hobby calculator
Estimate fish tank capacity in gallons, liters, and practical water volume from dimensions.
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Usable Gallons
40.39 gal
Save this page to size filters, heaters, water changes, and stocking plans from actual tank dimensions.
Formula
Volume = Length × Width × Height. US Gallons = Cubic Inches ÷ 231. Liters = Cubic Centimeters ÷ 1000.The calculator measures gross tank volume first, then applies a fill percentage to estimate practical water capacity.
Step by step
Related guides
Guide
Learn how to calculate aquarium volume in gallons and liters, estimate practical fill capacity, and size equipment using real tank dimensions.
Site Guide
Learn how to use calculators on Do The Calculation, enter inputs, read results, explore formulas, and navigate between related tools for comprehensive planning.
Summary
Usable Gallons
40.39 gal
Practical volume at the selected fill level.
Usable Liters
152.91 L
Metric practical volume.
Gross Gallons
44.88 gal
Full geometric capacity before displacement.
D3 visualization
Compare the full geometric volume against practical water volume at the chosen fill level.
Guide
An aquarium is fundamentally a three-dimensional container, so the starting calculation is rectangular prism volume: length multiplied by width multiplied by water height. This gives gross geometric capacity, which can then be converted into liters or gallons depending on the system used.
For a rectangular tank measured in inches, gross cubic volume is:
$$V = L \times W \times H$$
To convert cubic inches into US gallons, divide by 231:
$$\text{Gallons} = \frac{LWH}{231}$$
For metric dimensions measured in centimeters, cubic centimeters are converted into liters by dividing by 1000:
$$\text{Liters} = \frac{LWH}{1000}$$
These formulas produce the gross theoretical capacity before gravel, décor, safe fill margin, and filtration hardware reduce the actual usable water volume.
Advertised aquarium size is often a gross geometric number. Actual water volume is usually smaller because tanks are not filled perfectly to the rim and because substrate, rocks, driftwood, filters, and internal equipment displace water. Practical planning should therefore distinguish between gross capacity and usable volume.
Capacity affects filtration, heater selection, medication dosing, water conditioner calculations, and stocking decisions. A tank that is mismeasured by even a modest margin can produce underpowered equipment choices or incorrect chemical dosing. This is why capacity math is central to responsible aquarium setup and maintenance.
Gross capacity
The full geometric volume of the tank before displacement and fill-margin adjustments.
Usable volume
The actual water volume available after substrate, décor, and non-filled headspace are considered.
Turnover rate
The number of times per hour that the filter processes a tank’s water volume.
Multiply length, width, and water height to get cubic volume, then convert to gallons or liters.
Divide cubic inches by 231 to get US gallons.
Divide cubic centimeters by 1000.
Because substrate, decorations, equipment, and safe fill margin reduce usable water capacity.
Use actual water height, not total glass height, because tanks are rarely filled exactly to the top.
Heater wattage recommendations depend on water volume, so inaccurate capacity estimates can lead to underheating or overheating risk.
Filter turnover recommendations are usually expressed relative to total water volume per hour.
Yes. Once total volume is known, partial water-change percentages can be translated into exact amounts.
This version is best for rectangular tanks. Curved, cylindrical, or bow-front tanks need different geometry adjustments.
Because medication, conditioner, and fertilizer dosages should match the water actually present, not just the gross glass dimensions.