How this calculation works
The Aquarium Volume & Water Weight Calculator computes gross and net water capacity in US gallons and liters, total filled tank weight (water + glass + gravel), and minimum filter Gallons Per Hour (GPH) flow ratings.
Mathematical formula and logic
Gross Gallons = (Length × Width × Height) / 231. Net Water Height = Height - Substrate - 1 inch top rim. Water Weight = Net Gallons × 8.34 lbs/gal (or 1 kg per Liter).
Worked example
A 36 × 18 × 18 inch aquarium has a gross volume of 50.5 US Gallons (191.1 Liters). With 2 inches of gravel and a 1-inch top margin (15 inch water column), net water volume is 42.1 Gallons (351 lbs water weight), requiring a minimum 170-210 GPH filter.
Calculation assumptions
- 1 US Gallon equals 231 cubic inches = 3.78541 Liters.
- Water density = 8.34 lbs/gallon (1 kg/Liter). Fresh water standard.
Frequently asked questions
How much does a filled aquarium weigh per gallon?
Fresh water weighs 8.34 pounds per gallon. Factoring in the weight of the glass tank, gravel substrate, and decorations, budget approximately 10 to 12 pounds per gallon of total filled aquarium weight.
What filter flow rate (GPH) is needed for my tank?
Aquarium filters should turn over the total tank volume 4 to 5 times per hour. For a 50-gallon aquarium, choose a filter rated for at least 200 to 250 Gallons Per Hour (GPH).
How many fish can I keep in my aquarium ('one inch per gallon' rule)?
The traditional '1 inch of adult fish per gallon' is a rough beginner guideline for slender community fish (like tetras). Larger, wide-bodied fish (like cichlids or goldfish) produce much more bioload and require significantly more water volume and heavy filtration.
Does sand or gravel displace water volume?
Yes, 2 inches of substrate in a 50-gallon tank displaces approximately 5 to 6 gallons of water volume, reducing actual swimming space.
How does glass thickness affect internal volume?
Thick glass (1/2-inch or 12mm) on large tanks reduces internal capacity by 5% to 8% compared to outer exterior dimensions.
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