Aquarium Volume Calculator: Calculate Your Fish Tank's True Water Volume
Calculate the exact water volume of rectangular, bow-front, hexagonal, and cylindrical aquariums. Account for substrate and decorations for accurate dosing.
Knowing your aquarium's actual water volume isn't pedantic — it's essential. Medication dosing, fertilizer amounts, water conditioner quantities, and stocking limits all depend on accurate volume. And the volume printed on a tank's label is the total volume, not the actual water volume you're working with after adding substrate, hardscape, and decor.
The CalcHub Aquarium Volume Calculator calculates volume for multiple tank shapes and lets you subtract estimated displacement from substrate and decorations.
Volume Formulas by Tank Shape
Rectangular Tank
Volume (gallons) = Length × Width × Height (inches) ÷ 231A standard 55-gallon tank: 48" × 12" × 20" ÷ 231 = 49.8 gallons actual
(The nominal "55-gallon" is the full tank to rim; actual working volume is lower.)Bow-Front Tank
Bow-front tanks bow outward at the center. The calculator accounts for this using the maximum depth at the widest point of the bow.Hexagonal Tank
Volume = (3√3 / 2) × side length² × height ÷ 231Cylinder/Column Tank
Volume (gallons) = π × radius² × height ÷ 231| Tank Type | Nominal Size | Actual Water Volume |
|---|---|---|
| Standard rectangular | 10 gal | ~8.5–9 gal |
| Standard rectangular | 29 gal | ~25–27 gal |
| Standard rectangular | 55 gal | ~49–51 gal |
| Standard rectangular | 75 gal | ~68–70 gal |
| Tall hexagonal | 35 gal | ~28–30 gal |
Subtracting Displacement
After calculating geometric volume, subtract:
| Material | Approximate Gallons Displaced |
|---|---|
| 1 inch of gravel substrate | ~1.5 gal per 10 sq. inches of footprint |
| Large rocks/stones | Measure water level before and after |
| Driftwood (medium piece) | 0.5–1.5 gal |
| Coral/resin decor (medium) | 0.2–0.8 gal |
Why This Matters
Medication dosing: Most treatments specify "per gallon of tank water." Using the nominal size can under- or overdose your fish — overdosing is potentially fatal. Water conditioner: Sodium thiosulfate-based dechlorinators need precise volumes for full neutralization. Fertilizer/CO2 (planted tanks): Fertilizer dosing instructions depend on water volume for nutrient targets. Stocking rules: The common "1 inch of fish per gallon" rule (though oversimplified) uses actual water volume.Is the "1 inch per gallon" stocking rule reliable?
It's a very rough starting point and fails for larger fish (a 10-inch Oscar and a 10-inch school of Neon Tetras are obviously different bioloads). Use the fish tank stocking calculator for more meaningful guidance.
How do I measure my tank's actual volume without math?
Fill it with a known container (like a 1-gallon jug) and count. Slower but accurate. The calculator is faster once you have the dimensions.
Does water temperature affect volume calculations?
Not meaningfully for practical fishkeeping purposes — the expansion of water between 68°F and 80°F is less than 0.3%.
Related Calculators
- Fish Tank Stocking Calculator — Determine safe fish quantities for your volume
- Pet Medication Calculator — Accurate dosing based on true water volume
- Pet Calorie Calculator — Also handles fish feeding amounts