Aquarium Sizes Explained: Dimensions, Weights, and Which Size Fits Your Space

If you’ve ever stood in the fish aisle staring at two tanks that both say “20 gallons” but look nothing alike, you’re not imagining it. Aquarium sizes aren’t just about how much water a tank holds — the exact shape of that gallon count changes everything from which fish you can keep to whether your floor can handle it. This guide breaks down standard and specialty-shaped aquarium dimensions and weights by size, and helps you figure out which size actually fits your space and your fish.

Why Aquarium Dimensions Matter More Than the Gallon Number

Two tanks holding the same 20 gallons can be very different environments. A 20 gallon high is tall and narrow; a 20 gallon long is short and wide. Same water volume, different living space.

• Length is swimming room. Active, schooling fish — danios, tetras, rasboras — use horizontal space more than vertical space.
• Height adds volume and suits tall-bodied fish like angelfish and discus, but in planted tanks it also increases the distance light has to travel to reach plants near the substrate. How much that matters depends on the fixture’s output, spectrum, and mounting height, the plant species, and water clarity — there’s no single depth where lighting “stops working.”
• Footprint (length × width) determines how much substrate you need to cover the bottom, and how much room you have for rocks, wood, and plants.
• Surface area affects gas exchange at the water’s surface — genuinely relevant to water quality, but it’s one factor among several (filtration, species, feeding, maintenance) that determine safe stocking, not a shortcut for how many fish a tank can hold.

Common Rectangular Aquarium Sizes and Approximate Dimensions

These are common dimensions for traditional rectangular glass aquariums, cross-referenced against manufacturer and major retailer specifications. Exact measurements vary by manufacturer and model — “standard” is useful shorthand, not a guarantee that every 15-gallon or 30-gallon tank on the market matches these numbers exactly.

Small aquariums (2.5–15 gallon)

Aquarium Size	Dimensions (L × W × H, in)	Shape
2.5 gallon	12.25 × 6.25 × 8.13	Standard rectangular
5 gallon	16 × 8 × 10	Standard rectangular
5.5 gallon	16.25 × 8.38 × 10.5	Standard rectangular
10 gallon	20.25 × 10.5 × 12.63	Standard rectangular
15 gallon	24.25 × 12.5 × 12.75	Standard rectangular
15 gallon tall	20.25 × 10.5 × 18.75	Tall rectangular

The 5 gallon aquarium dimensions above are one of the most common nano sizes. 15 gallon aquarium dimensions vary more than most sizes because standard and tall 15-gallon configurations are genuinely different shapes — check which one you’re looking at before ordering a stand or lid.

Mid-size aquariums (20–45 gallon)

Aquarium Size	Dimensions (L × W × H, in)	Shape
20 gallon high	24.25 × 12.5 × 16.75	High rectangular
20 gallon long	30.25 × 12.5 × 12.75	Long rectangular
29 gallon	30.25 × 12.5 × 18.75	Standard rectangular
30 gallon	36.25 × 12.63 × 16.75	Standard rectangular
38 gallon	36.25 × 12.63 × 19.75	Standard rectangular
40 gallon breeder	36.25 × 18.25 × 17	Breeder rectangular (wide)
45 gallon	36.25 × 12.63 × 23.75	Tall rectangular

30 gallon aquarium dimensions are worth double-checking against manufacturer specs, since multiple 30-gallon configurations exist — don’t order a stand, canopy, or lid from the label alone. The 40 gallon breeder’s wider 18.25-inch depth (versus the standard tank’s roughly 12.6 inches) is exactly why it’s a popular choice for aquascaping and fry-raising: more usable floor space, not just more volume.

