Knowing your tank's exact size helps with almost every decision: the stand you buy, the heater and filter you choose, and how much medicine or fertiliser to dose. Measuring takes two minutes with a tape measure.

What you need

  • A tape measure, ideally a rigid one
  • A notepad or your phone
  • The aquarium volume calculator to turn the numbers into gallons and litres

Measure length, width and height

  1. Length is the longest side, measured across the front.
  2. Width (also called depth) runs from the front glass to the back glass.
  3. Height runs from the bottom glass to the top rim. Don't include the stand or the lid.

Write down each measurement to the nearest quarter inch or half centimetre. Most tanks are sold in imperial sizes in the US, so a standard 20 gallon long measures about 30 x 12 x 12 inches.

Inside or outside measurements?

It depends on what you need the numbers for:

  • Outside measurements tell you whether the tank fits a space, a stand or a cabinet. Size charts, including our aquarium size chart, list outside measurements.
  • Inside measurements give the true water volume. Aquarium glass is commonly 3 to 12 mm thick, thicker on bigger tanks, so a large tank holds noticeably less than its outside size suggests.

For dosing, go one step further and measure from the top of the substrate to the water line. The real water volume calculator does this for you if you enter glass thickness, substrate depth and the gap below the rim.

Measuring bowfront tanks

A bowfront has a curved front, so the width changes along its length. Take three measurements as well as the height:

  • The length along the straight back
  • The depth at each end, front to back
  • The depth at the middle of the bow, the widest point

Choose Bowfront in the volume calculator and enter all of them. It adds the curved section to the rectangle behind it.

Measuring corner, hexagon and cylinder tanks

  • Corner tanks: measure one of the two straight back sides and the height.
  • Hexagon tanks: measure the length of one of the six sides and the height.
  • Cylinder tanks: measure the diameter across the widest point and the height.

Turn measurements into gallons

For a rectangular tank, multiply length x width x height in inches and divide by 231. A 48 x 13 x 21 inch tank works out to 13,104 cubic inches, or 56.7 US gallons. That's the standard 55 gallon tank. In centimetres, multiply the three numbers and divide by 1,000 for litres.

Frequently asked questions

Do you measure a fish tank from the inside or outside?

Measure the outside when you need to know whether a tank fits a space or stand. Measure the inside when you need its true volume for dosing medicine or fertiliser.

How do I find out what size my aquarium is?

Measure length, width and height in inches, multiply them, and divide by 231 for US gallons. Then compare the result with the standard tank size chart.

How do I measure a tank that is already set up?

Measure the outside length and width, then measure from the top of the substrate to the water line for the water height. Use those in the volume calculator for the water you actually have.