Aquarium Volume Calculator (Feet to Gallons)
Use this tool to calculate your aquarium's volume in U.S. gallons by entering the tank’s dimensions in feet. Works for all standard rectangular tanks.
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How to Convert Aquarium Feet to U.S. Gallons
If your tank dimensions are in feet, the gallon volume is straightforward: multiply length × width × height (in feet) to get cubic feet, then multiply by 7.48052 (the number of U.S. liquid gallons in one cubic foot). Two caveats matter: this uses the dimensions you enter (outer or inner), and it produces nominal volume — the water volume after glass, rim gap, and substrate is smaller. This guide gives the formula, the corrections, and a quick-reference for standard tank sizes.
The Formula (U.S. Gallons)
Volume (ft³) = L × W × H (all in feet) Gallons = Volume (ft³) × 7.48052 Combined: Gallons = L × W × H × 7.48052 Example: 6 × 2 × 2 ft tank Volume = 6 × 2 × 2 = 24 ft³ Gallons = 24 × 7.48052 ≈ 179.5 gal
7.48052 is the U.S. liquid gallon factor. The Imperial gallon is larger: 1 ft³ = 6.22883 Imperial gallons. If a product label uses Imperial gallons (common in the UK and some Commonwealth countries), the same tank is 149.5 Imp gal, not 179.5 U.S. gal. Always confirm which unit the label uses.
U.S. Gallon vs Imperial Gallon
| Unit | Liters | Gallons per ft³ |
|---|---|---|
| U.S. liquid gallon | 3.78541 L | 7.48052 |
| Imperial gallon (UK) | 4.54609 L | 6.22883 |
The difference is about 20%. A "55-gallon" tank in the U.S. is about 45.8 Imperial gallons — which is why product labels sometimes quote different gallon figures for the same tank. When in doubt, convert to liters: U.S. gallons × 3.785 = liters.
Outer vs Inner Dimensions
The dimensions you measure determine whether the result is outer-tank volume or water volume. Internal water volume is smaller:
inner_length = outer_length − 2 × glass_thickness inner_width = outer_width − 2 × glass_thickness inner_height = outer_height − glass_thickness (bottom) − rim_gap (typically 1") Example: 6 × 2 × 2 ft tank, 1/2" glass, 1" rim gap inner_L = 6 − 2 × 0.0417 = 5.917 ft inner_W = 2 − 2 × 0.0417 = 1.917 ft water_D = 2 − 0.0417 − 0.0833 ≈ 1.875 ft Gallons = 5.917 × 1.917 × 1.875 × 7.48052 ≈ 159.3 gal → about 11% less than the "179.5 gal" outer estimate
Typical glass thickness: 3 mm (~0.01 ft) for small tanks; 1/4" (~0.021 ft) for 20–40 gal; 3/8" (~0.031 ft) for 55–75 gal; 1/2" (~0.042 ft) for 90–125 gal. The rim gap is usually 1" for freshwater, sometimes more for reef or open-top setups.
Substrate and Displacement
| Contents | Depth (in) | Volume on a 6 × 2 ft footprint (gal) |
|---|---|---|
| Substrate | 1 | ~15.0 gal |
| Substrate | 2 | ~29.9 gal |
| Substrate | 3 | ~44.9 gal |
| Rock / driftwood | — | ~5–15% additional (highly variable) |
| Internal filter / equipment | — | ~2–5% |
| Water level (1" rim gap) | — | ~7.5 gal |
On a 6 × 2 ft tank, 2" of substrate plus a 1" rim gap deducts about 37 gallons — a "179.5 gal" outer-tank figure becomes ~142 gallons of actual water before hardscape. This is the number to use for stocking, dosing, and filter sizing. The aquarium volume calculator deducts substrate precisely.
Standard Tank Sizes — Outer and Actual Water
| Dimensions (ft) | Outer gallons | Approx. actual water | Typical rated size |
|---|---|---|---|
| 1 × 1 × 1 | 7.5 | ~6–7 | Nano cube |
| 2 × 1 × 1.5 | 22.4 | ~19–21 | 20-gallon long |
| 2.5 × 1 × 1.33 | 24.9 | ~22–23 | 25-gallon |
| 3 × 1 × 1.5 | 33.7 | ~28–30 | 29-gallon (30"×12"×18" in) |
| 3 × 1.5 × 1.5 | 50.6 | ~44–47 | 50-gallon |
| 4 × 1.5 × 2 | 89.8 | ~78–82 | 90-gallon |
| 4 × 2 × 2 | 119.7 | ~103–108 | 120-gallon |
| 6 × 2 × 2 | 179.5 | ~140–150 | 180-gallon |
| 8 × 2 × 2 | 239.4 | ~190–205 | 240-gallon |
"Actual water" assumes 1/2" glass, 1" rim gap, 2" substrate, and light hardscape. Individual tanks differ; the manufacturer's spec sheet is authoritative for a specific model.
