Aquarium Weight Calculator
What your tank weighs when it is full, what that works out to per square foot of floor, and how that compares to the load a residential floor is designed for.
Last reviewed August 2026 · Free · Nothing you enter is stored
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The pounds-per-square-foot figure is the tank's own footprint load, not a verdict on your floor. Codes size floors for load spread evenly across a room, so a tank above the code figure is common and usually fine. Treat a high number as a reason to check the joists, not as a warning that the floor will fail.
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Water is nearly the whole answer
Every part of this calculation except one is a rounding error. Fresh water weighs 8.34 pounds per US gallon, and saltwater mixed to a specific gravity of 1.025 weighs about 8.55, which is the only reason a reef tank of a given size is heavier than a freshwater one. Multiply your gallons by that figure and you have most of the total already.
The glass adds a fixed amount on top. A 75-gallon tank holds around 626 pounds of water and the empty tank is about 140, so the water is 82 percent of the load before you have put a single stone in it. Substrate and rock might add another 100 to 150. The stand adds whatever the stand weighs. None of those shift the answer as much as getting the gallons right, which is why the first thing to do is work out the actual volume rather than trusting the number on the box.
Acrylic is the one variable people over-think. An acrylic tank is roughly half the weight of the glass equivalent when empty, though no manufacturer publishes an exact percentage and the real ratio moves with panel thickness. It makes the empty tank a one-person carry. It makes almost no difference to what is sitting on your floor a week later.
Common tank sizes and what they weigh filled
These are the standard US footprints, with the trade figures for empty glass weight and fresh water filled to the nominal volume. Saltwater adds about two and a half percent to the water column.
| Sold as | Dimensions (in) | Empty glass (lb) | Water (lb) | Filled total (lb) |
|---|---|---|---|---|
| 10 gallon | 20 × 10 × 12 | 11 | 83 | 94 |
| 20 gallon long | 30 × 12 × 12 | 25 | 167 | 192 |
| 29 gallon | 30 × 12 × 18 | 40 | 242 | 282 |
| 40 gallon breeder | 36 × 18 × 16 | 58 | 334 | 392 |
| 55 gallon | 48 × 13 × 21 | 78 | 459 | 537 |
| 75 gallon | 48 × 18 × 21 | 140 | 626 | 766 |
| 90 gallon | 48 × 18 × 25 | 160 | 751 | 911 |
| 125 gallon | 72 × 18 × 21 | 206 | 1,043 | 1,249 |
Fresh water weighs 8.34 lb per US gallon; seawater at a specific gravity of 1.025 weighs about 8.55. Empty glass tank weights are the standard Aqueon and Marineland trade figures. The floor loading comparison uses IRC Table R301.5, which sets a 40 lb per square foot minimum uniformly distributed live load for habitable rooms and 30 for sleeping rooms. Building codes size floors for distributed load, not point loads, so a tank that exceeds the figure is not automatically unsafe - it means the load deserves a look rather than an assumption.
What the floor is actually rated for
IRC Table R301.5 sets a minimum uniformly distributed live load of 40 pounds per square foot for habitable rooms and 30 pounds per square foot for sleeping rooms. Those are real numbers from a real code, and they are the figures every aquarium forum argument eventually cites. They are also routinely misused.
The critical word is distributed. That 40 psf describes furniture, people and belongings spread across the whole floor, which is the load case a joist span table is built around. A tank does something different: it puts a large weight on a small footprint, and the joists beneath it carry that load back to the walls at either end. A filled 75-gallon tank works out at roughly 140 pounds per square foot over its own six square feet, which sounds alarming until you notice that millions of 75s sit on ordinary wooden floors and have done for decades without anything happening.
So the honest position is this. Exceeding 40 psf under a tank is normal and is not by itself evidence of a problem. There is no aquarium clause in the building code, no size at which a tank becomes illegal, and nobody should be telling you your tank is about to go through the floor. What the number is good for is telling you when the load deserves five minutes of thought rather than none.
| Tank | Footprint (in) | Area (sq ft) | Filled weight (lb) | Load (lb/sq ft) |
|---|---|---|---|---|
| 10 gallon | 20 × 10 | 1.4 | 94 | 68 |
| 20 gallon long | 30 × 12 | 2.5 | 192 | 77 |
| 29 gallon | 30 × 12 | 2.5 | 282 | 113 |
| 40 gallon breeder | 36 × 18 | 4.5 | 392 | 87 |
| 55 gallon | 48 × 13 | 4.3 | 537 | 124 |
| 75 gallon | 48 × 18 | 6.0 | 766 | 128 |
| 90 gallon | 48 × 18 | 6.0 | 911 | 152 |
| 125 gallon | 72 × 18 | 9.0 | 1,249 | 139 |
Those figures are tank and water only. Add substrate, rock and a wooden stand and a 75 lands nearer 140 pounds per square foot, which is the number usually quoted for it.
