Aquarium Volume Calculator Litres

Work out gross and practical aquarium water volume in litres, UK gallons and US gallons, with allowances for shape, waterline gap, substrate and decor.

Measure The Tank

Tank shape Pick the closest shape. Rectangular covers most standard aquariums. Measurement unit Internal centimetres give the cleanest litre result. Length For a cylinder, this field is ignored.cm Width or diameter For a cylinder, enter the internal diameter. For a hex tank, enter flat-to-flat width.cm Tank height Measure to the top glass edge, then subtract the waterline gap below.cm Waterline gap Distance from rim to normal filled water level.cm Substrate depth Average gravel, sand or soil depth across the base.cm Decor displacement Estimate rocks, wood, internal filters and dense hardscape as a share of gross water space.% Bow depth Only used for bow-front tanks. Enter the extra curve depth at the centre.cm Planned water change Common values are 10%, 20% or 25%; follow advice for your tank.%

Rectangular tanks use length x width x usable water height, then subtract substrate and decor allowance.

Your Water Volume

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Enter the tank measurements to calculate litres.

Result Reading

Share the litre figure with a shop, vet, aquascaper or anyone checking treatment dosage.

Gross Litres Versus Practical Water Volume

Most aquarium dimensions describe the outside glass size or the total box shape. The amount of water the tank actually holds is lower once the normal waterline sits below the rim, the base contains gravel or sand, and rocks, wood, plants, internal filters or caves push water out of the tank. This calculator gives both figures. Gross litres help with comparing tank sizes, while practical water volume is usually the safer figure for water conditioner, salt mix, fertiliser, medication labels, water-change buckets and filter turnover checks.

The result is still an estimate unless you count the water added during a fill. A litre jug, marked bucket or water meter reading gives the best value. Use the calculated value when the tank is already running and draining it would be impractical, then adjust the decor percentage if the final number is clearly too high or too low.

Volume Formula By Tank Shape

Rectangular litres = length cm x width cm x usable water height cm / 1000 Cylinder litres = pi x radius cm x radius cm x usable water height cm / 1000 Hex litres = 0.866 x flat-to-flat width cm x flat-to-flat width cm x usable water height cm / 1000 US gallons = litres x 0.264172; UK gallons = litres x 0.219969

A rectangular tank is the simplest case. The calculator converts millimetres and inches into centimetres first, subtracts the waterline gap from height, then removes an estimated substrate volume and decor displacement. The cube option uses the same formula but makes it easier to load a common preset. The cylinder option uses diameter, not radius, because most owners measure across the top. The hexagonal option assumes the width is measured flat-to-flat. Bow-front tanks are approximated by adding half an elliptical curved front to a rectangular box, so they are best treated as planning estimates rather than laboratory measurements.

How To Measure Without Guessing

1. Measure inside glass

Internal dimensions are better than manufacturer outside dimensions because glass thickness does not hold water.

2. Use the filled height

Measure to the normal water surface, not to the very top rim, especially on open tanks.

3. Average the substrate

Sloped aquascapes need an average depth. A 2 cm front and 8 cm rear is roughly 5 cm.

4. Add a decor allowance

Dense stone and wood can displace a large amount. Light plastic decor may displace far less.

If you are setting up a new aquarium, record how many litres go into the tank after substrate and decor are added. That becomes your best practical value for future dosing. If the aquarium is already stocked, do not chase mathematical precision by disturbing hardscape or filter media. A careful estimate is usually enough for routine conditioner and water-change planning, while medication instructions should be checked with a suitable aquatic professional or the product supplier.

Common Aquarium Size Table

Internal Dimensions	Gross Litres	Practical Volume With 3 cm Gap, 5 cm Substrate And 8% Decor	20% Water Change
45 x 28 x 30 cm	37.8 L	27.8 L	5.6 L
60 x 30 x 30 cm	54.0 L	37.5 L	7.5 L
60 x 30 x 45 cm	81.0 L	56.3 L	11.3 L
80 x 35 x 40 cm	112.0 L	80.6 L	16.1 L
90 x 45 x 45 cm	182.3 L	132.7 L	26.5 L
100 x 40 x 50 cm	200.0 L	154.6 L	30.9 L
120 x 45 x 45 cm	243.0 L	176.9 L	35.4 L
150 x 60 x 60 cm	540.0 L	418.1 L	83.6 L

What The Litre Result Is For

Water Conditioner

Conditioner bottles often dose per litre or per gallon. Practical water volume is better than box volume because you are treating the water actually present.

