Fish Tank Volume Calculator (Litres)
Tip: Use internal dimensions for the most accurate water volume.
Why knowing litres in a fish tank matters
Tank volume is more than a number on a product label. It directly affects water quality, fish stocking levels, filtration requirements, heating, dosing, and maintenance frequency. If you underestimate volume, you can overdose water treatments or under-size your filter. If you overestimate volume, you may add too many fish and create unnecessary stress in the system.
This calculator helps you quickly find tank volume in litres, then converts the result to both US gallons and UK gallons for convenience.
How to calculate fish tank litres
1) Rectangular tanks
Most aquariums are rectangular. For these, multiply length × width × water height to get cubic volume, then convert to litres.
If your measurements are in inches, feet, or metres, convert them to centimetres first (the calculator does this automatically).
2) Cylindrical tanks
For round tanks, use diameter and water height:
Cylindrical calculations are useful for column tanks, round display tanks, and some custom builds.
Worked examples
Example A: Standard rectangular tank
- Length = 90 cm
- Width = 45 cm
- Water height = 45 cm
Volume = 90 × 45 × 45 = 182,250 cm³
Litres = 182,250 ÷ 1000 = 182.25 L
Example B: Cylindrical tank
- Diameter = 40 cm
- Water height = 50 cm
Radius = 20 cm
Volume = π × 20² × 50 = 62,832 cm³ (approx)
Litres = 62,832 ÷ 1000 = 62.83 L
Why actual water volume can differ from tank rating
The nominal size printed on a tank box often reflects external volume under ideal conditions. Real water volume can be lower due to:
- Glass thickness reducing internal dimensions
- Not filling all the way to the brim
- Substrate, rocks, wood, and decorations displacing water
- Internal filter compartments and backgrounds
For best accuracy, measure the internal water dimensions from the inside panes and to the real water line.
Using litre volume for better aquarium decisions
Filter sizing
A common freshwater target is 4x to 8x turnover per hour. For a 150 L tank, this means about 600 to 1,200 L/h total filtration flow (before real-world media and head-loss effects).
Heater selection
A quick planning rule is roughly 1 watt per litre in cooler rooms, and less in stable warm homes. Always verify with species needs and ambient room temperature swings.
Medication and conditioner dosing
Most products dose per litre. Accurate volume helps prevent overdosing sensitive species and keeps treatment effective. When in doubt, dose to actual water volume, not gross tank volume.
Frequently asked questions
Should I use external or internal tank dimensions?
Use internal dimensions for best accuracy, especially when dosing treatments.
What is the litres-to-gallons conversion?
- 1 US gallon = 3.785 litres
- 1 UK gallon = 4.546 litres
Do saltwater tanks use the same volume formulas?
Yes, geometric volume is the same. But reef systems often include sumps and complex rock displacement, so total system water volume needs extra measurement.
Can I calculate odd shapes?
For custom tanks, break the shape into simpler sections (rectangles/cylinders), calculate each volume separately, and add them together.
Final tip
Recheck tank volume whenever you redesign hardscape, change substrate depth, or alter water level. A small update in litres can improve stocking choices, dosing safety, and long-term fish health.