How do I calculate gallons in an aquarium?

Multiply interior length × width × height in inches, then divide by 231. A 48 × 18 × 21 in tank computes to 18,144 ÷ 231 ≈ 78.5 cubic-inch gallons — minus glass inset and substrate displacement, you'll see closer to 70 actual water gallons.

Why is my real volume lower than the tank's sticker?

Sticker volumes use external dimensions, ignore the glass thickness on five panels, and round generously. A 75-gal sticker often holds 65–72 gal of water once you subtract glass, rock displacement, sand, and the gap below the rim.

Should I use sticker volume or net volume for dosing?

Net volume. Every dosing constant on this site is in mL or grams per US gallon (or per litre) of actual water. Overstating volume by 15 % means under-dosing alkalinity by 15 %, which is how you get coral that won't grow despite 'normal' Alk numbers.

Does the fish tank volume calculator work for freshwater?

Yes — water volume is water volume regardless of salt content. The same numbers feed into a freshwater fertiliser calculator or a saltwater two-part dosing calculator. The geometry doesn't know whether you stock cichlids or coral.

How accurate is the bow-front estimate?

Within 1–2 % for typical residential bow-front tanks. The actual bulge varies by manufacturer — Marineland, Aqueon, and custom builders all cut slightly different curves. If your tank manufacturer publishes a real volume figure, trust that over the 6 % estimate.

Calculator

Aquarium Volume Calculator

Tank sticker volumes are always the outside-of-glass figure rounded to the nearest marketing number. Your real net water volume is what dosing math depends on. Enter interior dimensions (or external dimensions and a glass inset) and pick the geometry — the aquarium size calculator returns US gallons, litres, and cubic inches. Works as a fish tank volume calculator, aquarium gallon calculator, and aquarium capacity calculator: the math is the same, the words are interchangeable.

Aquarium volume

Shape

Glass / displacement inset (in per side)

Default 0.5 in to estimate interior water volume.

Length (in)

Width / front-to-back (in)

Height (in)

US gallons

69.18US gal

Litres

261.9L

Cubic inches

15980cu in

Centimetres³

261870cm³

How this is calculated

For a rectangular footprint we use L × W × H / 231 to convert cubic inches to US gallons (1 US gal ≡ 231 cubic inches). Other shapes:

rectangular   V = L × W × H
cube          V = s³
cylinder      V = π × (d/2)² × H
half-cylinder V = ½ × π × (d/2)² × H
hexagonal     V = (3√3 / 2) × s² × H
bow-front     V = L × W × H × 1.06   // +6% midpoint of 5–8% bulge

The glass-inset field subtracts panel thickness from each interior dimension so an externally-measured 48 × 18 × 21 in tank with ½ in glass comes out around 70 net gal, not the 75 sticker. The bow-front 6 % multiplier sits at the midpoint of the 5–8 % range you'll see quoted across manufacturers — no primary-source constant exists, so the methodology page flags it as a documented estimate.

Fish tank volume calculator

“Fish tank” and “aquarium” are the same thing — the tank doesn't care what the marketing department called it. The same formula gives gallons for a freshwater community fish tank, a saltwater reef, or a planted aquascape. The only thing that changes is how much of the displayed volume actually holds water: substrate, hardscape, and the gap below the rim eat into the net figure. A 75-gal sticker tank with a 2 in sand bed and 30 lb of rock holds closer to 62 actual gallons of water — and that's the number every dosing calculator on this site wants.

How to calculate gallons in an aquarium

For rectangular tanks measured in inches: multiply length × width × height, then divide by 231 (the legal definition of a US gallon — NIST). For measurements in cm, divide L × W × H by 1000 to get litres, then divide by 3.785 if you want US gallons. The aquarium dimensions calculator above does both directions and adds the geometry-specific corrections automatically.

FAQ

How do I calculate gallons in an aquarium?: Multiply interior length × width × height in inches, then divide by 231. A 48 × 18 × 21 in tank computes to 18,144 ÷ 231 ≈ 78.5 cubic-inch gallons — minus glass inset and substrate displacement, you'll see closer to 70 actual water gallons.
Why is my real volume lower than the tank's sticker?: Sticker volumes use external dimensions, ignore the glass thickness on five panels, and round generously. A 75-gal sticker often holds 65–72 gal of water once you subtract glass, rock displacement, sand, and the gap below the rim.
Should I use sticker volume or net volume for dosing?: Net volume. Every dosing constant on this site is in mL or grams per US gallon (or per litre) of actual water. Overstating volume by 15 % means under-dosing alkalinity by 15 %, which is how you get coral that won't grow despite 'normal' Alk numbers.
Does the fish tank volume calculator work for freshwater?: Yes — water volume is water volume regardless of salt content. The same numbers feed into a freshwater fertiliser calculator or a saltwater two-part dosing calculator. The geometry doesn't know whether you stock cichlids or coral.
How accurate is the bow-front estimate?: Within 1–2 % for typical residential bow-front tanks. The actual bulge varies by manufacturer — Marineland, Aqueon, and custom builders all cut slightly different curves. If your tank manufacturer publishes a real volume figure, trust that over the 6 % estimate.

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Sources & references

01
NIST — 1 US gallon = 231 cubic inches — Legal definition
https://www.nist.gov/pml/owm/si-units-volume
02
Randy Holmes-Farley — Specific Gravity (oh how complicated) — Cross-reference for volume-density conversion
https://reefs.com/magazine/chemistry-and-the-aquarium-specific-gravity-oh-how-complicated/
03
Omni Calculator — Aquarium Volume — Bow-front exact derivation
https://www.omnicalculator.com/other/aquarium-volume

Last reviewed 2026-05-25