the glass thickness — so it overstates the water. Your real water volume is typically 10–15% lower once you fill below the rim and account for substrate, rocks, driftwood, the filter and the heater. Dosing dechlorinator or medication against the gross box number is the classic way to overdose a tank. This tool always shows both numbers and uses the net one for the headline.
A standard 55-gallon rectangular tank measures about 48 × 13 × 21 inches internally. You fill to 20 inches and have a Moderate (10%) aquascape:
The tool shows volumes of 50 or more as whole numbers and smaller volumes to one decimal, so your live result here reads 57, 54 and 48.6.
When a product says "1 cap per 10 US gallons," you'd dose for ~48.6 ÷ 10 ≈ 4.9 caps, not the 5.5+ you'd get from the box number.
Work internally in cubic inches (input=in) or cm³ (input=cm).
Cross-section AREA by shape:
Rectangular/Cube : A = L × W
Bow Front : A = (L × W) + (2/3) × L × bowDepth // parabolic-segment approx
Cylinder : A = pi × (D/2)^2
Half-Cylinder : A = (pi × (D/2)^2) / 4 // quarter-round corner tank
Hexagon (side S) : A = (3 × sqrt(3) / 2) × S^2 ≈ 2.598 × S^2
VOLUME = A × height (use actual water height when provided)
Convert cubic inches Vci -> output:
US gal = Vci / 231 Imp gal = Vci / 277.42 L = Vci / 61.0237
Convert cm³ Vcc -> output:
L = Vcc / 1000 US gal = Vcc / 3785.41 Imp gal = Vcc / 4546.09
GROSS = volume at full height
FILLED = volume at actual water height (or full height if blank)
NET dosing = FILLED × (1 − displacement% / 100)
Water weight: US gal × 8.34 = lb ; Liters × 1.0 = kg (freshwater)No account, no coding — grab the copy-paste embed code from the "Add this calculator to your site" box below the guide. See it live: view a real embed example →
For a rectangular tank, multiply internal length × width × height in inches, then divide by 231 for US gallons (277.42 for Imperial, 61.0237 for liters). This tool handles bow-front, cylinder, half-cylinder and hexagon too, plus the usable dosing volume.
The box is the to-the-brim capacity and often uses external dimensions. Real water volume runs 10–15% lower after the waterline sits below the rim and substrate, rock, driftwood, filter and heater take up space. Dose against the lower net volume.
Yes. A US gallon is 231 in³ (3.78541 L); an Imperial gallon is 4.54609 L — about 20% bigger. North American bottles dose per US gallon, many UK ones per Imperial. Getting it wrong mis-doses by ~20%, so this tool labels every figure US gal or Imp gal.
Pick a displacement level — Light (~5%), Moderate (~10%, the default), Heavy (~15%) — or enter your own %. It's subtracted from your filled volume to give the net dosing volume you actually treat against.
Always internal, glass-to-glass. External/box dimensions include the glass and overstate volume — several percent on small tanks. For dosing, measure inside length, inside width, and the real water height (the waterline, not the rim).
About 8.34 lb per US gallon, or 1 kg per liter, for freshwater. Add ~10% for substrate and glass, and ~2.5% more for saltwater. A 55-gallon tank is roughly 460 lb of water — check your stand and floor can take it.
It's an estimate; the bow-front geometry is within ~1–2%. Good for dosing, water changes and stocking, not precision engineering. When unsure, dose to the lower net volume to avoid overdosing and follow the product label.
This tool gives an estimate. Use internal (glass-to-glass) measurements — external box/spec dimensions include glass thickness and overstate volume, and on small tanks the glass can be several percent. Real water volume is typically 10–15% below to-the-brim capacity once the fill line sits below the rim and substrate, rocks, driftwood, filter and heater displace water; the default 10% displacement reflects this — adjust it to your aquascape. The bow-front figure uses a geometric approximation of the front bulge (~1–2% vs the exact circular segment), and the hexagon formula assumes a regular hexagon with all six sides equal. A US gallon = 231 in³ = 3.78541 L; an Imperial gallon = 4.54609 L (~20% larger) — confirm which one your product means. Water weight figures (8.34 lb/US gal, 1 kg/L) are freshwater near room temperature; saltwater is ~2.5% heavier, which matters for whether a stand or floor can support the tank.
Disclaimer — not medical or livestock advice. For medication or chemical dosing, when in doubt dose to the lower (net) volume estimate to avoid overdosing, and always follow the product label. This calculator does not assess fish health, stocking safety, or water chemistry.
Everything is calculated in your browser. We don't store or sell your data. Estimate only — not medical or livestock advice.