Aquarium Calculator

What is aquarium volume and why does it matter?

Aquarium volume is the total three-dimensional space enclosed by a fish tank's walls, measured in gallons, liters, or cubic units. It determines how much water the tank holds and directly affects every aspect of fishkeeping, from chemical dosing to biological filtration capacity.

Accurate volume measurement matters because medication dosages, water conditioner amounts, and fertilizer concentrations are all calculated per gallon or per liter. A 10% error in volume estimation translates to a 10% overdose or underdose of treatment chemicals, which can stress or kill fish. Filtration systems are rated by flow rate relative to tank volume, so an incorrect volume reading leads to under-filtered or over-filtered water.

A critical distinction exists between total tank volume (the geometric capacity of the empty glass box) and actual water volume (the water remaining after substrate, rocks, driftwood, and decorations displace space). A 55-gallon tank with 2 inches of gravel and several decorations may hold only 42–45 gallons of actual water. This difference directly affects medication dosing and chemical treatment accuracy.

What are the fish tank volume formulas?

Each tank shape requires a different geometric formula. Below is a quick reference for all eight shapes supported by this calculator. Select your shape above to see the full step-by-step calculation.

where $L$ = length, $W$ = width, and $H$ = height of the tank.

where $a$ = side length (all sides equal).

where $d$ = arc depth of the curved front glass.

where $r$ = radius (half the diameter) and $h$ = height.

where $s$ = side length of the hexagonal base.

where $s$ = side length of the pentagonal base.

where $b$ = base, $d$ = depth of the right triangle, and $h$ = tank height.

where $a$ = major axis, $b$ = minor axis of the oval base.

How does the aquarium volume calculator work?

This calculator supports eight fish tank shapes and three calculation modes. Select your tank shape from the dropdown, enter the required dimensions in any unit (inches, centimeters, millimeters, meters, or feet), and the calculator returns the volume in your preferred output unit (liters, US gallons, UK gallons, cubic feet, cubic inches, cubic meters, or cubic centimeters).

The Aquarium Volume mode calculates total tank capacity and, optionally, the water volume at a specific fill level. The Actual Water Volume mode subtracts substrate and decoration displacement from the total volume to return the true water capacity. The Aquarium Weight Calculator mode estimates the combined weight of water, substrate, decorations, and glass for stand and floor safety verification.

All unit conversions happen within the calculator. Switching an input from centimeters to inches converts the displayed value to maintain the same physical measurement. Switching a result from liters to gallons recalculates the output in the new unit.

What's the formula for a rectangular aquarium's volume?

The rectangular tank is the most common fish tank shape, accounting for the majority of tanks sold worldwide. Standard sizes like the 10-gallon, 20-gallon, 55-gallon, and 75-gallon are all rectangular. The volume formula multiplies three perpendicular dimensions.

• $L$ = Length (longest horizontal dimension, typically front to back)
• $W$ = Width (shorter horizontal dimension, typically side to side)
• $H$ = Height (vertical dimension from bottom to water line)

Manufacturers round this to 75 gallons because the stated capacity accounts for glass thickness, silicone seams, and a water line approximately one inch below the rim.

How much water does a cube aquarium hold?

Cube tanks are popular for nano reef setups, planted aquascapes, and desktop displays. Because all three dimensions are equal, the volume calculation simplifies to a single measurement cubed.

Cube tanks offer equal viewing depth from all angles, making them well-suited for centerpiece placement where the tank is visible from multiple sides. The equal dimensions also simplify aquascaping because depth perception is consistent regardless of viewing angle.

What's the actual capacity of a bowfront tank?

Bowfront aquariums feature a curved front glass panel that bows outward, creating a wider viewing area and a more organic appearance. The curved glass adds volume beyond a standard rectangular tank of the same width. The arc depth ($d$) measures how far the curve extends beyond the flat back panel.

The $\frac{2}{3}d$ term comes from the geometric area of a parabolic segment. It approximates the additional volume created by the outward bow of the front glass. Measure the arc depth by placing a straight edge across the front opening and measuring the maximum gap between the edge and the glass.

How do you calculate a cylindrical tank's capacity?

Cylindrical aquariums are tall, round tanks often used as room dividers or centerpiece displays. The circular cross-section means the volume depends on the radius of the base and the height of the cylinder.

Cylindrical tanks present a unique aquascaping challenge: the curved glass distorts the view of objects near the edges. Fish and plants placed within the central 60% of the diameter appear with minimal distortion.

