acalculator

How many gallons is my aquarium?

Type the inside length, width and water depth of a rectangular tank, or the diameter and depth of a round one. The calculator gives the volume in US gallons or liters and the weight of the water, so you can plan the stand, the filter and the heater.

Your numbers

Units
Tank shape
Volume
19.95 gal

The tank holds 19.95 gal of water, which weighs 166.1 lb.

Liters
75.5
US gallons
19.95
Water weight
166.1 lb

Volume: 19.95 gal. The tank holds 19.95 gal of water, which weighs 166.1 lb.

How to calculate

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

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.

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.

  • 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.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Tank shape Rectangular, Length 24 in, Width 12 in, Water depth 16 in gives US gallons 19.948052, Liters 75.511591, Water weight 166.1 lb.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. Tank shape Rectangular, Length 47.24 in, Width 19.69 in, Water depth 19.69 in gives Liters 300, US gallons 79.251616, Water weight 660.1 lb.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. Tank shape Cylinder, Diameter 18 in, Water depth 24 in gives US 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)

How it works

  • Rectangular tank: V = length × width × height.
  • Cylinder tank: V = π × (diameter ÷ 2)² × height.
  • US gallons = V ÷ 231 in³ (1 gal = 231 in³ = 3.785411784 L exactly, since 1 in = 2.54 cm).
  • Liters = V in cm³ ÷ 1,000.
  • Water weight = liters × 0.998 kg, fresh water at 20 °C (1 lb = 0.45359237 kg).

The headline shows gallons in US units and liters in Metric.

Rules

  • Every size is 0.1 in to 1,000 in, typed in inches, feet or centimeters, and used as the exact decimal typed; only π is rounded.
  • Height is the inside height, or the depth of the water when the tank is not full.

Worked examples by hand

24 × 12 × 16 in. 24 × 12 × 16 = 4,608 in³. 4,608 ÷ 231 = 19.95 gal; 4,608 × 16.387064 cm³ = 75,512 cm³ = 75.51 L. Water: 75.51 × 0.998 = 75.36 kg = 166.1 lb.

120 × 50 × 50 cm. 120 × 50 × 50 = 300,000 cm³ = 300 L = 300 ÷ 3.785411784 = 79.25 gal. Water: 300 × 0.998 = 299.4 kg.

Cylinder, 18 in across, 24 in deep. π × 9² × 24 = 6,107.26 in³ ÷ 231 = 26.44 gal.

Other questions people ask

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.