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CalcMax

Aquarium Volume Calculator

Range: 0 cm – 1,000 cm

Range: 0 cm – 1,000 cm

Range: 0 cm – 1,000 cm

Result

54.0000 L

Volume in litres

Volume (US gallons)
14.2653 gal (US)

An aquarium volume calculator takes the three outside measurements of a rectangular tank and returns how much water it holds, in litres and in US gallons. The arithmetic is the volume of a box — length times width times height — and everything interesting about the page is in the units and in what the number means. Fish keeping is a hobby where the volume is the parameter that matters most and is quoted least reliably. Nearly every rule in the hobby is written per litre: how many centimetres of fish a tank supports, how much filter turnover it needs, how big a water change should be, what dose of conditioner or medication goes in, whether a heater is big enough. Getting the volume wrong by a third means every one of those is wrong by a third, and a tank sold as a forty-litre aquarium is often a forty-litre box — the glass has thickness, the rim sits above the water line, and gravel and wood take up space that water cannot. This page gives the capacity of the box as measured on the outside, which is the upper bound: the water you actually pour in will be a little less. The unit to be careful with is the gallon, because there are two of them and they differ by a fifth. This page uses the US gallon, 3.785411784 litres, which is the one on American products and in American hobby writing. The imperial gallon is 4.54609 litres, and a British or older aquarium book quoting gallons means that one. A tank called a hundred gallons holds 378 litres in the United States and 455 in Britain, so a figure quoted without saying which gallon is not a measurement. Litres have no such problem, which is why the litres reading is the primary one here.

The capacity of a rectangular tank from its three outside measurements

Length (cm)Width (cm)Height (cm)Volume (litres)Volume (US gallons)
6030305414.2653
100405020052.8344
120455529778.4591
45303040.510.699
302020123.1701
904545182.2548.1454
10101010.2642
2006060720190.2039

Eight tanks and five columns — three measurements in and two readings out. The first row is the tank the page loads with: a sixty-centimetre tank, the size the numbers in the boxes are set to. The seventh is the row that proves the conversion is the right way round: a ten-centimetre cube is a thousand cubic centimetres and a litre is a thousand cubic centimetres, so it has to come out as exactly one. Comparing the two readings down a row is worth doing once, because they are always in the same ratio — 54 litres is 14.2653 US gallons, and every other row divides by the same 3.785411784. The fourth row is a 45-centimetre tank, where the litres figure carries a half and is the first row on the table that is not a whole number of litres. The sixth is the same kind of thing one step further on: 182.25 litres, where the gallons reading runs to 48.1454 and neither figure is tidy. The last row is a two-metre tank, where the litres reading runs to 720 and the hobby stops being about a tank in a room and starts being about a tank in a wall. Every value here is recomputed from its three inputs when the page is built, in centimetres, and the four decimals are the same four decimals the results panel uses — which is why the litres column shows a whole number on six rows out of eight and the gallons column never does.

Formula

V(litres) = length × width × height ÷ 1000

Length
The longest outside measurement of the tank, in centimetres. Outside rather than inside, so the answer is the capacity of the box rather than of the water
Width
The depth front to back, in centimetres — the measurement people forget, since a tank looks like its length and its height
Height
The vertical measurement, in centimetres. The full outside height, not the depth of water, which sits below the rim
1000
The number of cubic centimetres in a litre, and the whole of the conversion. The three lengths multiply to cubic centimetres, and dividing by a thousand turns that into litres
3.785411784
The number of litres in a US gallon — 231 cubic inches, exactly. The gallons reading is the litres reading divided by this, so the two always agree with each other
Volume in litres
How much water the box holds, in litres. The reading to work from, since every rule in fish keeping is written in litres and no country has two of them
Volume in US gallons
The same quantity in American gallons. A British gallon is 4.54609 litres, a fifth larger, so a figure quoted in gallons without saying which is not a measurement

Anyone setting up a tank needs this number before buying anything else, because the volume decides everything downstream. Filtration is quoted as turnovers per hour, so the flow rate a tank needs is its volume multiplied by the number of turns wanted. Heating is quoted in watts per litre, and the difference between a fifty-litre tank and a hundred-litre one is the difference between one heater and two. Stocking is the argument the hobby has most often, and every version of it — centimetres of fish per litre, surface area, the inch-per-gallon rule the Americans use — starts from the volume. Water changes, dosing, the amount of salt for a marine tank, the weight of the substrate, the number of plants: all of them are per litre. The same calculation is also how people work out what a tank weighs, which matters the moment it goes on a floor that was not designed for it — a litre of water is a kilogram, so a two-hundred-litre tank is two hundred kilograms of water before the glass is counted. Outside the hobby it is the volume of any rectangular container: a sump, a quarantine tub, a water butt, a storage box, a shipping crate, a bath. And because the litres reading is the capacity of the box and not of the water, it is also the number to use when the question is how much the container would hold if it were filled to the brim.

Worked examples

  1. A 60 by 30 by 30 tank

    1. Multiply the three measurements: 60 × 30 × 30 = 54000 cubic centimetres
    2. Divide by 1000 to turn cubic centimetres into litres: 54000 ÷ 1000 = 54
    3. Divide the litres by 3.785411784 for US gallons: 54 ÷ 3.785411784 = 14.2653

    The commonest aquarium size there is, and the one the page loads with: sixty centimetres long, thirty deep, thirty high, which is 54 litres. If you have kept a tank, this is very likely the one you started with. The litres figure is exact — 54000 cubic centimetres divides by a thousand with nothing left over — and the gallons figure is not, which is a fair illustration of why the litres reading leads.