What's the volume of a hexagonal aquarium?

Hexagonal tanks have six equal sides and offer wider viewing angles than rectangular tanks of similar footprint. The base area of a regular hexagon is $\frac{3\sqrt{3}}{2} s^2$, where $s$ is the side length.

How much water fits in a pentagon aquarium?

Pentagonal (five-sided) tanks are specialty shapes found in custom builds and select manufacturer lines. The base area of a regular pentagon uses the formula $\frac{\sqrt{25 + 10\sqrt{5}}}{4} s^2$.

What's the capacity of a corner triangle tank?

Corner tanks are right-triangle prisms designed to fit into 90-degree room corners, making them a practical choice for small apartments and offices. The triangular base area is half the product of the two perpendicular sides.

Corner tanks maximize floor space usage in tight rooms. The right-angle corner of the tank sits flush against two walls, and the hypotenuse face provides the primary viewing window.

How do you measure an oval fish tank's volume?

Elliptical (oval) tanks have an elongated circular base defined by two axes: the major axis ($a$, the longest diameter) and the minor axis ($b$, the shortest diameter). The base area is $\pi \times \frac{a}{2} \times \frac{b}{2}$.

How do you calculate actual water volume with substrate?

Substrate (gravel, sand, or soil) occupies space inside the tank that would otherwise hold water. Failing to account for this displacement leads to over-dosing medications and water conditioners. Gravel displaces approximately 100% of its submerged volume, while fine sand displaces roughly 80% due to tighter packing and smaller air gaps between grains.

To calculate substrate displacement, multiply the tank's bottom area by the substrate depth, then apply the displacement factor:

where $f = 1.0$ for gravel and $f = 0.8$ for sand. Decorations (rocks, driftwood, ornaments) displace approximately 2.5 cubic inches per pound of material. Subtract both displacement values from the total tank volume to get the actual water volume.

How much does a full aquarium weigh?

A full aquarium's weight determines whether your floor, stand, and furniture can safely support it. Water is the dominant weight factor: freshwater weighs 8.34 pounds per US gallon, while saltwater weighs 8.55 pounds per gallon due to dissolved salts. A 75-gallon freshwater tank holds approximately 625 pounds of water alone.

Substrate adds significant weight. Gravel weighs roughly 100 pounds per cubic foot, and sand weighs about 95 pounds per cubic foot. A 2-inch layer of gravel across a 55-gallon tank base adds 40–50 pounds. Glass weight varies with thickness: tanks under 20 gallons typically use ¼-inch glass, 20–55 gallon tanks use ⅜-inch, and tanks above 55 gallons use ½-inch or thicker. Glass weighs approximately 1.4 pounds per square foot per ¼-inch of thickness.

Before placing a large aquarium, verify that your floor can support the concentrated load. Residential floors are typically rated for 40 pounds per square foot of distributed load, according to structural engineering standards. A 75-gallon tank on a 6-square-foot stand exerts roughly 143 pounds per square foot, which exceeds standard residential ratings. Place large tanks against load-bearing walls or over floor joists.

How do you calculate the weight of a saltwater reef tank?

Saltwater aquariums weigh more than freshwater tanks of the same size because dissolved salts increase water density. A gallon of saltwater weighs 8.55 pounds compared to 8.34 pounds for freshwater—a 2.5% difference that adds up quickly in larger tanks. Reef tanks also typically use heavier live rock and denser aragonite sand, further increasing the total weight.

Reef tanks over 100 gallons should only be placed on concrete slabs or floors specifically reinforced to handle the concentrated load. Consult a structural engineer before installing tanks over 150 gallons on upper floors.

What are the standard fish tank sizes?

Manufacturers produce tanks in standardized sizes. The table below lists the most common dimensions and their approximate filled weights for freshwater setups with 2 inches of gravel.

Size	Dimensions (in)	Dimensions (cm)	Volume (L)	Weight Full (lbs)
5 gal	16 × 8 × 10	41 × 20 × 25	18.9	62
10 gal	20 × 10 × 12	51 × 25 × 30	37.9	111
20 gal (long)	30 × 12 × 12	76 × 30 × 30	75.7	225
29 gal	30 × 12 × 18	76 × 30 × 46	109.8	330
40 gal (breeder)	36 × 18 × 16	91 × 46 × 41	151.4	458
55 gal	48 × 13 × 21	122 × 33 × 53	208.2	625
75 gal	48 × 18 × 21	122 × 46 × 53	283.9	850
90 gal	48 × 18 × 25	122 × 46 × 64	340.7	1,050
125 gal	72 × 18 × 21	183 × 46 × 53	473.2	1,400
150 gal	72 × 18 × 25	183 × 46 × 64	567.8	1,700
180 gal	72 × 24 × 25	183 × 61 × 64	681.4	2,100

How many gallons per inch of fish?