Large aquariums (55–210 gallon)

Aquarium Size	Dimensions (L × W × H, in)	Shape
55 gallon	48.25 × 12.75 × 21	Standard rectangular
60 gallon breeder	48.5 × 18.5 × 16	Breeder rectangular
65 gallon	36.38 × 18.38 × 25	Standard rectangular
75 gallon	48.5 × 18.5 × 21.13	Standard rectangular
90 gallon	48.5 × 18.5 × 25.13	Standard rectangular
125 gallon	72.5 × 18.5 × 23.13	Standard rectangular
150 gallon	72.5 × 18.5 × 29.5	Standard rectangular
210 gallon	72.5 × 24.5 × 29.63	Standard rectangular

A note on 50-gallon tanks specifically: this size isn’t standardized enough to state one universal measurement. Some rectangular 50-gallon models run around 36 × 18 inches of footprint, but manufacturers vary meaningfully here — check the exact model’s spec sheet before buying a stand or lid, rather than assuming a single “50 gallon” figure applies.

A note on the 60-gallon breeder’s empty weight: Aqueon’s own published spec sheet lists it at 43 lb empty (about 543 lb filled), but Aqueon’s product listing on Petco states the empty tank actually weighs closer to 74.8 lb and flags the lighter figure as an error on their information sheet. The 48.5 × 18.5 × 16-inch dimensions are consistent everywhere; only the empty-weight figure is genuinely disputed between the manufacturer’s own documents. Use the heavier estimate when planning a stand.

Sources:

Aqueon Standard Tank Sizes specifications (PDF)

Petco Aquarium Sizes, Weights & Dimensions Guide

Same Gallons, Different Shape: Three Direct Comparisons

Gallon capacity alone hides how a tank is actually built. These comparisons make the pattern concrete.

20 High vs. 20 Long

	20 High	20 Long
Length	~24.25 in	~30.25 in
Width	~12.5 in	~12.5 in
Height	~16.75 in	~12.75 in

Same nominal capacity, roughly six extra inches of horizontal length in the Long, versus more volume concentrated vertically in the High.

40 Breeder vs. 55 Gallon vs. 75 Gallon (footprint only)

Tank	Footprint (L × W)
40 Breeder	36.25 × 18.25 in
55 gallon	48.25 × 12.75 in
75 gallon	48.5 × 18.5 in

The 55 holds more nominal water than the 40 Breeder but is considerably narrower front-to-back. The 75 combines roughly four feet of length with substantially more depth than the 55 — which matters for aquascaping room, but also means a stand built for a 55 shouldn’t be assumed to fit a 75.

Neither shape is “better” in general — the right one depends on the fish you’re keeping, your aquascaping plans, and the space you have.

Beyond Rectangular: Hexagon, Bow Front, Cube & Corner Aquarium Dimensions

Standard rectangular tanks aren’t the only shape on the market, and gallon-per-shape confusion is even worse here than with rectangular tanks — a 20-gallon hexagon and a 20-gallon rectangular tank share almost nothing in common footprint-wise.

Hexagon aquariums

Size	Dimensions (W × H, in)	Shape
10 gallon	14.5 × 18.375	Hexagon
20 gallon	18.75 × 20.625	Hexagon
20 gallon (deluxe)	15 × 24	Hexagon (tall)
28 gallon	21 × 20 (footprint 21 × 18)	Hexagon
35 gallon	23.25 × 24.75	Hexagon
60 gallon	27.25 × 29.5	Hexagon

Hexagon tanks trade swimming length for a small footprint and a distinctive display shape, which is exactly why they’re popular for corners and tight rooms — but that same shape means less usable horizontal swimming room than a rectangular tank of equal volume, and light has to penetrate a taller water column in many hex models.

Bow front aquariums (curved front glass panel, flat back)

Size	Dimensions (L × W × H, in)	Shape
16 gallon	20 × 13 × 21	Bow front
26 gallon	24.2 × 14.8 × 20.9	Bow front
36 gallon	30.25 × 15.5 × 20.875	Bow front
46 gallon	36.375 × 16.375 × 20.875	Bow front
72 gallon	48 × 18 × 22	Bow front

The 26-gallon figure above is consistent across both Aqueon- and Marineland-branded bow fronts in independent retailer listings, suggesting shared tooling between the two brands (both are part of the same parent company) rather than a coincidence.