Placement does more than arithmetic
Where the tank goes matters more than what it weighs. Run it perpendicular to the floor joists so that several of them share the load, rather than parallel, where the whole tank can end up bearing on one or two. On typical 16-inch joist spacing, a 48-inch tank set across the joists spans three or four; the same tank turned ninety degrees may be carried almost entirely by one. That single decision changes the situation more than any hundred pounds of rock.
Beyond that, put the tank near a load-bearing wall rather than in the middle of a span, because a joist is stiffest at its ends and floppiest in the middle. Be more careful on an upper floor than on a ground floor over a slab, where the question does not really arise. Be more careful in an older house, where spans and joist sizes were not standardised and a century of settlement may already have used up some margin. And above about 125 gallons, particularly upstairs, it is worth getting someone to look underneath before you fill it rather than after.
The stand matters as much as the floor
Most tank failures that get blamed on floors are stand problems. A tank needs continuous support under its bottom rim, which is what a proper aquarium stand with a full-perimeter frame provides. Ordinary furniture, or a stand built with four legs and no perimeter, concentrates the entire load on four small points and leaves the bottom pane unsupported between them. Glass tolerates compression and hates being asked to bridge a gap.
That point loading also passes straight through to the floor. Six hundred pounds spread over a 48 by 18 inch frame is a very different thing from six hundred pounds on four contact patches the size of a coaster, and it is the second version that dents floorboards and crushes vinyl. If the stand is not purpose-built, that is the first thing to fix.
Level matters just as much, and for the same reason. Never fill a tank that is not level. Shim it and check it with a spirit level in both directions while it is empty and light enough to move, because once there is water in it the glass is being loaded unevenly and there is no fixing it without draining the whole thing. A tank sitting a quarter of an inch out over four feet puts a permanent twist through the silicone seams, and the visible water line will tell everyone about it for as long as the tank is up.
Frequently asked questions
How much does a filled aquarium weigh?
Take the gallons and multiply by 8.34 for fresh water, or 8.55 for saltwater at a specific gravity of 1.025, then add the empty weight of the tank. Water is so much heavier than everything else that this one multiplication gets you within a few percent of the answer. A 75-gallon freshwater tank comes to roughly 626 pounds of water and 140 pounds of glass, so a shade over 760 pounds before substrate, rock or the stand.
Will my floor hold a 75 gallon tank?
Almost certainly, and millions of them sit on ordinary wooden floors without incident. IRC Table R301.5 sets a minimum uniformly distributed live load of 40 pounds per square foot for habitable rooms, and a filled 75 works out at roughly 140 pounds per square foot across its footprint, but those two numbers are not directly comparable. The code figure describes load spread evenly over an entire room; a tank concentrates its weight over six square feet, and the joists underneath share it. What matters far more is which way the tank runs relative to the joists and how close it sits to a supporting wall.
Should the tank run across the joists or along them?
Across, so that the load is shared by several joists rather than resting on one or two. A 48-inch tank set perpendicular to joists spaced at 16 inches spans three or four of them. The same tank turned parallel can end up bearing almost entirely on a single joist, which is the arrangement that causes deflection and cracked plaster. Siting the tank against a load-bearing wall rather than in the middle of a span helps for the same reason.
At what size do I need to worry?
There is no threshold written into any code, because no building code addresses aquariums specifically. As a practical matter, tanks up to about 75 gallons on a ground floor over a slab or a well-supported crawl space are not worth losing sleep over. Above roughly 125 gallons, or anywhere upstairs, or in a house built before the war where joist sizes and spans are unknown, the sensible move is to look under the floor or have someone who does this for a living look at it.
How much lighter is an acrylic tank?
Roughly half the weight of the equivalent glass tank, empty. Manufacturers do not publish an exact percentage and the ratio drifts with panel thickness and bracing, so treat it as an estimate rather than a specification. It also barely matters once the tank is full, because the water dominates. Acrylic's real advantage is that one person can move the empty tank up a staircase; the filled weight on the floor is nearly identical.
Does substrate and rock add much?
Enough to notice, though nowhere near as much as the water. Gravel runs about 12 pounds per gallon of displaced volume and sand about 14, so two inches across a 75-gallon footprint is somewhere around 90 pounds. A rock scape can easily add another 50 to 100. The important consequence is not the total but the fact that this weight sits on the bottom pane and pushes down through the stand at whatever point you piled it, which is one more reason the tank needs to be level before anything goes in it.
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