Water Changes

The result shows your chosen change size in litres, so you can prepare buckets, dechlorinator and temperature-matched water before starting.

Medication Caution

A litre estimate does not decide treatment, diagnosis or safety. Check the product label and seek specialist aquatic advice if fish are unwell.

Volume also affects filter and pump selection. Many filters quote a tank-size range or litres-per-hour rating, but the correct choice also depends on fish species, waste load, media capacity, flow preference and maintenance. RSPCA and OATA guidance both link water volume to water quality, but they also warn that filtration, stocking and care routines matter. Do not treat any litre-per-fish rule as a complete welfare decision.

UK Gallons, US Gallons And Litres

UK aquariums and treatments usually use litres, but older books, imported products and overseas forums may mention gallons. There are two gallon systems. One UK imperial gallon is about 4.546 litres. One US gallon is about 3.785 litres. A 120 litre tank is about 26.4 UK gallons or 31.7 US gallons, so using the wrong gallon type can shift a dose or filter comparison by about 20%.

When a product label says “gallons”, look for the country or a metric line on the same label. If the product also gives litres, use litres. If a forum answer gives only gallons, convert the value before applying it to a UK aquarium. For aquarium care, the practical litre value is usually the clearest common language between owners, retailers and product labels.

Filter Turnover Reference

Practical Water Volume	Four Times Per Hour	Six Times Per Hour	Ten Times Per Hour
30 L	120 L/h	180 L/h	300 L/h
60 L	240 L/h	360 L/h	600 L/h
100 L	400 L/h	600 L/h	1,000 L/h
150 L	600 L/h	900 L/h	1,500 L/h
200 L	800 L/h	1,200 L/h	2,000 L/h
300 L	1,200 L/h	1,800 L/h	3,000 L/h
500 L	2,000 L/h	3,000 L/h	5,000 L/h

This table is a flow reference, not a purchase rule. Some fish need gentler flow, some tanks need stronger waste handling, and filter media volume can matter more than the number printed on a box. Rated flow can also fall after media, pipe bends and head height are added. Use it as a conversation starter with a reputable aquatic retailer, not as the only basis for stocking or filtration.

FAQs

Should I use internal or external aquarium dimensions?

Use internal dimensions where possible. External dimensions include glass thickness, trims and sometimes a lid frame. That space does not hold water. If you only have external dimensions from a product listing, the result is still useful for comparing tanks, but it may be a little higher than the true filled volume.

How much decor displacement should I enter?

Use 5% for a lightly decorated tank, 8% to 12% for a typical community tank, and more for heavy rock, large wood or dense aquascaping. The number is an allowance, not a lab measurement. If you filled the tank with measured buckets, use that filled volume as your practical value instead.

Does substrate always reduce the same volume?

No. The calculator estimates substrate as base area multiplied by average depth. Real gravel and sand hold small spaces between grains, and slopes are not perfectly even. The allowance is good for planning conditioner and water changes, but a measured fill is better when exact dosing matters.

Can I use the litre result for fish stocking?

Use it only as one part of stocking research. Volume matters, but fish size, adult behaviour, oxygen needs, waste load, filtration, temperature and swimming space also matter. RSPCA guidance notes that simple volume guidelines are only guidelines because water quality depends on many factors.

Why does the bow-front result say approximation?

Bow fronts vary by manufacturer. The calculator treats the curved section as half an ellipse added to a rectangular tank. That is a practical approximation for planning water changes and conditioner. If the manufacturer provides a tested litre volume, compare it with the result and adjust the decor allowance if needed.

Which gallon should I use for aquarium products?

Use litres whenever the label gives them. If a label or article gives gallons only, check whether it is UK imperial or US. UK gallons are larger, so the same litre tank has fewer UK gallons than US gallons. This page shows both to reduce unit mistakes.

Sources

Bureau International des Poids et Mesures. (2019). The International System of Units (SI), 9th edition. BIPM. https://www.bipm.org/en/publications/si-brochure
Royal Society for the Prevention of Cruelty to Animals. (n.d.). Choosing an aquarium for pet fish. RSPCA. https://www.rspca.org.uk/adviceandwelfare/pets/fish/environment
Ornamental Aquatic Trade Association. (n.d.). How to set up and look after a freshwater tank / aquarium. OATA. https://ornamentalfish.org/what-we-do/advice-information/care-sheets/caresheets-tropical-freshwater-fish/how-to-set-up-and-look-after-a-freshwater-tank-aquarium/