Aquarium Volume Calculator

Enter a fish tank's inside dimensions — rectangle, bow front, cylinder, hexagon or corner — and get the water volume in gallons and litres after substrate and rim gap, plus its weight.

Health & EverydayWorldwideSource: Volume geometry; NIST HB 44 gallon/litre; USGS 8.34 lb per gallonFree, no sign-up

Tank shape

Units (inside measurements)

Length (in)

Width (front to back) (in)

Height (in)

Gap from water to rim (in)

Substrate depth (in)

The answer updates as you type; nothing is sent anywhere.

Water volume

48.6 US gallons · 184.1 litres

Water weighs about 406 lb (184 kg), before glass, stand, rock or substrate. Filled to the rim the tank holds 56.7 gal (214.7 L).

The arithmetic

Footprint 624 sq in × water height 18 in ÷ 231 cubic inches per gallon = 48.62 gal. Litres = gallons × 3.785. Weight = gallons × 8.34 lb.

What this page cannot tell you

Use inside measurements: outside measurements include the glass and overstate the volume. Rock, wood and equipment displace water, so the real water volume is lower. Saltwater is slightly heavier than the fresh-water weight shown.

How to calculate aquarium volume

Pick the shape. Choose rectangle, bow front, cylinder, hexagon, quarter-circle corner or pentagon corner.
Measure inside the glass. Enter inside dimensions in inches or centimetres; outside measurements include the glass and overstate the volume.
Allow for water level and substrate. Enter the gap between the water and the rim and the depth of sand or gravel.
Read gallons, litres and weight. The result is the water volume in US gallons and litres, the full-to-rim volume, and the water's weight.

10 situations where an aquarium volume calculation helps

Dosing medication or water conditioner

Doses are per gallon or litre; dosing by the label's nominal size instead of the real water volume can overdose fish.

Checking a second-hand tank's size

A tank with no label can be measured and its real capacity worked out.

Sizing a filter or heater

Equipment is rated by tank volume; the real figure avoids an undersized heater.

Planning a stand or floor load

Water alone weighs 8.34 lb per gallon, so a 55-gallon tank holds over 400 lb of water before glass and rock.

Working out a bow front tank

The curved front adds a circular segment that simple length × width misses.

Measuring a cylinder or hexagon tank

Odd shapes are where guesses go most wrong.

Water changes

Knowing the true volume turns a 25% change into a number of gallons or buckets.

Salt mixing for a marine tank

Salt is mixed per gallon; the water volume after substrate is what matters.

Converting between gallons and litres

European tanks in litres and US equipment in gallons, shown side by side.

Building a custom tank or sump

Try dimensions before ordering glass to hit a target volume.

How the volume is worked out

Volume is the footprint area times the water height, divided by 231 cubic inches per US gallon. One gallon is 3.785411784 litres. Both conversions are from NIST Handbook 44, Appendix C. The water's weight uses 8.34 pounds per gallon, the figure the USGS Water Science School gives; the metric weight takes one litre of fresh water as one kilogram.

Footprints: rectangle = length × width; cylinder = π × diameter² ÷ 4; regular hexagon measured flat to flat = (√3 ÷ 2) × width²; bow front = length × depth at the ends plus the circular segment formed by the bow; quarter-circle corner = π × side² ÷ 4; pentagon corner = side² minus the two triangles cut off the front, side² − cut² ÷ 2.

The water height is the tank height minus the gap below the rim and the substrate depth. Everything is calculated in your browser.

Limits: rock, wood, plants and equipment displace water, so the real volume is lower still. Glass thickness matters if you measure outside. Saltwater is denser than fresh water, so it weighs a little more than shown.

Sources

NIST Handbook 44, Appendix C — 1 gallon = 231 in³ = 3.785411784 L
USGS Water Science School — A Million Gallons of Water — 8.34 pounds per gallon

Frequently asked questions

How do I calculate the volume of a fish tank in gallons?

Multiply length × width × water height in inches and divide by 231. A 48 × 13 × 21 in tank holds 56.7 gallons filled to the rim.

How do I calculate aquarium volume in litres?

Multiply length × width × height in centimetres and divide by 1,000. A 100 × 40 × 50 cm tank is 200 litres.

Should I measure inside or outside the glass?

Inside. Outside measurements include the glass and make the tank seem larger than it is.

How much does a full aquarium weigh?

Water alone weighs about 8.34 lb per gallon, so 50 gallons of water is about 417 lb, plus the glass, stand, substrate and rock.

Does substrate reduce the water volume?

Yes. The calculator subtracts the substrate depth and the gap below the rim from the height.

How do I work out a bow front tank?

Take the rectangle up to the ends of the bow, then add the curved segment: the calculator does this from the length, end depth and how far the front bows out.

Why is my tank's real volume less than its label?

Labels are usually nominal and measured outside; the water level, substrate and decorations all take space.