Aqueon Bow Front Aquariums (manufacturer specifications)

NAFB Aquarium — Aqueon 36-gallon bow front spec (independent confirmation)

NAFB Aquarium — Aqueon 46-gallon bow front spec

Cube aquariums

Rimless cube tanks (Marineland’s line is a common reference point) commonly run 25, 45, and 60 gallon — all built on a roughly equal-sided cube footprint rather than a long rectangle, which concentrates volume in a smaller floor area but also means less horizontal swimming room per gallon than a standard or long tank.

Corner and column aquariums

Corner-fit tanks (like Marineland’s Deep Dimension corner-flo line) and column tanks solve a specific placement problem — filling an unused room corner — but their triangular or narrow-footprint shape reduces usable swimming length more than any other common shape, so they suit slow-moving or smaller fish better than active swimmers.

Looking for a specific branded kit’s dimensions (Aqueon, Marineland, Tetra) or a reef-ready/drilled tank for sump filtration? Those live in a separate companion reference, since branded product specs change more often than standard tank sizes do and deserve their own page.

A Quick Aquarium Shape Glossary

• Standard rectangular — the default balanced-proportion box shape for a given gallon count.
• Long — stretched horizontally, shorter in height, same or similar volume as the standard version.
• High / Tall — same footprint as standard, taller water column.
• Breeder — noticeably wider front-to-back than a standard tank of the same length, adding usable floor space.
• Bow front — a curved front glass panel with a flat back; adds visual depth without a fully rectangular footprint.
• Hexagon / flat-back hexagon — six-sided footprint, often used in corners; a flat-back version sits flush against a wall.
• Cube — roughly equal length, width, and height; concentrates volume in a small footprint.
• Corner / column — narrow or triangular footprint designed to fill an unused room corner.
• Reef-ready / drilled — pre-drilled for an overflow box and external sump, common on larger saltwater setups.
• Rimless — no visible top or bottom frame, common on nano and cube tanks for an unobstructed view.

Converting Between US Gallons, UK Gallons & Liters

Many aquarium searches come from outside the US, where tanks are usually sized in liters or, in the UK, sometimes in imperial (UK) gallons. The conversions below are fixed unit definitions, not estimates:

Unit	Equivalent
1 US gallon	3.785 liters
1 UK (imperial) gallon	4.546 liters
1 UK gallon	≈1.201 US gallons
1 liter	≈0.264 US gallons

So a tank advertised as “100 liters” is roughly a 26-gallon US tank, and a UK-market tank listed in “gallons” will hold noticeably more than the same number in US gallons — worth checking which gallon a listing means before comparing it to the US-based charts above.

How Much Water Is Actually in Your Tank?

If you know a rectangular tank’s internal dimensions in inches, you can estimate its gallon capacity with:

Length × Width × Height ÷ 231 = US gallons

(One US liquid gallon equals 231 cubic inches — this is a fixed unit conversion, not an approximation.)

Example: an internal water space measuring 30 × 12 × 12 inches gives 30 × 12 × 12 = 4,320 cubic inches, and 4,320 ÷ 231 ≈ 18.7 gallons.

That number, though, is the tank’s raw capacity — not necessarily how much water ends up in your finished setup. Several things reduce usable volume below the advertised rating:

• Glass and frame thickness — advertised capacity is often based on external dimensions; water only occupies the internal space.
• Fill line — most setups leave a gap between the water surface and the rim.
• Substrate — gravel, sand, and planted-tank substrate all displace water.
• Hardscape — rocks, wood, and dense décor can meaningfully reduce actual water volume in a heavily aquascaped tank.
• Internal equipment — internal filters and similar components take up space too.

Treat the advertised gallon number as a useful size category, not an exact measurement of the water that will physically be in the tank once it’s set up.

How Much Does a Fish Tank Weigh?

