Aquarium water volume:
litres from length, width and water height

Why a tank's label size is not the water you have

A tank sold as 120 litres may only contain about 105 once it is set up. The label usually reports the brimful capacity of a bare glass box, measured to the top rim. In daily use you have glass thickness, a water level a few centimetres below the rim, a layer of gravel or sand, rocks, wood and filters taking up space. That gap matters when you dose water conditioner, medication or salt, all of which are specified per litre.

The way round it is to measure for yourself and do the multiplication.

The formula in centimetres

Measure inside the glass, not the outside. Two sheets of 6 mm glass take 12 mm from each dimension, which on a small tank is a visible fraction of the total. For the height, measure from the inner base to the waterline you actually run, not the rim.

Worked example: 80 × 35 × 40 cm

In US gallons that is 112 ÷ 3.785 = about 29.6. Once you subtract decor and substrate, expect something closer to 100 litres of real water, so a rule of thumb of 10 to 15 % off the geometric volume is reasonable for a planted or rock-heavy tank.

Subtracting what takes up space

Substrate is the biggest item. A 5 cm layer of sand across the base of this tank is 80 × 35 × 5 = 14,000 cm³, which is 14 L of volume. Because water fills the gaps between grains, the water displaced is less than that: perhaps 8 to 10 L of real loss, depending on how tightly the grains pack.

Hardscape is harder to estimate. For rocks and wood, fill a bucket to a known mark, submerge the item and note how much the level rises, or weigh it dry and divide by about 2.5 for typical rock to get a rough litre figure. Add all of it and subtract from the geometric result.

Other tank shapes

• Cylinder: π × radius² × water height, with the radius in cm, then ÷ 1000. A 30 cm wide, 40 cm tall bowl is about 3.1416 × 15² × 40 ÷ 1000 = 28.3 L.
• Bow-front: measure the straight part as a rectangle, then add the curved bulge, roughly one half the bulge depth times width times height.
• Hexagon and corner tanks: split into triangles or use manufacturer figures.
• Tanks with a sump: add the sump's water volume at its working level, since it also holds fish and needs dosing.

A rectangle-area and cylinder-volume calculation can reproduce the base maths if you prefer to double-check the arithmetic.

Dosing, water changes and what volume means for them

Treatments are written per litre, and so are water changes. A 25 % weekly water change on a 100 L working volume is 25 L, which is two and a half 10 L buckets. Conditioner is usually dosed on the new water you add, not the whole tank, but medication and salt are dosed on the full volume, which is where the real figure matters most.

To take a worked case, a medication that asks for 1 tablet per 40 litres would be 112 ÷ 40 = 2.8 tablets using the geometric volume, but only 100 ÷ 40 = 2.5 for the real water. Since the tablets can rarely be split exactly, the safer course is to round the real volume down and follow the manufacturer's advice for borderline cases. A difference of 12 L is also easy to measure directly by filling with a marked bucket.

A bucket test for the true figure

If you need a precise number, the most reliable method is to measure the water you add. When setting up the tank, fill it using a bucket of known size, such as 10 litres, and count the buckets. Mark the waterline you run. Do this after adding substrate and hardscape, and your count is the true working volume, including every displacement.

If the tank is already running, you can still do it during a large water change. Drain to a mark, note the number of litres removed, and compare that with the geometric volume between the mark and the water surface. Dividing the litres removed by the height drop in centimetres gives litres per centimetre of height, which for the 80 × 35 cm tank is 80 × 35 ÷ 1000 = 2.8 L per cm. That figure lets you convert any change in level straight into litres.

Keep in mind that tank glass bows slightly under load, so a very long tank may hold a litre or two more than its straight-edged geometry suggests, and a tank sitting on an uneven stand may have a shallow end that overflows before the deep end is full. A bubble level on the rim before filling saves trouble later.

Write the working volume and the date on a sticker or in a notebook along with the filter and heater models. When you need to treat a sick fish at midnight, having the figure to hand removes a source of mistakes.

Using the volume for fish and equipment

Do not use volume alone to decide stocking. Filtration, oxygen exchange and the fish's adult size matter more than a litres-per-fish rule. The old one-inch-of-fish-per-gallon rule is unreliable and best avoided.

Heaters are normally sized by watts per litre, with 1 W per litre a common starting point for rooms at moderate temperatures; a 100 L real volume suggests a 100 W heater, rounded up to a standard size. Check the manufacturer's guidance, and for filtration look at the rated turnover, usually 3 to 5 times the tank volume per hour.