Aquarium volume calculator

Common aquarium sizes: gallons, litres, and water weight

Manufacturer tank labels are nominal — “20 gallon” is the outside-dimension geometry, not what the tank actually holds once glass thickness, an inch or two of air below the rim, and a shallow substrate bed are accounted for. Stated gal converts cleanly to litres (1 US gal = 3.785 L) and water weight (1 US gal = 8.34 lb / 3.79 kg). The working column below assumes a light setup (1.5″ air gap, shallow substrate, no aquascape); heavier setups land lower — see “Why box gallons lie” further down for the stacked math.

Water weight is for fresh water only — saltwater is ~2.5% denser. For full setup weight including glass, stand, and substrate, use the water weight calculator. For a side-by-side of two specific tanks, use compare tanks.

Use inside dimensions, not outside

Manufacturer-stated tank gallons are computed from outside dimensions. Glass thickness (1/4" on small tanks, up to 5/8" on larger ones), the 1–2 inches of air below the rim, and substrate displacement reduce the actual water volume by 10–20%. A “20-gallon long” sold at 30 × 12 × 12 actually holds about 17 gallons of water once it's set up. Use water height (not tank height) and inside dimensions for accurate stocking, dosing, and water-change math.

Why box gallons lie

Three layers of error stack between the "20 gallons" printed on the box and the water you're actually treating with dechlorinator:

1. Outside-vs-inside dimensions.A 20-long's stated 30 × 12 × 12 outside reduces to roughly 29.5 × 11.5 × 11.5 inside once you subtract glass thickness on each face. That alone cuts volume by ~7%. The calculator's Inside / Outside toggle handles the conversion automatically; see glass thickness by water column height for the height-driven recommendation.
2. Fill below the rim. Most aquarists fill 1–2 inches below the top edge to leave room for the lid, surface agitation, and to prevent splash-out. On a 12-inch-tall tank, 1.5 inches of air gap is another 12.5% reduction in usable height.
3. Substrate + hardscape displacement. Two inches of substrate over a 30 × 12 footprint displaces ~3 gallons of potential water. A medium aquascape adds another 5% loss.

Two reasonable readings show up depending on setup. The compact top-of-page table assumes a light setup (1.5″ air gap, shallow substrate, no aquascape) and lands a 20-long around ~17 working gallons. A heavier setup — 2″+ air gap, 2″+ substrate, a medium-rock aquascape — stacks all three losses: stated 20 → ~17 inside-dimensions → ~14.5 after fill drop → ~13 working volume. That's why the calculator surfaces both gross (geometric capacity) and working (what you actually treat) numbers — for the same tank, the two can differ by 30%+.

Working volume vs gross volume

The headline of the calculator is working volume— that's the number to use for dosing chemicals (dechlorinator, ferts, medication) and for sizing weekly water changes. Gross volume only shows up for floor-load planning, where the geometric capacity matters because the tank weighs the same whether you put substrate in it or not.

Most online aquarium calculators only report a single "volume" number (typically gross). That's fine if you understand the gap, fragile if you don't. Beginner stories of overdosing medication usually trace back to dosing for the box gallons, not the working gallons.

Formulas this calculator uses

• Rectangular:L × W × H, divided by 231 (cubic inches per US gallon, NIST exact). The 231 number isn't a rounding — it's the legal definition of a US liquid gallon.
• Bow-front: rectangular base + a half-elliptical bulge on the front. Half-ellipse area is (π/2) × (L/2) × bow depth, where bow depth is measured from the flat-front line to the deepest tip of the bulge (NOT from the back wall to the tip).
• Cylindrical: π × r² × H, where r is half the inside diameter. Simple, but the 0.5-inch glass thickness on a 12-inch outside diameter cuts internal volume by ~16%.
• Hex (regular six-sided): (√3 / 2) × F² × H, where F is the flat-to-flat distance. Hex tanks are typically advertised by point-to-point diameter, which is larger than F by a factor of 2/√3 ≈ 1.155 — verify which dimension your tank uses.

Where this calculator falls short

• Heavy aquascape with substantial rock or driftwood: the qualitative decor preset is a planning estimate, not a survey-grade measurement. For dosing-precision on a stocked aquascape, drain and measure refill volume directly.
• Saltwater tanks: math is identical, but water weight differs. Saltwater is ~2.5% denser than freshwater. For floor-load on a reef build, multiply the freshwater readout by 1.025.
• Acrylic tanks with bowed or asymmetric panels: not modeled. Use the rectangular shape and accept ±2–3% on bowed panels.
• Drilled / overflow tanks: the math here is gross capacity. If your display has an overflow box that constrains water level below the rim, set the "air above water" slider to that constraint.

Where volume feeds the rest of the math

Volume is the gateway input for almost every other tank-sizing decision. Once you have it, here's where it goes next.

Setup math from this volume

• Water weight calculator — for floor-load planning on tanks above 75 gallons.
• Heater sizing calculator — wattage from volume + room/target temperature delta.
• Substrate calculator — converts tank dimensions + bed depth into pounds and bags.
• Filter flow rate calculator — recommended GPH from working volume + stocking density.
• Air pump size calculator — required L/min by tank gallons, filter mode, and depth.

Once it's running

• How to cycle a new aquarium — the foundational beginner guide. Volume is the main input.
• Fishless cycling calculator — day-by-day ammonia dosing plan tied to your volume.
• Aquarium compatibility checker — multi-species stocking analyzer with a verdict + fixes.
• Stocking density calculator — honest verdict on a proposed stocking, not just inches-per-gallon.
• Water change calculator — per-session gallons, weekly cadence, hauling weight.
• Dechlorinator dosage calculator — Prime / Stress Coat / AquaSafe per water change.

If something goes wrong

• Ammonia / nitrite emergency calculator — exact water-change % + Prime emergency dose by severity.
• Nitrite spike calculator — water-change % + targeted aquarium-salt dose.
• High nitrate calculator — change-percent by tank context, source-water aware.
• New tank syndrome calculator — cycle stage + estimated days remaining from your readings.
• Fish gasping at surface — diagnostic — ranked diagnosis from test readings + tank conditions.

Sources: NIST gallon definition (1 US liquid gallon = 231 cubic inches, exact), Aqueon and Marineland published tank spec sheets for standard sizes, peer-reviewed aquaculture literature on substrate-displacement effects. Working-volume math layer in lib/aquarium/tankVolume.ts on GitHub.