An aquarium volume calculator exists to correct a number almost everyone uses wrongly. A tank described as 240 litres is being described by the box it makes in the air — outside length times outside width times outside height. The water inside it sits behind glass panes, above a bed of substrate, below the rim, and around whatever rock, wood and equipment you have put in. By the time all four of those are taken out, a 240 litre tank commonly holds somewhere near 180 litres of water, which is a quarter less than the label.
Arb Digital keeps a free tools library for the practical measurement problems people meet at home, and this one has consequences: every dose, every water change and every filter rating is expressed per unit of water, not per unit of tank. This page computes the real figure for the five tank shapes that account for nearly every aquarium sold, and shows you exactly where the missing volume went.
What This Aquarium Volume Calculator Does
Choose a shape, enter the outside dimensions in centimetres or inches, and set glass thickness, substrate depth, the gap you leave below the rim and a percentage for hardscape displacement. The headline result is the water volume in litres, with US and imperial gallons underneath. The supporting figures give the nominal size a retailer would quote from the same outside dimensions, the volume the substrate occupies, the volume lost to the rim gap and hardscape together, and the weight of the water on its own.
The boundary with the closest tool on this site is worth stating. The tank volume calculator handles industrial storage vessels — horizontal cylinders, capsule and cone-bottom tanks, and the partial volume at any liquid depth. It knows nothing about glass thickness or substrate. This page handles the shapes aquariums are actually built in and subtracts the four things that stop a display tank holding its rated volume. For a plain unit conversion with nothing subtracted, the volume converter is the simpler tool.
How to Use It
- Measure the outside, not the inside. Outside dimensions are what you can reach with a tape measure and what the manufacturer quotes. The calculator subtracts the glass for you.
- Pick the shape carefully. A bow front needs its bow depth — the distance the curved pane bulges past the line of the front edge. Cylinder and corner tanks use the length field as a diameter, and a hexagon uses it as one side length.
- Enter glass thickness in the same units. Common panes are 6, 8, 10 and 12 mm, which is 0.6, 0.8, 1.0 and 1.2 cm. On a large tank the glass alone can account for several litres.
- Set substrate depth honestly. Measure it at the front glass with a ruler after it settles, not from the bag's contents.
- Read the water figure, not the nominal figure. The gap between the two is the point of the page.
The Formula / How It's Calculated
Every shape reduces to a cross-sectional area multiplied by a water depth. For a rectangular tank the internal area is (length − 2 × glass) × (width − 2 × glass). A cylinder uses π × internal radius², a corner tank a quarter of that, and a regular hexagon (3√3 ÷ 2) × side². A bow front is a rectangle plus a circular segment, whose area comes from the chord and the bulge height using the standard segment formula set out in the MathWorld entry on the circular segment.
The water depth is outside height − base glass − rim gap − substrate depth. Multiply by the internal area, then reduce by the hardscape displacement percentage. Litres are cubic centimetres divided by 1,000; US gallons are litres divided by 3.785412 and imperial gallons litres divided by 4.546090, and inches convert at exactly 25.4 mm as defined by NIST's reference on SI units of length.
Worked example, using the values the page loads with. A 120 × 40 × 50 cm tank with 8 mm glass has an internal footprint of 118.4 × 38.4 cm = 4,546.6 cm². Interior height is 49.2 cm; take off 3 cm of rim gap and 5 cm of substrate and the water column is 41.2 cm deep. That gives 187,318 cm³, or 187.3 litres, and a 5% hardscape allowance brings it to 178.0 litres — about 47.0 US gallons. The nominal size from the outside dimensions is 240 litres. The tank holds 74% of what its label implies.
Why the Number on the Box Is Always Too Big
Four subtractions separate the label from reality, and they compound. Glass thickness removes a shell from every dimension: on the example above the panes alone account for roughly 8 litres. The rim gap removes another 13.6 litres, because you cannot fill to the very top — the water line has to sit below the rim so the surface does not slop over, and on a rimless tank it must sit below the point where surface tension gives out.
Substrate is the largest single loss in a planted tank. Five centimetres across a 118 × 38 cm footprint is 22.7 litres of gravel, and deeper aquascaping beds of 8 to 10 cm can take double that. The calculator treats substrate as fully displacing, which is slightly conservative: sand and gravel beds hold water in the gaps between grains, typically a third to two fifths of their bulk volume, so a little of that volume is still water. It is not water you can dose into or that circulates freely, so leaving it out of the total is the safer assumption.
Hardscape is the one people underestimate most. A single large piece of dragon stone or a substantial spider wood centrepiece can displace several litres on its own, and a rock-heavy scape easily reaches 15 to 20% of the water column. Equipment counts too — an internal filter, a heater and a canister filter's intake and outflow all sit in the water.
