Tool
Aquarium volume calculator
Estimate rectangular aquarium capacity from inside dimensions and fill level. See the formula and why geometric volume is not an exact medication volume.
Your result
About 49 litres
Geometric estimate: about 12.8 US gallons / 10.7 UK gallons. This does not subtract displacement or add connected equipment water. Not an exact medication volume.
What this calculator estimates
This tool estimates the geometric volume of a rectangular tank from its internal dimensions and the percentage of its height filled with water. It does not subtract the water displaced by substrate, rocks or equipment, and it does not add water in a connected filter, sump or pipework. The result is not a measured treatment volume or a medication dose.
Formula, units and defaults
Estimated litres = inside length in cm × inside width in cm × inside height in cm × (filled-to percentage ÷ 100) ÷ 1,000. For a rectangular tank, the fill percentage describes water height, not a deduction for decoration. If you enter the actual water depth as the height, use 100% so that you do not reduce the waterline twice.
The starting inputs are 60 × 30 × 30 cm and 90% filled. They are editable examples, not a recommended fill level. They give 48.6 L, displayed as about 49 litres. Gallon outputs use 0.264172 US gallons or 0.219969 UK gallons per litre and round to one decimal place.
Model: rectangular geometric volume v1; wording and limitations reviewed October 14, 2026. The calculation has not been calibrated against your aquarium.
When to use it
- Comparing rectangular tank capacities in litres and gallons.
- Getting a starting volume estimate when checking equipment specifications, alongside the equipment instructions and the needs of the inhabitants.
- Checking how an entered waterline changes geometric capacity. Volume alone does not determine suitable stocking or treatment.
How to use it
- Measure the inside dimensions in centimetres. Outside dimensions include glass and may overstate the internal space.
- Enter the waterline percentage. Use the proportion of internal height actually filled, or enter water depth as height with fill set to 100%.
- Read the result as an estimate. Check what occupies that space and which connected compartments belong to the system you are assessing. Those adjustments are not made by this calculator.
Worked example
For inside dimensions of 60 × 45 × 45 cm and 90% fill: 60 × 45 × 45 × 0.9 ÷ 1,000 = 109.35 L, displayed as about 109 litres. This is the space below the entered waterline before displacement, not proof that a decorated aquarium contains 109.35 L of water.
Before using a volume for medication
Merck's treatment considerations require accurate measurement of the water volume to be treated for a bath treatment. Do not treat this estimate as close enough for precise dosing. Verify the actual water in the treatment system, including only connected equipment that will share the treatment, and follow the exact product label and treating professional's instructions.
If the volume is uncertain, ask how to establish it before calculating a dose. The amount replaced during a partial water change does not establish the total treatment volume. Do not empty an occupied aquarium just to measure it. This tool cannot choose a medicine, check compatibility or determine a safe dose.
Limitations
- Shape matters. Bow-front, cylindrical and corner tanks need a shape-appropriate calculation; their bounding rectangle is not their water volume.
- Displacement is not measured. Substrate and solid objects reduce water within the measured space. The full outline volume of a porous gravel bed is not the same as its solid displacement, because water occupies the gaps.
- Connected volume is not included. A sump or external filter may add water if it is connected to the system being assessed; this tool has no input for it.
- Input and rounding affect the result. Check units and dimensions. A rounded number on screen is not additional measurement precision.
Frequently asked questions
Why does this differ from the size on the box?
Check whether the manufacturer's figure refers to internal capacity, external dimensions or a rounded nominal size. This estimate uses your internal measurements and fill percentage, but still does not account for displacement.
Do I use litres or gallons for dosing?
Follow the product's units and verify the actual treatment volume first. US and UK gallons are different; the conversions here do not make the underlying volume exact or establish a dose.
Does the fill percentage account for substrate?
No. It accounts only for water height in a rectangular tank. Displacement and connected equipment must be assessed separately; do not subtract a guessed gravel-bed volume and call the result precise.