What the Aquarium Volume Calculator does
This calculator works out how much water a fish tank holds, in litres, US gallons and UK gallons, for rectangular, cube, cylindrical, bow-front and hexagonal tanks. A 60 × 30 × 30 cm tank is 54 litres by its outside measurements; take away 6 mm glass, 5 cm of gravel and a 2 cm gap below the rim and it holds about 38 litres of water.
It also estimates what the full tank weighs - water, glass and substrate - so you can check that a stand, shelf or floor can carry it, and gives the usual rules of thumb for heater wattage and filter flow. Measurements can be in centimetres, millimetres, inches or feet.
How to use it
- Pick the tank shape and the unit you measured in.
- Enter the dimensions. For a bow-front, measure the straight back length, the depth at the ends, and how far the curved front bows out at its centre. For a hexagon, measure one side or the width across two flat sides.
- If you measured the outside of the tank, enter the glass thickness; then add the substrate depth and how far below the rim the water stops.
- Choose fresh or salt water and read the volume, the filled weight and the heater and filter guidance. Copy the result or print it.
Reading the results
Three volumes are shown. The outside volume is what many tanks are sold as; inside the glass is the true capacity; the water figure is what you actually fill once the gravel and the gap at the top are taken away. Use the water figure for dosing treatments, water conditioner and salt.
The filled weight adds water, glass and substrate. Water alone is about 1 kg per litre, so even a 200-litre tank weighs well over 200 kg once glass, gravel and rocks are added. The floor load figure spreads that over the tank's footprint.
Heater and filter figures are common hobby rules of thumb - about 1 watt per litre and a turnover of 4-10 tank volumes per hour - not manufacturer ratings. Room temperature, fish species and the filter's real flow under load all change what you need.
Worked example: a 60 × 30 × 30 cm tank with 6 mm glass
Outside: 60 × 30 × 30 = 54,000 cm³ = 54 litres, or 14.3 US gallons.
Inside, the glass takes 0.6 cm off each side and the base: 58.8 × 28.8 × 29.4 = 49,787 cm³ ≈ 49.8 litres.
With 5 cm of gravel and the water 2 cm below the rim, the water is 22.4 cm deep: 58.8 × 28.8 × 22.4 = 37,933 cm³ ≈ 37.9 litres - 10 US gallons. It weighs 37.8 kg; the gravel adds about 13.5 kg and the glass about 10.8 kg, so the stand carries roughly 62 kg before rocks and equipment.
Formulas and scoring rules
- Rectangle and cube
volume = length × width × heightcm³ ÷ 1,000 = litres.- Cylinder
volume = π × (diameter ÷ 2)² × height- Hexagon (regular)
volume = (3√3 ÷ 2) × side² × height; side = width across flats ÷ √3- Bow-front
area = length × end depth + segment; segment = r² × acos((r − b)/r) − (r − b) × √(2rb − b²), r = (length²/4 + b²) / 2bb is the bow depth at the centre. The front is treated as a circular arc.- Inside the glass
inside length = outside − 2 × glass; inside height = outside − glass (base only)- Gallons
US gal = litres ÷ 3.785411784; UK gal = litres ÷ 4.54609- Weight
water kg = litres × 0.997 (fresh) or × 1.025 (sea); glass kg = glass volume × 2.5Substrate uses an editable rough bulk density of 1.6 kg per litre. Results are rounded for display.
Why the label volume is more than the water you add
Tanks are usually sold by their outside dimensions, which include the glass. On a small tank, 6 mm glass on every side and the base removes nearly 8% of the volume; 10-12 mm glass on a large tank takes even more. Then the substrate fills the bottom few centimetres, the water stops below the trim, and rocks, wood and equipment displace more. A tank sold as 54 litres commonly holds 38-45 litres of water.
That difference matters when a medicine, a fertiliser or a salt mix is dosed per litre or per gallon: dosing for the label volume can give the fish noticeably more than intended.
Limitations: what the result does not prove
- Shapes are idealised: vertical sides, flat base, and a bow-front treated as a circular arc. Curved corners, sloping fronts and cabinets with built-in filters need adjusting by eye.
- The glass weight is an estimate from the wall area and thickness; acrylic tanks are about half as heavy. Rocks, wood, decor, lids, lights and the cabinet are not included.
- Heater and filter sizes are general rules of thumb. Follow the equipment maker's rating and your species' needs.
- This is not a structural assessment. For large tanks on upper floors, ask a qualified person whether the floor can take the load.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- NIST SP 811 - Guide for the Use of the International System of Units - checked 19 Sep 2026
- NIST Handbook 44 Appendix C - definitions of the US gallon, cubic inch and inch
- UK Weights and Measures Act 1985, Schedule 1 - the gallon (4.54609 litres)
- USGS - Water density
Frequently asked questions
How do I calculate how many litres my fish tank holds?
Multiply length × width × height in centimetres and divide by 1,000. A 80 × 35 × 40 cm tank is 112,000 cm³, or 112 litres. Measure inside the glass, or enter the glass thickness here, to get the true capacity rather than the label figure.
How many gallons is my aquarium?
Divide the litres by 3.785 for US gallons or by 4.546 for UK (imperial) gallons. In inches, length × width × height ÷ 231 gives US gallons directly: 48 × 13 × 21 in is 13,104 cubic inches, about 56.7 US gallons.
How much does a full aquarium weigh?
Allow about 1 kg per litre for the water (8.3 lb per US gallon), plus the glass and gravel. A 200-litre tank weighs roughly 250-280 kg filled, before rocks and the cabinet. The calculator breaks the weight down so you can check a stand.
How do I work out the volume of a bow-front tank?
Treat it as a rectangle (back length × depth at the ends) plus the curved part in front, which is a segment of a circle. Measure how far the bow sticks out at its centre and the calculator adds the segment for you.
Does gravel reduce the amount of water in a tank?
Yes. Substrate occupies the bottom of the tank, so the water column is shorter. Five centimetres of gravel in a 60 cm tank removes about 8.5 litres of space. Gravel holds a little water between the grains, but dose medication for the volume above it.
What size heater does my aquarium need?
A common rule of thumb is about 1 watt per litre (3-5 watts per US gallon) for tropical fish in a normally heated room, more in a cold room. It is guidance only; follow the heater maker's tank-size rating.
How strong a filter do I need for my tank?
Hobby guidance is a filter that moves 4-10 times the tank volume per hour, so a 100-litre tank needs roughly 400-1,000 litres per hour. Heavily stocked tanks and messy fish need the upper end; shrimp and small fish prefer gentler flow.
Last reviewed by the A2Z.Tools team against the sources listed above.