Why Actual Volume Matters More Than Nominal
Anything expressed per litre or per gallon is expressed per litre of water. Dechlorinator, fertiliser, medication and mineral additives are all dosed that way, and dosing 240 litres of product into 178 litres of water is a 35% overdose. In the other direction, filter and heater ratings quoted against tank size have already built in an assumption about how much water a tank of that size holds; if your scape is unusually rock-heavy, the equipment is oversized rather than under, which is the harmless direction.
Water changes are the other place the difference shows. A "50% water change" on a 240 litre tank sounds like 120 litres, but half of the actual 178 litres is 89 litres. Getting that wrong means either mixing far more replacement water than you need or, on a marine tank, mismatching salinity because the salt was measured against the wrong volume.
Water Weight and Whether the Floor Can Take It
Fresh water is close to one kilogram per litre, a density the USGS Water Science School's page on water density gives as roughly one gram per millilitre, varying slightly with temperature and with anything dissolved in it. Salt water at a specific gravity of 1.025 is about 2.5% heavier. The example tank's 178 litres therefore weigh about 178 kg, or 392 pounds.
That is only part of the load. The glass itself on a 240 litre tank is commonly 40 to 60 kg, the substrate is heavier than water — gravel runs around 1.5 to 1.6 kg per litre dry — and the cabinet adds more. The example's 22.7 litres of gravel is roughly 35 kg. A filled 240 litre setup landing on a domestic floor is therefore a load in the region of 300 kg concentrated over about half a square metre. The water weight calculator handles the volume-to-weight conversion on its own, and the mass to volume converter covers substrates and other materials with a density you can supply.
Odd Shapes and How to Approximate Them
Not every tank is one of the five options. Two approaches work for anything else. The first is to split the tank into shapes the calculator does handle and add the results — a peninsula tank with a curved end is a rectangle plus half a cylinder, and an L-shaped corner unit is two rectangles minus their overlap. The cylinder volume calculator and the square footage calculator help with the pieces.
The second approach is empirical and beats any formula: fill the tank with a container of known volume and count. A 10 litre bucket filled to a marked line and tipped in twenty times is 200 litres, measured rather than estimated, and it accounts for every irregularity, brace bar and silicone bead at once. Do it before the substrate goes in, note the number on a label stuck to the cabinet, and you never have to calculate again. For converting the answer between systems, the volume converter covers the US and imperial gallon difference, which is a 20% gap and a common source of error.
Arb Digital's free tools library covers measurement, volume and cost calculations for home and business projects, and our team is happy to talk through anything the tools cannot answer.
Browse Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Dosing against the tank's rated size — the real water volume is routinely 20 to 30% smaller, which turns a correct dose into a significant overdose.
- Forgetting the glass — thick panes remove a shell from every internal dimension, and on a large tank that is several litres before anything else is subtracted.
- Measuring substrate from the bag — a bag's stated litres are loose volume before settling. Measure the actual bed depth at the front glass.
- Confusing US and imperial gallons — an imperial gallon is about 20% larger, so the same tank has two very different gallon figures.
- Ignoring hardscape — a rock-heavy scape can take 15 to 20% of the water column, and a large piece of wood several litres on its own.
Related Free Tools From Arb Digital
Use the volume converter to move between litres, US gallons and imperial gallons, the tank volume calculator for industrial vessels and partial fill depths, the water weight calculator for the load on a floor or a stand, the cylinder volume calculator for round sections of an unusual build, and the length converter when a manufacturer quotes in inches. Everything else is in the free online tools hub.
Frequently Asked Questions
Because the stated size is calculated from outside dimensions. Glass thickness, the gap you leave below the rim, the substrate bed and any rock, wood or equipment all take volume out. A tank sold as 240 litres commonly holds around 178 litres of water.
Outside, in the same units throughout, and enter the glass thickness separately. The calculator subtracts two pane thicknesses from length and width and one from the height, which is the same result you would get measuring internally but far easier to do accurately.
Yes. A 5 cm bed across a 118 by 38 cm footprint is nearly 23 litres. Sand and gravel do hold water in the gaps between grains, usually a third to two fifths of their bulk, but that water does not circulate freely, so this tool leaves it out.
Fresh water is close to one kilogram per litre, so 178 litres is about 178 kg. Add the glass, the cabinet and the substrate — gravel runs about 1.5 kg per litre — and a 240 litre setup can total near 300 kg over roughly half a square metre.
A US gallon is 3.785 litres and an imperial gallon is 4.546 litres, so the imperial measure is about 20% larger. This calculator shows both, because equipment and dosing instructions do not always say which one they mean.
Select the bow front shape and enter the bow depth — how far the curved pane bulges past the line of the front edge. The tool adds the area of that circular segment to the rectangular section behind it before applying the water depth.
Yes. Fill the empty tank using a container of known volume and count how many it takes. That measures every brace, bead and irregularity at once. Do it before the substrate goes in and write the number on the cabinet.
This tool performs geometric arithmetic on the dimensions you supply. It is not advice on aquarium husbandry, stocking, dosing or livestock care, and any product should be used according to its own label instructions.
