How Much Water a Tank Really Holds: A 60 cm Standard Tank Is About 52 L, 20% Below the Catalog

Tank volume = inner width × depth × water level ÷ 1,000

To get volume in liters, multiply width, depth and height in centimeters and divide by 1,000, because 1,000 cm³ is exactly 1 L. For a 60 cm standard tank (outside 60×30×36 cm), that's 60 × 30 × 36 ÷ 1,000 = 64.8 L. The "about 65 L" in catalogs and shops is close to this figure.

However, this calculation uses the tank's outside dimensions. Water only fits inside the glass, and it isn't filled right to the rim. Substrate takes up room with its grains, and stones and driftwood take up room too. It's more accurate to think of the outside figure as the size of the box, not the amount of water it holds.

• Glass thickness: width and depth lose both sides, and height loses the one bottom pane
• Surface gap: water usually stops 2–3 cm below the top rim (for lids, lighting and splashing)
• Substrate: water falls by the volume of the grains themselves. Water fills the gaps between grains
• Layout: stones, driftwood and plants take up space. In a rock-built tank, this can be 10% or more

Checking a 60 cm standard tank: from 64.8 L down to about 52 L

Here is the volume of a 60 cm standard tank (5 mm glass) with 4 cm of Oiso sand and the surface 3 cm below the rim, subtracting one step at a time. It uses the same formula as the Aquarium Water Volume Calculator.

The largest drop comes from the surface gap: just 3 cm removes about 5 L. Four centimeters of substrate has a volume of about 6.8 L, but water fills about 40% of the space between Oiso sand grains, so the water only falls by about 4.1 L. As a result, a 60 cm standard tank with substrate actually holds about 52 L, around 80% of the catalog figure.

Actual water volume by standard size

For commonly sold sizes, this table shows the outside volume, the volume inside the glass with the surface 3 cm down (bare tank) and the volume with 4 cm of Oiso sand. Glass thickness is calculated at 4–10 mm according to size.

The smaller the tank, the bigger the gap from the outside figure. In a 20 cm cube, the glass thickness and the 3 cm at the surface take up a large share of the 20 cm height, so the actual volume is about two-thirds of the outside figure. When medicating or salting a small tank, be especially careful not to use the outside figure.

Even a "60 cm tank" can differ in height and depth by manufacturer. For your own tank, measure the outside dimensions with a tape measure and check the glass thickness before calculating. Amounts for other substrate types and depths are covered in Calculating How Much Substrate You Need.

How many fish the volume allows

You can estimate how many fish to keep from common water-per-fish guidelines: 1 L per 1 cm of body length for small tropical fish, 1 L per medaka, 10 L per goldfish and 0.5 L per small shrimp such as cherry shrimp. Counting by actual volume with 4 cm of Oiso sand gives the following.

• Count body length at adult size, not at the size when you buy them
• For about the first month after setup, start at half the guideline or less until the filter bacteria have grown
• Tanks with strong filtration or frequent water changes have more leeway. If you tend to skip water changes, keep fewer
• These guidelines don't directly apply to territorial fish, fish that grow large or fish you want to swim in schools

Base water changes and dechlorinator on actual volume too

Water changes are usually described as a proportion, such as "one-third a week," and that proportion also applies to the actual volume. A 30% change on a 60 cm standard tank (51.5 L of actual water) is about 15.5 L, or 2 loads of a 10 L bucket. Using the outside 64.8 L gives about 19.4 L, which throws off how many buckets you prepare.

1. 1

   Work out how much to change

   Actual volume × proportion changed. For 30% on a 60 cm standard tank, 51.5 × 0.3 = about 15.5 L.

2. 2

   Work out the dechlorinator dose

   For a product labeled "1 mL per 10 L of water," that's 15.5 L ÷ 10 × 1 = about 1.5 mL. Calculate from the new water going in; there's no need to dose for the whole tank.

3. 3

   Use the full volume for setups and full changes

   When setting up a tank, dose for the full actual volume (about 5.2 mL for a 60 cm standard tank).

Once you know the volume, you can also decide on heater wattage and filter flow. Read on in Choosing Heater Wattage and Filter Flow; for concerns about where to put the tank, see Tank Weight and Floor Load; and for salt amounts when fish are unwell, see How Much Salt for a Salt Bath.

FAQ

How many liters does a 60 cm tank really hold?

A 60 cm standard tank (60×30×36 cm, 5 mm glass) holds about 55.6 L with the surface 3 cm below the rim, and about 51.5 L with 4 cm of Oiso sand. That's about 80% of the 64.8 L calculated from the outside dimensions.

How do I calculate a tank's water volume?

Inner width × depth × water level (all in cm) ÷ 1,000 gives liters. Subtract the glass thickness from the outside dimensions to get the inner size, and use the height up to the water surface. If you've added substrate, subtract the volume of the grains themselves.

How much does substrate reduce the water volume?

By the substrate's volume minus the water that gets into the gaps between grains. With 4 cm of Oiso sand in a 60 cm standard tank, the substrate has a volume of about 6.8 L, but water fills the gaps, so the water only falls by about 4.1 L.

How many liters does a 30 cm cube tank hold?

It's 27 L from the outside dimensions of 30×30×30 cm, but about 22.3 L after subtracting 5 mm glass and a 3 cm surface gap, and about 20.3 L with 4 cm of Oiso sand.