Aquarium Volume Calculator - Seven Fish Tank Shapes

An aquarium volume calculator is a free, multi-shape planning tool that converts the inside dimensions of a fish tank into the four numbers a fishkeeper needs: total water volume in US gallons, the same volume in liters, the same volume in cubic meters for aquarium-room load, and a quick stocking estimate based on the 1 cm of fish per 1 L hobby rule. It supports seven common tank geometries - rectangular prism, cube, cylinder, half cylinder, quarter cylinder, bow front, and corner bow front - and lets you swap the dimension inputs to match the chosen shape.

• • Sizing a new tank purchase: matching interior dimensions to the marketed gallon size before committing to a stock tank.
• • Cross-checking a manufacturer label: comparing the actual interior volume of a used tank against the gallon rating.
• • Sizing equipment and water change math: computing volume to size a filter, heater, and weekly water change.
• • Planning stocking density: estimating the maximum number of small community fish.

The aquarium volume drives filter flow rate, heater wattage, medication dosing, fertilizer dosing for planted tanks, and the weekly water change percentage. Most tanks hold 90 to 95 percent of the marketed rating once gravel, substrate, and equipment are subtracted.

Once the volume is known, the next planning step is sizing the glass panels for a custom build, and our Aquarium Glass Thickness Calculator handles side, front, and bottom glass thickness plus total filled weight.

The calculator routes the input dimensions to the matching geometric formula for the selected tank shape, computes the raw volume in cubic inches, then divides by 231 in^3 per US gallon and 61.0237 in^3 per liter. The fish stocking estimate divides the liter result by 3 and rounds down.

• length: Interior length in inches. Used as the rectangle side for rectangular and bow front, as the cube side for cube.
• width: Interior width in inches (straight-side width for bow front).
• height: Interior water height in inches. Used by every shape.
• diameter: Interior diameter in inches. Only used by cylinder, half cylinder, and quarter cylinder.
• fullWidth: Maximum width of the curved front in inches. Only used for bow front and corner bow front.

The bow front formula is the only non-trivial piece of geometry on the page. The curved front panel is a circular segment whose chord is the tank length and whose sagitta is the difference between the full bow-front width and the straight width.

According to the NIST Guide for the Use of the SI, one US liquid gallon is exactly 231 in^3 and one cubic inch equals 0.016387064 L, giving the conversion constants 231 in^3 per US gallon and 3.785411784 L per US gallon used in every result on this page.

For larger rectangular and cylindrical water containers like stock tanks, our Tank Volume Calculator applies the same length-times-width-times-height logic plus a fill-height override.

Four concepts explain why the same labelled tank can hold different amounts of water.

Subtract 5 to 10 percent from the calculator result for substrate and equipment, especially for rimless tanks.

Custom rectangular and bow front builds need silicone at every glass joint, and our Sealant Calculator sizes the silicone bead length and cartridge count for the seams implied by the volume and footprint the calculator just measured.

Six steps to size any of the seven aquarium shapes and read the volume in four units at once.

1. 1 Pick the tank shape: Use the Tank Shape dropdown to pick rectangular prism, cube, cylinder, half cylinder, quarter cylinder, bow front, or corner bow front.
2. 2 Measure the inside dimensions: Measure length, width, and height on the inside of the glass in inches. For a bow front, also measure the full width at the widest point of the curved front. For a cylinder, measure the inside diameter and inside height.
3. 3 Enter the matching fields: Enter each measurement into the form. The calculator ignores unused inputs.
4. 4 Read the volume outputs: Volume in US gallons is the primary reading for North American stocking and equipment sizing. Volume in liters and cubic meters is the primary reading for metric equipment and aquarium-room load.
5. 5 Read the fish stocking estimate: The fish count uses the 1 cm of fish per 1 L hobby rule with a 3 cm average fish length. Multiply by the average adult length of your species to refine.
6. 6 Sanity-check against the label: Compare the calculator's gallon reading to the gallon rating on the tank. Most tanks hold 90 to 95 percent of the rating once substrate is subtracted.

Try a 48 in x 18 in x 21 in rectangular tank: the calculator returns 78.5 US gallons, 297.1 liters, 0.297 cubic meters, and a 99 fish stocking estimate. Subtract 5 percent for substrate and you get about 282 L of actual water, which lines up with the marketed 75 gallon rating on most 48 in x 18 in x 21 in tanks.

If the aquarium volume is being planned alongside a backyard water feature, our Pond Calculator applies the same length, width, and depth logic to a flexible liner and returns the pump flow and liner area needed to keep the feature healthy.

Six practical benefits for fishkeepers, pet store staff, and anyone buying a used tank.

• • Seven shapes, one tool: Rectangular prism, cube, cylinder, half cylinder, quarter cylinder, bow front, and corner bow front are all handled.
• • Four output units at once: US gallons for stocking and equipment, liters for metric filters, cubic meters for room load, and a fish count.
• • Verifies manufacturer labels: A 75 gallon label is rarely exactly 75 US gallons of water.
• • Drives equipment sizing: Filter flow rate, heater wattage, CO2 dosing, and medication dosing all scale with volume.
• • Plans water change math: A 10 to 25 percent weekly water change is easy to compute once the volume is known.
• • Compares tank footprints: Try two footprints with the same target volume and the calculator tells you which one fits the stand.

Volume drives tank price, equipment price, substrate price, and livestock price, so the four-unit reading from this calculator feeds straight into our Hobby Cost Calculator to size the full project budget.

A few variables do most of the work in the calculator and a few extras matter in unusual builds.

• • The seven supported shapes cover the vast majority of consumer and hobby aquariums but not custom polyhedra, hex tanks, or L-shaped builds.
• • The calculator uses interior dimensions, not exterior dimensions. Subtract twice the glass thickness from each exterior dimension if the tank is not yet built.
• • Substrate, equipment, plants, and decorations all reduce the fillable volume.
• • Salt water is slightly denser than fresh water but the volume is essentially the same; the calculator reports liters at 1 kg per L.

According to The Engineering Toolbox, fresh water at 20 C has a density of 998.2 kg/m^3, which is why 1 L of aquarium water weighs almost exactly 1 kg and the calculator's liter reading is a usable proxy for dosing and water-change mass.

According to NIST Guide for the Use of the SI, one US liquid gallon is exactly 3.785411784 liters, which is the conversion factor used in this calculator.

If the build scales up to a swim pond or a small plunge pool, our Pool Volume Calculator extends the same length, width, and depth approach to larger water containers.