Quick answer: To find your aquarium’s volume in gallons, multiply length x width x height in inches and divide by 231. For liters, multiply the three sides in centimeters and divide by 1,000. That gives you the full tank to the rim, though, not the water actually in it. Glass, the gap at the top, substrate and rocks usually take up 10 to 25 percent of the space. This calculator works out both numbers for 7 tank shapes, then tells you the substrate you need, how much the tank weighs, and what size heater and filter to buy.
Real water volume (optional but recommended)
Lengths, widths and depths use the unit you picked above, glass thickness is always in mm. Results are estimates. Filters, plants and fish take up a little space too.
How to measure your tank
Length. The long side, left to right across the front.
Width. Front to back, sometimes called depth. For a bowfront, measure the depth at one of the flat sides and again at the middle of the curve.
Height. From the bottom glass to the top rim. Measure the glass, not the stand or the lid.
Inside or outside. Measuring the outside is easier. If you do, enter the glass thickness and the calculator takes it off. Most tanks under 40 gallons use 4 to 6 mm glass, while bigger tanks and rimless tanks use 6 to 12 mm.
The extras. Enter how far the water sits below the rim, how deep your substrate is on average, and roughly how much rock and wood you have. That’s how you get the real water volume.
Label size vs real water volume
| Tank | Outside size (inches) | Full to the rim | Real water, planted setup |
|---|---|---|---|
| 10 gallon | 20 x 10 x 12 | About 9.5 gal | About 7.5 gal |
| 20 gallon long | 30 x 12 x 12 | About 18 gal | About 13 to 14 gal |
| 29 gallon | 30 x 12 x 18 | About 27 gal | About 22 gal |
| 40 gallon breeder | 36 x 18 x 16 | About 43 gal | About 35 gal |
| 55 gallon | 48 x 13 x 21 | About 54 gal | About 46 gal |
| 75 gallon | 48 x 18 x 21 | About 75 gal | About 63 gal |
The “real water” column assumes 5 mm glass, water an inch below the rim, 2 inches of substrate and a normal amount of rock and wood. Your tank will differ, which is exactly why the calculator asks.
Here’s the honest part most volume calculators skip. The size on the box is a rounded sales number. A 20 gallon long set up with soil and driftwood often holds around 13 or 14 gallons of water. That gap matters most when you’re dosing. Medications, fertilizers, dechlorinator and salt are all dosed per gallon, so dosing for the box size can mean a 30 to 40 percent overdose. That’s harmless with dechlorinator and risky with copper medication or shrimp. Once you have your real number, the button under the results sends it straight to our aquarium fertilizer calculator.
Tank shapes this calculator handles
| Shape | What to measure | Formula it uses |
|---|---|---|
| Rectangle or cube | Length, width, height | L x W x H |
| Bowfront | Length, side depth, center depth, height | Rectangle plus the curved front section |
| Cylinder | Diameter, height | 3.14 x radius x radius x height |
| Half cylinder | Width of the flat back, height | Half of a cylinder |
| Corner quarter circle | Length along each wall, height | A quarter of a cylinder |
| Hexagon | One side, height | 2.6 x side x side x height |
| L shaped | Length and depth of each arm, height | Two rectangles joined at the corner |
What the results mean
Substrate needed. Liters and kilograms for the depth you entered. Aquarium soil weighs about 1 kg per liter, while sand and gravel run about 1.5 to 1.6 kg per liter, so check the bag size in liters, not just weight. Planted tanks usually want 2 to 3 inches, sloped deeper at the back. If you use plain sand or gravel, plan on root tabs, and our DIY root tabs guide shows how.
Total weight. Water weighs about 8.3 pounds per US gallon, and that’s before glass, substrate and rock. A set up 55 gallon is close to 500 pounds. Put anything over about 400 pounds on a real aquarium stand, near a wall that carries weight, never on a regular dresser.
Heater size. The calculator uses 3 to 5 watts per gallon of real water, which suits most rooms. Go to the higher end in a cold room. Tanks over 60 gallons do better with two smaller heaters at each end, so one failing doesn’t cook or chill the tank.
Filter flow. Pick your tank type and it shows the minimum flow in gallons and liters per hour. Bettas and shrimp like gentle flow, planted tanks with CO2 like more, and messy fish like goldfish or a big pleco need a lot.
Water change amounts. 25 and 50 percent of your real water volume, so you know how many buckets to fill.
Common tank volume mistakes
Dosing medication for the label size. The most common one. Use the real water number, and take out the carbon from your filter before medicating.
Measuring the stand or the trim. Rimmed tanks have a plastic frame that adds a little to the outside size. Measure the glass if you can.
Forgetting the sump or canister. A sump or a big canister filter adds water to the system. For dosing, add its volume on top of the tank’s.
Buying substrate by weight. Two bags of the same weight can fill very different volumes. Buy by liters.
Ignoring floor strength. Big tanks upstairs or on old floors need some thought. If you’re unsure, ask a builder before filling anything over 75 gallons.
Planning a new tank from scratch? Our fish tank setup guide walks through the whole process, and our 20 gallon fish tank guide compares the long and high versions.
FAQ
How do I calculate the gallons in a fish tank?
Multiply length x width x height in inches and divide by 231. A 30 x 12 x 12 inch tank is 4,320 cubic inches, which is about 18.7 gallons to the rim.
How do I calculate aquarium volume in liters?
Multiply length x width x height in centimeters and divide by 1,000. A 60 x 30 x 36 cm tank holds about 65 liters to the rim before glass and substrate.
How much does a fish tank weigh full of water?
Roughly 8 to 10 pounds for every gallon of tank size once you count water, glass, substrate and rock. A set up 20 gallon long is around 170 pounds, a 55 gallon around 500 pounds, and a 75 gallon close to 700 pounds, more with heavy rock.
How much substrate do I need for my tank?
Multiply length x width in centimeters by the depth you want in centimeters, then divide by 1,000 to get liters. The calculator does this for you and converts it to kilograms.
What size heater do I need for my aquarium?
About 3 to 5 watts per gallon of real water. A 20 gallon long with around 14 gallons of water needs a 50 to 75 watt heater.
Why is my tank smaller than the size on the box?
The box size is a rounded number for the whole glass box. Glass thickness, the gap under the rim, substrate and decor all reduce the real amount of water inside.