Calculator

Aquarium Calculator

Work out true water volume, water-change amounts, and unit conversions — with every step shown.

What you’ll need

Your tank’s internal dimensions, or a volume you already know

What your tank actually holds

1 Start from a standard tank

Or enter your own measurements below.

2 Tank shape

3 Measure inside the glass

Length

Width (front to back)

Water height

Use the water height, not the full height of the tank.

4 What isn’t water

Substrate

Rock & wood

Gap below rim

Defaults are rough estimates, not measurements. Adjust them to your tank.

Enter length, width (front to back), water height to see your volume.

These are arithmetic tools. They do not know what is in your tank and make no claim about what is safe to add to it.

Why the number on the box is the wrong number

Aquariums are sold by approximate gross capacity: the space inside the glass, filled to the brim, with nothing in it. Nobody runs a tank that way. Substrate sits in the bottom, hardscape displaces more, and there is always a gap below the rim.

The gap between those two figures is larger than most people expect. A standard 30 × 12 × 12 inch tank sold as 20 gallons measures about 18.7 US gallons brim-full and empty — and holds closer to 15 gallons of water once a normal substrate bed and rim gap are accounted for.

That matters when a dose is given per unit of water. Treating a nominal 20 gallons when the tank holds 15 means adding roughly a third more product than the label intends. This calculator exists to give you the figure the label actually wants.

Common questions

Why is my tank’s real volume lower than the number on the box?: A tank is sold by its approximate gross capacity — the volume inside the glass if you filled it to the very top with nothing in it. Substrate, rocks, wood and the gap you leave below the rim all take up space that never holds water. On a standard 30 × 12 × 12 inch "20 gallon" tank with a typical substrate bed, the water volume is nearer 15 US gallons than 20.
Does that matter when dosing a treatment?: It can matter a great deal. Dose instructions are given per unit of water. If you dose a "20 gallon" tank that actually holds 15 gallons of water, you have added roughly a third more product than the label intends. Always dose against your real water volume, and follow the product label rather than any figure produced here.
What should I enter for substrate and hardscape?: Those fields are your estimate, not ours. The defaults are conservative starting points, not measurements. A bare tank might be close to zero; a deeply planted tank with a thick soil bed and large hardscape could be well above the defaults. Adjust them to what you can see in your own tank.
Should I measure the outside of the tank?: No — measure the internal dimensions, inside the glass, and use the height of the water rather than the full height of the tank. Glass thickness is not water.

This tool performs arithmetic only. It holds no information about your livestock and makes no claim about what is safe to add to your tank. For that, use the treatment records — every verdict there is quoted from the manufacturer.

Related tools

Tank Profile

Save what lives in your tank once, then check any treatment against all of it at once.

Treatment Comparison

Put two or more treatments side by side: documented verdicts, ingredients, evidence quality and gaps.

Why the number on the box is the wrong number

Common questions

Why is my tank’s real volume lower than the number on the box?

A tank is sold by its approximate gross capacity — the volume inside the glass if you filled it to the very top with nothing in it. Substrate, rocks, wood and the gap you leave below the rim all take up space that never holds water. On a standard 30 × 12 × 12 inch "20 gallon" tank with a typical substrate bed, the water volume is nearer 15 US gallons than 20.

Does that matter when dosing a treatment?

It can matter a great deal. Dose instructions are given per unit of water. If you dose a "20 gallon" tank that actually holds 15 gallons of water, you have added roughly a third more product than the label intends. Always dose against your real water volume, and follow the product label rather than any figure produced here.

What should I enter for substrate and hardscape?

Those fields are your estimate, not ours. The defaults are conservative starting points, not measurements. A bare tank might be close to zero; a deeply planted tank with a thick soil bed and large hardscape could be well above the defaults. Adjust them to what you can see in your own tank.

Should I measure the outside of the tank?

No — measure the internal dimensions, inside the glass, and use the height of the water rather than the full height of the tank. Glass thickness is not water.