The "one inch of fish per gallon" rule is a widely cited stocking guideline, but it is a rough starting point rather than a precise formula. It was developed for small tropical community fish (tetras, rasboras, guppies) in filtered, heated tanks. It does not account for fish body mass, swimming behavior, waste production, or territorial requirements.

A 6-inch oscichthys cichlid produces far more waste than six 1-inch neon tetras, despite occupying the same "inch count." A more reliable approach factors in the fish's adult body volume, swimming level (top, middle, bottom), and territorial radius. Overstocking increases ammonia and nitrite levels, stresses fish, and shortens their lifespan regardless of what the inch-per-gallon rule suggests.

Use this calculator to determine your tank's actual water volume (accounting for substrate displacement), then research the specific stocking requirements for each species you plan to keep. Reputable fishkeeping databases list minimum tank sizes per species based on adult dimensions and behavior.

How do you calculate medication dosage for your fish tank?

Medication dosages are specified per gallon or per liter of water. Using the total tank volume instead of the actual water volume (after substrate and decoration displacement) causes overdosing. A 55-gallon tank with 2 inches of gravel and decorations may hold only 48 gallons of actual water, meaning a medication dose calculated for 55 gallons delivers 14.6% more chemical than intended.

Always use the Actual Water Volume mode of this calculator when preparing medication treatments. Enter your tank dimensions, substrate depth, substrate type, and decoration weight. The calculator returns the true water volume to use for dosing. For a medication rated at 1 ml per 10 gallons, a tank with 48.72 gallons of actual water requires $48.72 \div 10 \times 1 = 4.87$ ml, not the 5.5 ml you would calculate using the nominal 55-gallon rating. The FDA requires precise dosing for approved fish medications.

What filter size do you need for your aquarium?

Filter sizing is based on turnover rate: the number of times the filter processes the entire tank volume per hour. Most freshwater community tanks require a turnover rate of 4–6 times per hour. A 55-gallon tank needs a filter rated at 220–330 gallons per hour (GPH). Planted tanks and tanks with high bioloads (many fish, heavy feeders) benefit from 6–10 times turnover, according to aquarium filtration experts.

Saltwater reef tanks typically require 10–20 times turnover due to the sensitivity of corals and invertebrates to water quality. A 75-gallon reef tank needs a flow rate of 750–1,500 GPH. Use the actual water volume from this calculator (not the nominal tank size) when calculating your required GPH, because the filter only processes water, not substrate or glass.

How much substrate do you need for your fish tank?

Substrate depth depends on the tank's purpose. A standard community tank requires 1–2 inches of gravel. A planted tank benefits from 2–3 inches of nutrient-rich substrate. A bare-bottom tank (common for cichlid and hospital tanks) uses no substrate at all.

To calculate the substrate volume, multiply the tank's bottom area by the desired depth. A 55-gallon tank (48 × 13 inches base) with 2 inches of gravel requires $48 \times 13 \times 2 = 1{,}248$ cubic inches, or approximately 0.72 cubic feet. Gravel weighs roughly 100 pounds per cubic foot, so this layer weighs about 72 pounds. Use the Aquarium Weight Calculator mode to include this weight in your total load calculation for stand and floor safety.

For Further Reading

The following authoritative sources provide additional information on aquarium volume calculations, fishkeeping best practices, and structural safety:

• University of Florida IFAS Extension – Calculating Area and Volume of Ponds and Tanks – Comprehensive guide to volume calculations for aquaculture and ornamental fish keeping.
• U.S. Food and Drug Administration – Aquaculture Drugs – Official guidelines on medication dosing and water treatment for fish.
• American Society of Civil Engineers – Structural Engineering – Professional resources on load-bearing calculations and structural safety.
• Penn State Extension – Aquatic Plants, Soils, and Substrates – Detailed information on substrate selection and its impact on water chemistry.
• Engineering Toolbox – Tank Volume Calculations – Engineering reference for tank capacity calculations.

Note: Always verify calculations with a qualified professional before making structural modifications or installing large aquariums. The information provided here is for educational purposes and should not replace professional engineering advice.