Aquarium Volume by Size

The number printed on an aquarium box — "55 gallon," "75 gallon" — is a nominal rating, not a precise measurement of how much water you will actually put in the tank. This page provides all 16 standard aquarium sizes with their true calculated volumes, realistic water volumes after displacement, freshwater and saltwater weights, and liter equivalents. For brand-specific published dimensions alongside nominal vs. calculated volumes, see the aquarium volume by brand reference table. For tanks under 10 gallons — including cube, shallow, and rimless nano formats — see the nano aquarium volume guide. Working in metric units? The aquarium volume in litres page lists international tank sizes in centimetres with calculated litre volumes.

Standard Aquarium Volume Table

Calculated gallons = (L × W × H) ÷ 231. Actual water gallons = calculated × 0.90 (accounts for substrate, decor, and fill level). Weights are for calculated volume at room temperature.

Size Name Dimensions (in) Nominal gal Calculated gal Actual water gal FW weight (lbs) SW weight (lbs) Liters
5 Gallon 16 × 8 × 10 5 5.54 4.99 46.2 47.4 21.0
10 Gallon 20 × 10 × 12 10 10.39 9.35 86.7 88.8 39.3
15 Gallon 24 × 12 × 12 15 14.96 13.46 124.8 127.9 56.6
20 Gallon High 24 × 12 × 16 20 19.95 17.95 166.4 170.6 75.5
20 Gallon Long 30 × 12 × 12 20 18.70 16.83 155.9 159.9 70.8
29 Gallon 30 × 12 × 18 29 28.05 25.25 233.9 239.8 106.2
30 Gallon Breeder 36 × 18 × 12 30 33.66 30.30 280.7 287.8 127.4
40 Gallon Breeder 36 × 18 × 16 40 44.88 40.39 374.3 383.7 169.9
55 Gallon 48 × 13 × 21 55 56.73 51.06 473.1 485.0 214.8
65 Gallon 36 × 18 × 24 65 67.32 60.59 561.5 575.6 254.9
75 Gallon 48 × 18 × 21 75 78.55 70.69 655.1 671.6 297.4
90 Gallon 48 × 18 × 24 90 89.77 80.79 748.7 767.5 339.9
100 Gallon 60 × 18 × 20 100 93.51 84.16 779.9 799.5 354.1
120 Gallon 48 × 24 × 24 120 119.69 107.72 998.2 1023.4 453.1
125 Gallon 72 × 18 × 21 125 117.82 106.04 982.6 1007.4 446.2
150 Gallon 72 × 18 × 28 150 157.09 141.38 1310.1 1343.1 594.9
180 Gallon 72 × 24 × 24 180 179.53 161.58 1497.3 1535.0 679.7

Why Advertised Gallons Differ from Actual Water Volume

Aquarium manufacturers calculate nominal volume using exterior dimensions — the full outer length, width, and height of the glass box — and assume the tank is filled completely to the brim. Neither assumption reflects real-world use. Glass thickness subtracts from every interior dimension: a standard ¼-inch glass panel reduces the interior by ½ inch per dimension pair, and ½-inch glass (common on tanks over 75 gallons) removes a full inch from each paired dimension. On a 55-gallon tank, this alone can reduce true volume by 2–3 gallons. See the dedicated glass thickness volume loss table for precise interior volumes across all common glass thicknesses.

Substrate — gravel, sand, or soil — sits on the bottom of every stocked aquarium and displaces a meaningful amount of water. Two inches of gravel in a 55-gallon tank displaces roughly 3–4 gallons. Use the aquarium displacement calculator to calculate exact displacement for your tank. Sand displaces more because its fine particles leave fewer void spaces. Equipment including internal filters, heaters, and powerheads adds additional displacement, typically 0.5–1.5 gallons in a mid-size tank.

Finally, no responsible aquarist fills a tank to the absolute brim. A water surface 1–2 inches below the rim is standard practice to prevent overflow from surface agitation and splashing from pumps and powerheads. On a 55-gallon tank that is 21 inches tall, a 2-inch air gap reduces water volume by approximately 9.5% — roughly 5 gallons. Combined with glass thickness and substrate displacement, a "55-gallon" tank typically holds 46–50 gallons of actual water in normal operation.

Water Weight for Common Tank Sizes

Freshwater weighs 8.34 lbs per US gallon at room temperature. Saltwater at standard reef salinity (1.025 specific gravity) weighs approximately 8.55 lbs per gallon — about 2.5% heavier. For a 75-gallon reef tank, that difference adds up to over 16 lbs of extra water weight compared to freshwater. Always use the calculated gallon figure (not nominal) when estimating weight for floor load calculations, and add the weight of the empty tank, stand, substrate, and rock.

10 Gallon

20×10×12 in

Freshwater:87 lbs
Saltwater:89 lbs

55 Gallon

48×13×21 in

Freshwater:473 lbs
Saltwater:485 lbs

75 Gallon

48×18×21 in

Freshwater:655 lbs
Saltwater:672 lbs

How to Use This Table

If your tank matches one of the 16 standard sizes, read the actual water gallons directly from the table — this is the figure to use for medication dosing, water change volumes, and stocking density calculations. If your tank dimensions do not match any standard size (common for custom, imported, or older tanks), use our rectangular aquarium volume calculator to enter your exact inside dimensions. For non-rectangular shapes, browse the full list on the calculator hub.

To convert any volume result between US gallons, liters, imperial gallons, and cubic units, use the aquarium volume unit converter.

Size and Volume Questions

Why does my 55-gallon tank actually hold less than 55 gallons?
The 55-gallon label is a nominal rating based on exterior dimensions and assumes the tank is filled to the brim. In practice, glass thickness reduces interior space, substrate and equipment displace water, and you fill 1-2 inches below the rim to prevent overflow. Actual usable water volume is typically 85-90% of the nominal rating.
What is the difference between calculated gallons and actual water gallons?
Calculated gallons is the geometric volume based on inside dimensions measured to the top of the tank. Actual water gallons accounts for substrate, decorations, equipment displacement, and a typical fill level 1-2 inches below the rim — approximately 90% of calculated capacity.
Which standard aquarium size gives the best volume for the footprint?
The 40-gallon breeder (36 x 18 x 16 in) is widely regarded as the best all-around value in the hobby. Its wide footprint provides excellent swimming space, the 18-inch depth allows proper aquascaping, and 40 gallons is enough volume for a diverse community of fish.
How much does a 75-gallon aquarium weigh when full?
The water alone weighs about 655 lbs for freshwater (78.55 gal x 8.34 lbs/gal). Add roughly 140-180 lbs for the glass tank, stand, substrate, and rock. A fully set up 75-gallon freshwater aquarium can weigh 800-900 lbs total.
What is the lightest large aquarium option?
Acrylic tanks of the same dimensions weigh roughly 50% less than glass tanks. A 75-gallon acrylic tank weighs about 60 lbs empty vs. 120-140 lbs for glass. However, acrylic scratches more easily. For very large tanks (120+ gallons), acrylic is commonly preferred for structural weight reasons.
How do I know what size aquarium I have if there is no label?
Measure the inside length, width, and height in inches and multiply them together, then divide by 231. Compare the result to the nominal gallon ratings in the standard size chart. Most common tanks have dimensions that closely match established standard sizes.