Calculate Aquarium Volume
Select your tank shape, enter dimensions, and get water volume, weight, substrate displacement, stocking guidelines, and equipment recommendations instantly.
These are general guidelines. Always research specific species requirements. Overstocking stresses fish and degrades water quality.
Bar shows stocking level if you keep the recommended number of small tropical fish. Adjust based on your actual fish species and filtration.
Common Aquarium Sizes Reference
Click any row to auto-fill the calculator with that tank's dimensions.
| Tank Name | Gallons | L x W x H (in) | Weight (full) | Heater |
|---|---|---|---|---|
| 10 Gallon | 10 gal | 20 x 10 x 12 | ~111 lbs | 50W |
| 20 Gallon High | 20 gal | 24 x 12 x 16 | ~225 lbs | 75W |
| 20 Gallon Long | 20 gal | 30 x 12 x 12 | ~225 lbs | 75W |
| 29 Gallon | 29 gal | 30 x 12 x 18 | ~330 lbs | 100W |
| 40 Gallon Breeder | 40 gal | 36 x 18 x 16 | ~455 lbs | 150W |
| 55 Gallon | 55 gal | 48 x 13 x 21 | ~625 lbs | 200W |
| 75 Gallon | 75 gal | 48 x 18 x 21 | ~850 lbs | 250W |
| 90 Gallon | 90 gal | 48 x 18 x 24 | ~1050 lbs | 300W |
| 125 Gallon | 125 gal | 72 x 18 x 21 | ~1400 lbs | 2x200W |
How Aquarium Volume Is Calculated
Rectangular Tank
Volume = Length x Width x Height / 231 (US gallons)
For metric: Length (cm) x Width (cm) x Height (cm) / 1000 = liters. The inner dimensions are used after subtracting glass thickness from each side.
Bow-Front Tank
A bow-front tank is modeled as a rectangular section plus a curved (half-ellipse) section at the front. The extra volume from the bow is calculated as:
Bow extra = (pi/4) x Length x (BowDepth - FlatWidth) x Height
Cylindrical Tank
Volume = pi x (Diameter/2)^2 x Height / 231 (US gallons)
Hexagonal Tank
Volume = (3 x sqrt(3) / 2) x Side^2 x Height / 231 (US gallons)
Substrate Displacement
Substrate (gravel, sand, soil) occupies space that would otherwise hold water. This calculator estimates substrate volume based on depth and base area, then subtracts ~60% for the water that still fills the gaps between substrate particles (porosity factor of ~0.4 for typical aquarium gravel).
Water Weight
Freshwater weighs 8.34 lbs per US gallon (1 kg per liter). Saltwater at typical reef salinity (1.025 specific gravity) weighs approximately 8.56 lbs per gallon. Total setup weight includes the tank, stand, substrate, rocks, and decorations -- plan for the full weight when choosing a location.
Aquarium Setup Tips
Choosing the Right Tank Size
- Beginners: Start with at least 20 gallons. Larger tanks are more forgiving of water quality mistakes because they dilute toxins better.
- Planted tanks: Longer, shallower tanks provide better light penetration and more planting area.
- Saltwater/reef: Minimum 30 gallons recommended for stability. Larger volumes maintain more consistent salinity and temperature.
Weight Considerations
- A filled aquarium weighs roughly 10--12 lbs per gallon including tank, stand, substrate, and decorations.
- A 55-gallon tank can weigh over 600 lbs when fully set up. Verify floor load capacity before placement.
- Place tanks on a level surface near a wall, ideally on a ground floor for tanks over 40 gallons.
Equipment Sizing Rules
- Heater: 3--5 watts per gallon. Use two heaters for tanks over 75 gallons for redundancy and even heating.
- Filter: Turnover rate of 4x tank volume per hour for freshwater, 6--10x for saltwater. Oversizing filtration is always better than undersizing.
- Lighting: 1--2 watts per gallon for fish-only, 2--5 watts per gallon for planted tanks (LED equivalent).
Frequently Asked Questions
How do I measure the water level height?
Measure from the inside bottom of the tank to the actual water surface, not the top of the tank. Most tanks are not filled to the very top -- leave about 1 inch of space below the rim for splashing, surface agitation, and to prevent overflow.
Should I use inner or outer dimensions?
Use the outer dimensions you can easily measure with a tape measure. This calculator automatically subtracts glass thickness to compute the true inner volume.
How accurate is the 1-inch-per-gallon stocking rule?
It is a rough starting point for small, slim-bodied tropical fish like tetras and guppies. It does not work well for large-bodied fish (goldfish, cichlids), territorial species, or saltwater fish. Always research the specific needs of each species you plan to keep.
Why is actual water volume less than the rated tank size?
Manufacturers rate tanks by outer dimensions. Glass thickness, substrate, decorations, equipment (filters, heaters), and the air gap at the top all reduce the actual water volume. Expect 10--20% less water than the nominal rating.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.
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Privacy & Limitations
- All calculations run entirely in your browser -- nothing is sent to any server.
- Results are estimates and may vary based on actual conditions.
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Aquarium Volume Calculator FAQ
How do I calculate the volume of a rectangular aquarium?
Multiply length x width x height in inches, then divide by 231 to get US gallons. For centimeters, multiply length x width x height and divide by 1000 to get liters. A standard 20-gallon tank is typically 24 x 12 x 16 inches.
How much does aquarium water weigh?
Fresh water weighs approximately 8.34 pounds per US gallon (1 kg per liter). Salt water is slightly heavier at about 8.56 pounds per gallon due to dissolved salts. A 50-gallon freshwater tank weighs roughly 417 pounds of water alone, plus the weight of the tank, substrate, and decorations.
How many fish can I put in my aquarium?
A common rule of thumb is 1 inch of fish per gallon for freshwater tropical fish, though this is a rough guideline. Larger fish, aggressive species, and saltwater fish need more space. Always research specific species requirements and consider filtration capacity.
What size heater do I need for my aquarium?
The general guideline is 3-5 watts per gallon of aquarium water. For rooms kept at a stable temperature, 3 watts per gallon is usually sufficient. For cooler rooms or larger temperature differentials, use 5 watts per gallon. Tanks over 75 gallons benefit from two smaller heaters rather than one large one.
How much does substrate reduce aquarium volume?
Substrate typically displaces 5-15% of the total tank volume depending on depth and grain size. A 1-inch layer of gravel displaces about 5-7% of volume, while a 2-3 inch planted tank substrate can displace 10-15%. This calculator accounts for substrate displacement when estimating actual water volume.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.