How to Calculate Aquarium Volume in Gallons & Liters
Calculating exact aquarium water capacity is essential for dosing water conditioners, sizing canister or hang-on-back filters, and choosing proper heater wattage. The fundamental conversion constant for liquid US gallons is 231 cubic inches per US liquid gallon.
Rectangular Tank Formula
Gallons = (L × W × H) ÷ 231
Multiply length, width, and height in inches, then divide by 231 cubic inches.
Cylinder Tank Formula
Gallons = (π × Radius² × H) ÷ 231
Multiply Pi (3.14159) by squared radius and height in inches, then divide by 231.
Standard Fish Tank Dimensions, Volume & Weight Chart
Use this master reference table to look up standard aquarium sizes sold by major manufacturers (Aqueon, Marineland, Fluval).
| Standard Size | Dimensions (L x W x H) | Actual Volume | Filled Water Weight | Substrate (2" Bed) |
|---|---|---|---|---|
| 10 Gallon Standard | 20" x 10" x 12" | 10.4 Gal / 39 L | 87 lbs (39 kg) | 16 lbs |
| 15 Gallon Rimless Cube | 15" x 15" x 15" | 14.6 Gal / 55 L | 122 lbs (55 kg) | 18 lbs |
| 20 Gallon Long | 30" x 12" x 12" | 18.7 Gal / 71 L | 156 lbs (71 kg) | 29 lbs |
| 29 Gallon Standard | 30" x 12" x 18" | 28.1 Gal / 106 L | 234 lbs (106 kg) | 29 lbs |
| 40 Gallon Breeder | 36" x 18" x 16" | 44.9 Gal / 170 L | 374 lbs (170 kg) | 52 lbs |
| 55 Gallon Standard | 48" x 13" x 21" | 56.7 Gal / 215 L | 473 lbs (215 kg) | 50 lbs |
| 75 Gallon Standard | 48" x 18" x 21" | 78.5 Gal / 297 L | 655 lbs (297 kg) | 69 lbs |
| 125 Gallon Long | 72" x 18" x 22" | 123.4 Gal / 467 L | 1,029 lbs (467 kg) | 104 lbs |
Aquarium Water Weight & Floor Structural Safety Guide
Water weighs exactly 8.34 pounds per US liquid gallon (1.0 kg per liter). When planning large aquariums in residential homes, you must account for the weight of the water, heavy glass tank walls, substrate bed, hardscape rocks, and wooden stand.
Floor Joist Placement Checklist
- Place aquariums over 55 gallons perpendicular across multiple floor joists rather than parallel on a single joist.
- Position large tanks against interior load-bearing structural walls for maximum load distribution.
- Ensure wooden aquarium stands feature a flush flat bottom base to prevent localized floor pressure points.
Frequently Asked Questions
How do I calculate the volume of a rectangular fish tank in US gallons?
To calculate the volume of a rectangular aquarium in US gallons, multiply length by width by height in inches (L x W x H), then divide the total cubic inches by 231. For example, a 30" x 12" x 12" tank equals 4,320 cubic inches / 231 = 18.7 US Gallons.
How much does a filled 55-gallon fish tank weigh?
A 55-gallon freshwater aquarium filled with water weighs approximately 458 lbs (208 kg) for the water alone. When including the glass tank frame, gravel substrate, and rocks, the total filled weight reaches 600 to 650 lbs (272 to 295 kg).
How much gravel or sand substrate do I need for my aquarium?
A general rule of thumb is 1.5 to 2 pounds of gravel per gallon of water, or approximately 0.08 pounds of substrate per square inch of bottom surface area to achieve a standard 2-inch (5 cm) deep substrate bed.
Can my wooden floor or second-floor room hold a 75-gallon aquarium?
A filled 75-gallon tank weighs around 850 to 900 lbs. Placing a tank of this weight parallel across multiple floor joists against a load-bearing wall is safe, but tanks over 100 gallons placed in the middle of a room may require floor joist reinforcement.
What size filter (GPH) and heater (Watts) do I need for my tank?
Choose an aquarium filter with a turnover rate of at least 5 times your tank volume per hour (e.g. a 20-gallon tank requires a 100 GPH filter). For heating, use 5 Watts per gallon of water in standard room temperatures.
What is the 1 inch of fish per gallon stocking rule?
The 1 inch of slender fish per 1 to 1.2 gallons rule applies to small community fish like neon tetras. Deep-bodied fish like goldfish or cichlids create much higher bio-loads and require 1 inch of fish per 2 to 3 gallons.