Standard Fish Tank Sizes: Dimensions & Weights Chart

Almost every glass aquarium sold in the United States comes in one of about a dozen standard sizes, and those sizes have barely changed in 50 years. A “55 gallon” is 48 inches long whether it comes from Aqueon, Marineland, or a local glass shop, which is why stands, lids, and lights are interchangeable. Water weighs 8.34 pounds per gallon, so even a modest tank becomes seriously heavy once filled — a rule of thumb that holds across the whole chart is about 10 pounds per gallon for a filled, decorated tank.

This reference lists the exact dimensions and weights for the ten standard sizes from 5.5 to 125 gallons, explains which sizes suit beginners, and covers when a tank gets heavy enough that you should think about your floor. To check the true volume of any tank from its measurements, use the free fish tank volume calculator.

Standard Fish Tank Sizes: Dimensions and Weights

Dimensions below are the industry-standard nominal sizes (length × width × height, in inches). Filled weight includes water, the tank itself, and a typical gravel bed — which is where the ~10 lb per gallon rule comes from. Exact trim dimensions vary by up to an inch between manufacturers.

Tank size	Dimensions (L × W × H)	Empty weight	Filled weight (approx.)
5.5 gallon	16" × 8" × 10"	8 lb	62 lb
10 gallon (“Leader”)	20" × 10" × 12"	11 lb	111 lb
20 gallon High	24" × 12" × 16"	25 lb	225 lb
20 gallon Long	30" × 12" × 12"	25 lb	225 lb
29 gallon	30" × 12" × 18"	40 lb	330 lb
40 gallon Breeder	36" × 18" × 16"	58 lb	458 lb
55 gallon	48" × 13" × 21"	78 lb	625 lb
75 gallon	48" × 18" × 21"	140 lb	850 lb
90 gallon	48" × 18" × 24"	160 lb	1,050 lb
125 gallon	72" × 18" × 22"	206 lb	1,400 lb

Suffixes matter more than gallon counts. “High” and “Long” tanks hold the same water in different shapes: a 20 High stacks its volume vertically (better for a small footprint), while a 20 Long spreads it across 30 inches (more swimming length and more surface area for gas exchange). A “Breeder” tank is extra deep front-to-back — the 40 Breeder’s 18-inch depth is why it is the favorite of both fish breeders and aquascapers.

How Filled Weight Is Calculated

The filled weights above break down into three parts. Take the 75 gallon: water alone is 75 × 8.34 = 626 lb, the empty glass tank adds 140 lb, and a typical 1–2 inch gravel bed adds another 75–100 lb. Total: roughly 850 lb — about 11 lb per gallon, before you add a stand, canopy, or rockwork.

Substrate is the component people forget. The standard dosing rule is 1 pound of gravel per gallon for a 1-inch bed, 2 pounds per gallon for 2 inches — so a 40 Breeder planted tank with a 2-inch bed carries about 80 lb of substrate alone. The aquarium substrate calculator converts your tank’s footprint and target depth into pounds of gravel or sand so you buy the right number of bags the first time.

Best Sizes for Beginners (Bigger Is More Stable)

The counterintuitive rule of fishkeeping: larger tanks are easier, not harder. Water chemistry problems are concentration problems, and more volume means more dilution. Consider what happens when a fish dies unnoticed or you overfeed:

• In a 5.5 gallon, an ammonia spike can reach toxic levels in hours, and the water temperature can swing several degrees when a room heats up or cools down.
• In a 29 or 40 gallon, the same event is diluted 5–7×, giving you days instead of hours to notice and fix it, and 300+ lb of water changes temperature slowly.

Specific recommendations by size:

• 20 gallon Long — the most commonly recommended first tank. Its 30-inch length gives real swimming room, the 12-inch height makes planting and cleaning easy, and it fits standard 30-inch stands and lights.
• 29 gallon — same 30" × 12" footprint as the 20 Long with 6 more inches of height, so it upgrades capacity without needing more floor space.
• 40 gallon Breeder — the best value per gallon in the hobby and a frequent $50-sale item at chain pet stores; its 36" × 18" footprint is ideal for community tanks and aquascapes.
• 55 gallon — a classic showpiece, but note the narrow 13-inch width; wide-bodied fish and dense aquascapes fit better in a 75.
• 5.5 and 10 gallon — best reserved for a single betta, shrimp, or a quarantine tank. They are the cheapest to buy and the easiest to crash.

The practical ceiling for most beginners is the 40 Breeder or 55: big enough to be forgiving, small enough that a 25% weekly water change is a bucket or two, not a siphon-and-hose operation.

Floor Loading: When to Think About It

A filled aquarium concentrates a lot of weight on a small footprint. A 55 gallon puts about 625 lb on 4.3 square feet (roughly 145 lb per square foot); a 125 gallon puts about 1,400 lb on 9 square feet. For comparison, U.S. residential floors are typically designed to a 40 lb-per-square-foot live load — but that figure describes load spread across an entire room, not a point load, so the real question is how many floor joists carry the tank.

Practical guidance used by aquarists and contractors:

• Up to 55 gallons: safe on virtually any code-built floor, upstairs included, with a proper stand.
• 75–125 gallons: place the tank’s long side perpendicular to the floor joists so the weight spans several of them, and put it against a load-bearing wall where joists are supported. Avoid the middle of a long room span.
• 125 gallons and up: strongly prefer a ground floor, ideally over a slab or a beam line. For 180+ gallon tanks on wood-framed floors, have the framing checked or reinforced — a filled 180 exceeds 1,700 lb.

Also check the stand itself: weight must transfer through the stand’s perimeter to the floor evenly. A tank that is level and fully supported at all corners is at far lower risk of a seam failure than one twisted on an uneven base.

Checking a Tank’s Real Volume

Nominal gallon ratings are approximate — the honest number comes from the glass. The formula: gallons = (length × width × height in inches) ÷ 231, since one U.S. gallon is 231 cubic inches. A “55 gallon” at 48" × 13" × 21" works out to 56.7 gallons of raw box volume; subtract glass thickness, the substrate, and the 1–2 inches you leave below the rim, and actual water volume is typically 10–15% less than the label. That difference matters when you dose medication or fertilizer, size a heater, or rate a filter.

Measure your tank, then let the fish tank volume calculator do the conversion — it handles rectangular, bowfront, and cylinder tanks and returns both nominal and true fill volume. Pair it with the substrate calculator when planning a new setup, and you will know the tank’s final weight before you ever add water.