Other Tank Shapes
| Shape | Formula | Notes |
|---|---|---|
| Rectangle | L × W × H × 7.48052 | Base case |
| Cylinder | π × r² × H × 7.48052 | r = inner radius; D/2 where D is inner diameter |
| Bowfront | L × W × H × 7.48052 × 0.85 | 0.85 is a conservative approximation — bowfront holds 10–20% less than its outer box. Averaging front and back lengths (a common method) overstates the true volume. |
| Regular hexagon | 2.598 × S² × H × 7.48052 | S = side length. Example: S=1.5 ft, H=1.5 ft → 2.598 × 2.25 × 1.5 × 7.48052 ≈ 65.6 gal |
| Corner (right triangle) | (L × W / 2) × H × 7.48052 | For triangular footprints |
For an irregular tank, use the fill-and-measure method: fill the tank with a known volume (e.g., a 5-gal bucket, counted), marking each addition until full. Slower but exact. Alternatively, use the manufacturer's stated volume from the specification sheet.
Why the Difference Matters
- Dosing: conditioners, medications, and fertilizers are dosed per gallon — using outer volume over-doses by 15–25%.
- Stocking: bioload limits are based on actual water volume. See the stocking calculator.
- Filtration: filters are rated in gallons per hour; you want 4–8× actual water volume. See the filter size calculator.
- Heaters: heaters are dosed at ~3–5 W per gallon of actual water, not outer volume. See the heater size calculator.
Related Tools
- Aquarium volume calculator — with substrate deduction
- Dimensions to gallons (inches) — same conversion in inches
- Feet to liters — metric equivalent
- Gallon to liter — unit conversion
- Liter to gallon — reverse
- Tank weight calculator — floor loading
- Substrate calculator — depth and bag count
Frequently Asked Questions (Feet to Gallons)
How do I convert cubic feet to gallons?
Multiply cubic feet by 7.48052 for U.S. liquid gallons. For Imperial gallons, multiply by 6.22883. The two differ by about 20%; the tank in liters is the same in both cases.
Does glass thickness affect the result?
Yes — if you measure outer dimensions. Subtract twice the glass thickness from length and width, and once (plus rim gap) from height. For a 6 × 2 × 2 ft tank with 1/2" glass and a 1" rim gap, outer volume (179.5 gal) becomes about 159 gallons of inner-tank water.
What's the difference between outer gallons and actual water?
Outer gallons use outer dimensions. Actual water also excludes glass, rim gap, substrate, hardscape, and equipment. On a 6 × 2 × 2 ft tank with typical substrate and rock, actual water can be 25–35 gallons less than outer gallons — roughly 140–150 gallons.
Can I use this for round tanks?
Yes — use π × r² × H × 7.48052 with inner radius and inner height. For a cylinder with
inner diameter 2 ft (r = 1 ft) and water depth 2 ft: π × 1 × 2 × 7.48052 ≈ 47.0 U.S. gallons.
What if my tank isn't a perfect rectangle?
Use the shape-specific formula in the "Other Tank Shapes" table. For bowfront tanks, use the 0.85 factor rather than averaging front and back — averaging overstates the volume. For truly irregular tanks, use the fill-and-measure method or the manufacturer's stated volume.
Is U.S. gallon the same as Imperial gallon?
No. 1 U.S. gallon = 3.785 L; 1 Imperial gallon = 4.546 L. That is about a 20% difference. This calculator uses U.S. gallons. If a product label uses Imperial gallons, convert to liters for comparison (or divide the U.S. gallon figure by 1.201).
How do I convert to Imperial gallons?
Multiply cubic feet by 6.22883 (instead of 7.48052), or multiply U.S. gallons by 0.8327. For a 6 × 2 × 2 ft tank: 24 ft³ × 6.22883 ≈ 149.5 Imperial gallons (vs 179.5 U.S. gallons).
Can I convert gallons to liters?
Yes. Multiply U.S. gallons by 3.78541. For a 6 × 2 × 2 ft tank at 179.5 U.S. gallons, that is ~679 liters. The feet to liters calculator does this directly.
Is this calculator a substitute for the manufacturer's specification?
No. The manufacturer's spec sheet gives the exact outer dimensions, glass thickness, and rated volume for a specific model. Use this calculator for a general estimate or to check a spec sheet — but for stocking or dosing, rely on measured (or spec-sheet) actual water volume. Related tools: volume calculator, dimensions to gallons, gallon to liter.
Sources and references
- NIST Handbook 130 — Uniform Regulation for the Method of Sale of Commodities (unit definitions)
- Manufacturer dimension and volume specifications — Aqueon, Seapora, Waterbox
- OATA — Ornamental Aquatic Trade Association husbandry guidance