# How many gallons is my aquarium?

Works out how many gallons or liters a rectangular or cylinder fish tank holds from its inside size, and how much the water weighs.

- Page: https://www.acalculator.org/everyday-life/aquarium-calculator
- JSON spec: https://www.acalculator.org/everyday-life/aquarium-calculator.json
- Version: f98b4b11dcb4

## Default answer

Example with the default inputs (Tank shape Rectangular, Length 24 in, Width 12 in, Water depth 16 in): The tank holds 19.95 gal of water, which weighs 166.1 lb.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| shape | Tank shape | A rectangular (box) tank or a round cylinder tank. |
| l | Length | The inside length of the tank, left to right. |
| w | Width | The inside width of the tank, front to back. |
| dia | Diameter | The inside diameter of the round tank. |
| h | Water depth | The inside height of the tank, or the depth of the water in it. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| volume | Volume | The water the tank holds: US gallons in US units, liters in Metric. |
| liters | Liters | The volume in liters. |
| gallons | US gallons | The volume ÷ 231 in³. |
| weight | Water weight | Fresh water at 0.998 kg per liter; salt water, gravel and the tank add more. |

## Method

Rectangular: volume = length × width × height; cylinder: volume = π × (diameter ÷ 2)² × height; 1 US gallon = 231 in³; water weight = volume × 0.998 kg/L.

## Assumptions

- Sizes are inside sizes; outside sizes include the glass and give a little more.
- The tank is filled to the height typed; most tanks are filled an inch or two below the rim.
- The water is fresh water at 20 °C; gravel, rock and decorations take up some of the volume.

## Worked examples

1. shape = box, l = 0.6096, w = 0.3048, h = 0.4064 gives gallons = 19.948052, liters = 75.511591, weight = 75.360568. Source: NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement (1 US gallon = 231 cubic inches; 1 inch = 2.54 centimeters exactly), https://www.nist.gov/document/2026-nist-handbook-44-appendix-c (retrieved 2026-10-03); USGS Water Science School, Water Density (about 0.998 g/cm³ at 20 °C), https://www.usgs.gov/water-science-school/science/water-density (retrieved 2026-10-03).
2. shape = box, l = 1.2, w = 0.5, h = 0.5 gives liters = 300, gallons = 79.251616, weight = 299.4. Source: NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement (1 US gallon = 231 cubic inches; 1 inch = 2.54 centimeters exactly), https://www.nist.gov/document/2026-nist-handbook-44-appendix-c (retrieved 2026-10-03); USGS Water Science School, Water Density (about 0.998 g/cm³ at 20 °C), https://www.usgs.gov/water-science-school/science/water-density (retrieved 2026-10-03).
3. shape = cylinder, dia = 0.4572, h = 0.6096 gives gallons = 26.438338. Source: NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement (1 US gallon = 231 cubic inches; 1 inch = 2.54 centimeters exactly), https://www.nist.gov/document/2026-nist-handbook-44-appendix-c (retrieved 2026-10-03).

## FAQ

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

Multiply the inside length, width and height in inches, then divide by 231, the cubic inches in a US gallon. A 24 × 12 × 16 in tank holds 4,608 ÷ 231 = 19.95 gallons.

### How do I work out liters?

Measure in centimeters, multiply length × width × height, and divide by 1,000. A 120 × 50 × 50 cm tank holds 300,000 cm³, which is 300 liters.

### How much does the water in an aquarium weigh?

Fresh water weighs about 0.998 kg per liter at room temperature, about 8.33 lb per US gallon. A 20 gallon tank holds about 166 lb of water, before the glass, gravel and rock.

### Why does my tank hold less than its label says?

Labels often use the outside size, and tanks are filled below the rim. Glass, gravel and decorations take up space too, so the water volume is often 10% to 20% less than the label.

### How do I measure a round tank?

Measure the inside diameter across the middle and the water depth. The volume is π × (diameter ÷ 2)² × depth.

## Sources

- NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement (1 US gallon = 231 cubic inches; 1 inch = 2.54 centimeters exactly). https://www.nist.gov/document/2026-nist-handbook-44-appendix-c (retrieved 2026-10-03)
- USGS Water Science School, Water Density (fresh water is about 0.998 g/cm³ at 20 °C). https://www.usgs.gov/water-science-school/science/water-density (retrieved 2026-10-03)