Fresh water weighs approximately 8.34 lb per US gallon at typical reference conditions (water density shifts slightly with temperature, so treat this as a highly reliable estimate rather than an exact figure). Total operating weight also includes the empty tank, substrate, hardscape, equipment, and any water held in external filtration (a canister filter, for instance, holds its own water volume separately from the tank). Manufacturer “filled weight” figures below are useful planning references, but your actual setup may weigh more or less depending on what’s inside it.

Tank Size	Empty Tank Weight	Manufacturer Filled Weight*
10 gallon	~11 lb	~111 lb
15 gallon	~21 lb	~170 lb
20 gallon	~25 lb	~225 lb
29 gallon	~40 lb	~330 lb
40 gallon breeder	~58 lb	~458 lb
55 gallon	~78 lb	~625 lb
75 gallon	~98–140 lb	~785–965 lb
90 gallon	~135–160 lb	~1,050 lb
125 gallon	~177–206 lb	~1,400 lb
150 gallon	~281 lb	~1,800 lb
210 gallon	~343 lb	~2,450 lb

*Figures reflect manufacturer-published tank-plus-water weight; ranges reflect brand variance. These do not include substrate, décor, or equipment — add those separately using their listed weights.

The 75-gallon range is wider than most sizes for a specific reason: older manufacturer charts (including Aqueon’s own published spec sheet) list a lighter ~98 lb empty / ~785 lb filled 75-gallon tank, while other current retailer charts list a heavier ~140 lb empty / ~965 lb filled version. Both appear to be genuine standard 75-gallon glass tanks — the difference likely comes from glass thickness and frame construction differing by manufacturer and production era, not an error in either figure. Check your specific tank’s listed weight rather than assuming either end of the range.

To answer the specific question directly: a 20 gallon fish tank weighs roughly 225 lb as a manufacturer-rated filled weight (tank plus water), before substrate, decorations, or equipment are added.

Acrylic aquariums are generally lighter than comparable glass tanks when empty, but water usually accounts for most of the operating weight in a finished setup. Always calculate your complete setup weight using the specifications of your actual tank rather than a general glass-vs-acrylic rule of thumb.

Aqueon Standard Tank Sizes specifications (official PDF)

Standard vs. Long vs. High vs. Breeder: What the Names Mean

• Standard — the tank’s “default” shape for that gallon count.
• Long — similar volume, stretched horizontally, shorter in height. Favors active swimmers and surface oxygen exchange.
• High / Tall — same footprint, taller water column. Suits tall-bodied fish and vertical décor.
• Breeder — noticeably wider front-to-back than a standard tank of the same length, adding usable floor space. Popular for fry-raising and bottom-dwelling species.

Planning for the Weight of a Large Aquarium

Large aquariums place a substantial, concentrated load on a floor. Don’t assume a tank is safe in a given spot based only on its gallon capacity or a generic pounds-per-square-foot rule — actual floor safety depends on building design, joist span and spacing, material and condition, the aquarium’s footprint, and exactly where it sits, none of which a gallon number can tell you.

What you can reasonably do yourself:

1. Use a stand designed and rated for that specific aquarium, not general-purpose furniture.
2. Make sure the stand is level and fully supported across its base before filling.
3. Plan around the complete operating weight — tank, water, substrate, décor, and equipment — not just the empty tank or the water alone.
4. For large aquariums, upper-floor installations, older buildings, or any situation where you’re unsure about structural capacity, consult a qualified building professional before installation rather than relying on a general online rule.

IMAGE SUGGESTION: Real photo of a large aquarium on a properly rated aquarium stand. Type: real photo to source or take. Alt text: “large aquarium on a load-rated stand”

How to Choose the Right Aquarium Size for Your Space

Measuring “will it fit” takes more than checking the tank’s length.

• Measure the full footprint, including the stand — a 75-gallon tank can be nearly the same length as a 55 but need meaningfully more depth.
• Leave room for equipment. Filters, plumbing, and cords often extend behind or beside the tank; an alcove that fits the glass perfectly may leave nowhere to route a canister hose.
• Check maintenance access. Can the lid open fully? Can you reach the whole tank? Can equipment come out without moving the aquarium itself?
• Check the route into the room — door openings, hallways, stairs, corners, and elevators, measured for the aquarium and stand separately. Discovering a six-foot tank fits the room but not the doorway is an expensive mistake.

