Water & Containers
Fish Tank Volume Calculator
Estimate aquarium capacity from internal dimensions, water depth and displacement.
Enter your measurements
Results update as you type.
About this calculator
Enter the tank's internal length, width and height, plus the actual water depth. Internal dimensions exclude glass thickness. Subtract the volume of substrate and decorations as displacement to see the real water volume.
How to use this calculator
- Measure inside the glass: internal length, width and height.
- Measure the actual water depth.
- Estimate displacement from substrate and decorations if significant.
- Calculate for full capacity and actual water volume.
Formula
Full capacity = internal L × W × H
Water volume = L × W × water depth − displacement
1 L = 1000 cm³ exactly; 1 US gal = 231 in³; 1 imp gal = 4.54609 L
Assumptions
- Internal dimensions only — glass thickness is excluded by measuring inside.
- Displacement larger than the water volume is rejected as impossible.
- US and imperial gallons are labeled separately.
Common mistakes
- Using exterior glass dimensions for water volume.
- Ignoring the waterline, substrate and decoration displacement.
- Using litres alone to decide safe animal stocking.
Worked example
Try it: press “Load example” above the form.
36 × 18 × 24 in is 15,552 in³; at 231 in³ per US gallon, that is about 67.3 US gal.
Limitations
No livestock, dosing or water-treatment advice.
Frequently asked questions
Why is actual water volume lower than tank capacity?
The internal tank geometry gives a gross capacity. Fill level, substrate and decorations can reduce the water volume. Use the actual waterline and any exclusions rather than exterior glass measurements.
Can this determine how many fish I can keep?
No. Stocking depends on species, adult size, filtration, oxygen, behaviour and maintenance. A geometric water volume is only one input into aquarium planning.
What does the worked example show?
36 × 18 × 24 in is 15,552 in³; at 231 in³ per US gallon, that is about 67.3 US gal.
Which assumptions should I check before using the result?
No livestock, dosing or water-treatment advice.
Sources
Sources explain the methods and any named product presets. Check your product label where coverage, yield or density differs.
Calculation documentation · Updated 2026-11-08 · Version 1.6.4. Estimates are planning aids. Confirm quantities, coverage and prices with your supplier before purchasing. Found an error? Report it.
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