Aquarium volume calculator

How to calculate fish tank volume

For a rectangular tank, multiply length × width × height. With inches, divide the result by 231 to get US gallons; with centimeters, divide by 1,000 to get liters. A 36 × 18 × 16 inch tank (a "40 breeder") is 10,368 cubic inches, or 44.9 gallons.

Shape	Footprint area	Measure
Rectangular	L × W	Length, width, height
Bow-front	L × end depth + circular segment	Length, depth at ends and center, height
Cylinder	π × (D ÷ 2)²	Diameter, height
Hexagon	2.598 × side²	One side panel, height

Volume = footprint area × height. Real tanks hold 10–20% less than their outside dimensions because of glass, the gap below the rim, substrate and decor — the calculator works this out when you add them. Read the full guide: how to calculate aquarium volume.

How to use the calculator

1. Choose the shape. Pick rectangular, bow-front, cylinder or hexagon, and inches or centimeters. For a standard glass tank, choose its size from the list to fill in the dimensions.
2. Enter the measurements. Measure the outside of the tank with a tape measure. For a bow-front tank, measure the depth at the ends and at the center of the bow.
3. Refine to real water volume. Open Glass, water level & substrate and enter the glass thickness, how far below the rim you fill, and the substrate depth to get the water the tank actually holds.
4. Carry the result forward. Use the links under the result to size a heater, filter, substrate and stocking for the same volume, or copy a link to share your numbers.

All aquarium calculators

• Weight calculatorAdd up water, glass, wet substrate, rock and the stand to see what a set-up tank really weighs — and how hard it presses on your floor.
• Substrate calculatorEnter the inside footprint and the depth you want at the front and back. The calculator returns weight, volume and the number of bags.
• Heater size calculatorHeater size depends on how far the water must be lifted above the coldest room temperature, not just on gallons.
• Filter flow rate calculatorTurnover — how many times per hour the filter moves the tank’s volume — is the number that matters. Here is the flow to aim for and the filter rating to buy.
• Water change calculatorEvery water change removes the same share of nitrate. Enter your test results to see how many changes it takes to reach a target and where a weekly routine settles.
• Dosing calculatorLabels give a dose per 10, 40 or 50 gallons; your tank is something else. Enter the label dose and the volume you are treating.
• Glass thickness calculatorPlanning a DIY or custom tank? Enter the size to estimate how thick the front, end and bottom glass should be — then have a glass supplier or engineer check it.
• Stocking calculatorAdd the fish you keep or plan to keep. The calculator estimates bioload against your filtration and flags tank size, group size and temperature problems.

Quick charts: heater size by tank and the water change schedule.

Standard fish tank sizes

Outside dimensions of common US glass tanks, the water they really hold an inch below the rim, and their weight once set up with gravel. Open a size for heater, filter, substrate and stocking details.

Tank	Dimensions (in)	Rated volume	Real water	Set-up weight
5 gallon	16 × 8 × 10	5.5 gal	4.7 gal	55 lb
10 gallon	20 × 10 × 12	10.4 gal	9.1 gal	100 lb
15 gallon	24 × 12 × 12	15.0 gal	12.9 gal	150 lb
20 gallon high	24 × 12 × 16	19.9 gal	17.6 gal	195 lb
20 gallon long	30 × 12 × 12	18.7 gal	16.1 gal	185 lb
29 gallon	30 × 12 × 18	28.1 gal	25.1 gal	270 lb
40 gallon breeder	36 × 18 × 16	44.9 gal	39.7 gal	435 lb
40 gallon long	48 × 12 × 16	39.9 gal	34.9 gal	390 lb
55 gallon	48 × 13 × 21	56.7 gal	50.8 gal	545 lb
65 gallon	36 × 18 × 24	67.3 gal	59.6 gal	660 lb
75 gallon	48 × 18 × 21	78.5 gal	69.2 gal	770 lb
90 gallon	48 × 18 × 24	89.8 gal	79.8 gal	870 lb
125 gallon	72 × 18 × 21	117.8 gal	104.4 gal	1,145 lb
150 gallon	72 × 18 × 28	157.1 gal	138.5 gal	1,555 lb

Guides

• How to calculate aquarium volume (every tank shape)The formulas behind tank volume, how to measure each shape, and why a "55-gallon" tank holds closer to 46 gallons once it is set up.
• How much does a fish tank weigh?A set-up aquarium weighs about 10 pounds per gallon. Here is where that weight comes from, a chart for every standard size, and how to think about your floor.
• How much substrate do I need?Substrate depends on the floor area of your tank, not its gallons. Here is the formula, the right depth for your set-up, and a chart for standard sizes.
• Aquarium heater size guide: how many watts per gallon?The 5-watts-per-gallon rule is a rough start. The real answer depends on how cold the room gets. Here is how to size, place and run a heater safely.
• Aquarium filter flow rate: GPH and turnover explainedFilter boxes advertise gallons per hour. What your fish feel is turnover — and the real flow is lower than the box says. Here is how to choose.
• Aquarium water changes: how much and how often?Water changes are the single most important part of fishkeeping. The maths shows why 25% a week and 50% a week give very different tanks.

Frequently asked questions

How do I calculate how many gallons my fish tank holds?

Measure the length, width and height in inches, multiply them together and divide by 231, the number of cubic inches in a US gallon. A 30 × 12 × 18 inch tank is 6,480 cubic inches, or 28.05 gallons.

How do I calculate aquarium volume in liters?

Measure in centimeters, multiply length × width × height and divide by 1,000. A 60 × 30 × 36 cm tank holds 64.8 liters. To convert gallons to liters, multiply by 3.785.

Why does my tank hold less water than its gallon rating?

Ratings use outside dimensions. Glass thickness, the gap between the water line and the rim, substrate and decorations reduce the real water volume by 10–20%. A standard 55-gallon tank holds about 51 gallons filled to an inch below the rim, and about 46 gallons with a 2-inch gravel bed.

How much does aquarium water weigh?

Fresh water weighs about 8.34 lb per US gallon (1 kg per liter). Saltwater at a specific gravity of 1.025 weighs about 8.55 lb per gallon. A full set-up tank, including glass and gravel, comes to roughly 10 lb per gallon.

What is the difference between US and imperial gallons?

A US gallon is 3.785 liters (231 cubic inches); an imperial (UK) gallon is 4.546 liters, about 20% larger. US tank sizes such as "55 gallon" are always US gallons.

How do I measure a bow-front aquarium?

Measure the straight length, the front-to-back depth at the ends, the depth at the center of the bow, and the height. The calculator treats the bow as a circular arc, which matches how bow-front glass is bent.