If You Already Know Which Fish You Want

Work backward from the species rather than picking a gallon number first. Check its adult size, activity level, social behavior, territoriality, and preferred swimming zone — a longer tank often suits active horizontal swimmers better than a taller tank of similar capacity, while other species benefit from extra height or front-to-back depth instead. Stocking decisions should follow from the specific fish you’re keeping, not a generic “gallons per fish” formula, since the same nominal size can be right for one species and wrong for another.

For a betta specifically, many modern care guides recommend starting around 5 gallons or larger, with appropriate heating, filtration, and enrichment — this is a common husbandry recommendation, not a fixed biological minimum.

Before Buying a Stand, Lid, or Light: Measure the Actual Tank

Manufacturer dimensions can differ by fractions of an inch between models — enough to matter when you’re buying something meant to fit directly on the frame. Before ordering a stand, lid, canopy, light mount, background, or custom cabinet, check the manufacturer and model number and, where possible, measure the actual tank. This matters especially for rimless, bow-front, cube, column, and other specialty shapes that don’t follow standard rectangular dimensions.

Aquarium Sizes FAQ

What are the dimensions of a 5 gallon aquarium?

There’s no single measurement for every model. A common rectangular 5-gallon tank runs around 16 × 8 × 10 inches; a standard 5.5-gallon model is closer to 16.25 × 8.38 × 10.5 inches.

What are the dimensions of a 20 gallon aquarium?

It depends on the shape: a 20 High is about 24.25 × 12.5 × 16.75 inches, and a 20 Long is about 30.25 × 12.5 × 12.75 inches.

What is the size of a 30 gallon aquarium?

A common standard configuration is around 36.25 × 12.63 × 16.75 inches, but 30-gallon tanks vary by manufacturer — verify before buying fitted equipment.

What are the measurements of a 50 gallon fish tank?

50-gallon dimensions vary substantially by manufacturer and style; some rectangular models run around 36 × 18 inches of footprint, but check the specific model.

What are the dimensions for a 75 gallon tank?

A standard framed 75-gallon aquarium is about 48.5 × 18.5 × 21.13 inches.

What are standard 125 gallon tank measurements?

A common configuration is about 72.5 × 18.5 × 23.13 inches — a little over six feet long, which is worth confirming will actually get through your doors and hallways before you buy.

How much does a 20 gallon fish tank weigh?

Around 225 lb as a manufacturer-rated tank-plus-water figure, before substrate, décor, or equipment.

Are aquarium sizes exact?

Treat the gallon rating as a size category rather than an exact spec. Exterior dimensions, internal dimensions, and usable water volume can all vary between models and manufacturers.

About This Guide

This guide compiles and cross-checks published manufacturer specifications and major-retailer data (primarily Aqueon and Marineland, whose product lines cover the majority of standard glass aquariums sold in the US) rather than restating any single source. Every dimension and weight figure is sourced inline, and places where sources genuinely disagree — such as the 75-gallon weight range and the 60-gallon breeder’s empty weight — are flagged rather than smoothed into one number. Last fact-checked September 2026.

[Editor’s note for site publication: this section is the natural home for a named reviewer with hands-on fishkeeping or aquarium-retail experience — their name, credentials, and a line on what tanks they’ve personally set up. That kind of first-hand signal is something a research pass like this one can’t manufacture, and it’s worth adding before this goes live rather than leaving the guide anonymous.]

Final Takeaway

Start with gallon capacity, but don’t stop there. Length and width tell you the actual floor and swimming footprint; height adds volume without necessarily adding usable swimming space. Before buying, check three things: the exact dimensions, the realistic operating weight, and the manufacturer’s specific model — that’s what prevents the frustrating discovery that a “right size” tank doesn’t actually fit your stand, your lid, or your room.