  2. A metre-long tank

    1. Multiply the three measurements: 100 × 40 × 50 = 200000 cubic centimetres
    2. Divide by 1000: 200000 ÷ 1000 = 200 litres
    3. Divide by 3.785411784: 200 ÷ 3.785411784 = 52.8344 US gallons

    Two hundred litres, which is the size at which the hobby changes character: the tank is heavy enough that the floor has to be thought about, and big enough that a water change is measured in buckets. Two hundred litres of water is two hundred kilograms, and the glass and the stand add more on top of that.

  3. A 120 by 45 by 55 tank

    1. Multiply the three measurements: 120 × 45 × 55 = 297000 cubic centimetres
    2. Divide by 1000: 297000 ÷ 1000 = 297 litres
    3. Divide by 3.785411784: 297 ÷ 3.785411784 = 78.4591 US gallons, which is 65.3 imperial gallons

    A four-foot tank, the standard community aquarium size. The litres reading lands on a whole number here and the gallons reading does not, which is worth noticing: 297 litres is 78.4591 US gallons and would be 65.3 imperial gallons. Three quite different numbers for one tank, and the only one that is unambiguous is the litres one.

  4. A small 30 by 20 by 20 tank

    1. Multiply the three measurements: 30 × 20 × 20 = 12000 cubic centimetres
    2. Divide by 1000: 12000 ÷ 1000 = 12 litres
    3. Divide by 3.785411784: 12 ÷ 3.785411784 = 3.1701 US gallons

    Twelve litres, the sort of tank sold as a starter kit or used as a quarantine or shrimp tank. Small tanks are the ones where the difference between the box and the water matters most: a 12-litre box holds perhaps 9 litres once the glass, the rim and the substrate are accounted for, and every dose on the bottle is written for the litres of water rather than the litres of box.

  5. A 10 cm cube

    1. Multiply the three measurements: 10 × 10 × 10 = 1000 cubic centimetres
    2. Divide by 1000: 1000 ÷ 1000 = 1 litre
    3. Divide by 3.785411784: 1 ÷ 3.785411784 = 0.2642 US gallons

    The row that proves the conversion is the right way round. A ten-centimetre cube is a thousand cubic centimetres and a litre is a thousand cubic centimetres, so the answer has to be exactly one. It is also the row to use if you ever want to check the arithmetic yourself: a litre is a ten-centimetre cube, and that is the definition rather than a coincidence.

Limitations

This page gives the capacity of a rectangular box as measured on the outside. It is not the amount of water you will put in: the glass has thickness, the water sits below the rim, and gravel, sand, wood and rock all take up space that water cannot. For a typical tank the real figure is ten to twenty per cent lower than the one printed here, and for a small tank with a deep substrate it can be worse than that. Sloped or bow-fronted tanks, cylindrical tanks, corner tanks and tanks with a built-in filter compartment are not boxes and will not come out right. The gallons reading is the US gallon, 3.785411784 litres; the imperial gallon is 4.54609 litres, about a fifth larger, and any figure quoted in gallons without saying which one cannot be compared with this. The answer is a volume of water and says nothing about weight, though a litre of water is a kilogram if you need that — but the glass and the stand are on top of it, and the whole thing is heavier than the water alone. Three measurements are all taken in the same units, so a tank measured in inches should have all three entered in inches; the reading comes back in litres and gallons either way.

Frequently asked questions

Is this the water volume or the tank volume?
The tank volume, measured on the outside, which is the larger of the two. The water volume is what is left after the thickness of the glass, the gap below the rim and everything on the bottom of the tank. For a typical setup that is ten to twenty per cent less, and the difference matters because every dose and every rule in fish keeping is written for the water.
US gallons or imperial gallons?
US. This page divides the litres reading by 3.785411784, which is the American gallon of 231 cubic inches. The imperial gallon is 4.54609 litres, about a fifth larger, so the same tank reads 78.4591 US gallons and 65.3 imperial ones. If a figure you are comparing against came from a British source or an older book, it is almost certainly imperial and will look twenty per cent bigger.
Why is the litres reading the main one?
Because there is only one litre. The gallon is two different units with the same name, differing by a fifth, and a figure quoted in gallons without saying which one is not a measurement you can act on. Every rule in the hobby — stocking, filtration, heating, dosing — is written per litre in any case, so the litres reading is the one that plugs straight into the next calculation.
How much does a tank this size weigh?
The water alone is a kilogram per litre, so a two-hundred-litre tank is two hundred kilograms of water. On top of that go the glass, which for a tank that size is substantial, the stand, the substrate and the rock. A filled tank is routinely two or three times the weight of its water, and it is worth doing that arithmetic before deciding where the tank goes rather than after.
My tank is bow-fronted or has a curved corner. Will this work?
No, and the answer will be too low rather than too high. This page computes a box, and a curved front adds volume that a box does not have. The way round it is to measure the tank as if it were a box at its widest and tallest, work that out, and treat the result as a lower bound; for a slightly bowed front the error is small, and for a corner tank it is not.
Does this count the filter and the gravel?
It counts nothing inside the tank, because it is measuring the outside. Gravel, sand, wood and rock all displace water and all reduce what the tank actually holds, and a built-in filter compartment is volume you can see but cannot use. There is no formula for this — it depends what you put in — but a few litres of substrate is normal for a small tank and ten or more for a large one.

References

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