Aquarium Glass Thickness Calculator

Why water depth sets glass thickness, not gallons

Gallons tell you how much water a tank holds. They do not tell you how hard that water pushes on the glass. Pressure at any point depends on the depth of water above it, and it rises in a straight line: fresh water adds 0.0361 psi for every inch of depth. A tank with 12 inches of water sees 0.43 psi at the bottom, and a tank with 24 inches sees 0.87 psi, no matter how long either tank is.

What changes with length is how far the panel has to span between supports. A long, shallow tank can use thinner glass than a short, deep one, which is why a 125 gallon tank at 21 inches of water often uses the same 1/2 inch glass as a 75 gallon tank at the same depth. The number that matters is the water column, measured from the top of the water to the bottom of the pane.

Two corrections matter in practice. First, the water line sits 1 to 2 inches below the rim on most tanks, so use the water depth, not the tank height. Second, the bottom pane carries the whole column plus the weight of rock and sand, so it is built thicker than the sides in almost every design.

The plate formula behind the calculator

Aquarium glass is sized with a plate-bending formula, the same family of math used for windows and structural glazing. The version used by tank builders solves for thickness when the panel is supported on four edges:

t = square root of (beta x p x H squared / sigma)

In that expression, t is thickness in inches, beta is a coefficient near 0.5 for a panel supported on all four edges, p is the pressure at the bottom in psi, H is the water depth in inches, and sigma is the design stress of the glass. For annealed soda-lime float glass the design stress is about 1,000 psi, a figure that already carries the hobby's long-standing safety factor of 3.8 against rupture. Custom builders sometimes work to 5.0 and end up one size thicker.

Substituting p equals 0.0361 times H gives a short form that is easy to check by hand: t = 0.00425 x H to the power 1.5. At 20 inches that is 0.38 inch, so 3/8 inch glass. At 24 inches it is 0.50 inch, so 1/2 inch. At 30 inches it is 0.70 inch, so 3/4 inch.

The formula ignores the top brace, the silicone seam and the frame. Those details are why a braced tank can run one size thinner than a rimless tank of the same depth, and why a manufacturer's chart beats a calculator when the two disagree.

Glass thickness by tank size

Pre-solved charts are how most tanks get built, because the formula needs a coefficient most people do not have. The table below follows published hobby charts for annealed float glass at a safety factor of 3.8, with the length correction this calculator applies above 48 inches.

Notice how little the gallon count matters next to the height. The 55 and 75 gallon tanks share 3/8 inch sides because both are 21 inches tall, and the 90 and 120 gallon tanks share 1/2 inch sides because both are 24 inches tall. The 125 gallon tank at 72 inches long needs 1/2 inch sides at only 21 inches of water, purely because of the span.

Rimless tanks go one size up from these numbers. Without a top brace the panel deflects more, so a 48 inch tank at 23 inches of water runs 5/8 inch rimless where a braced build stays at 1/2 inch. That extra step is what you pay for the clean top edge.

Worked example: a 48 by 18 by 24 inch tank

Take a 48 inch long, 18 inch wide, 24 inch tall tank with the water line 1 inch below the rim, so 23 inches of water. That is a common 90 gallon footprint.

• Pressure at the bottom: 23 x 0.0361 = 0.830 psi.
• Side thickness: 0.00425 x 23 to the power 1.5 = 0.00425 x 110.3 = 0.469 inch, which rounds up to 1/2 inch.
• Length correction: the tank is 48 inches, so no extra thickness is added.
• Bottom: one size up from the sides, so 5/8 inch.
• Water volume: 48 x 18 x 23 = 19,872 cubic inches, divided by 231 = 86.0 gallons.
• Water weight: 86.0 x 8.34 = 717 pounds.

Now push the same tank to 30 inches of water. The pressure climbs to 1.08 psi, the side thickness goes to 0.70 inch, and the build needs 3/4 inch glass on the sides and 7/8 inch on the bottom. The tank holds 112 gallons and the water alone weighs 936 pounds, before rock, sand and the stand. That is the point where a wood floor needs a check from a contractor, not just a level.

The 8.34 pounds per gallon figure is the density of fresh water at room temperature, and the NIST unit conversion tables give the metric equivalents if you work in liters and kilograms.

Run the numbers the other way and a shallow tank looks cheap. A 48 by 24 inch footprint at 16 inches of water needs 0.27 inch sides, so 5/16 inch glass, and holds 80 gallons. The same footprint at 24 inches needs 1/2 inch glass for 120 gallons. Doubling the height does not double the glass cost, it roughly quadruples the glass volume and the weight of the empty tank.

Safety factor, bracing and the bottom pane

Glass has no yield point. It bends until it breaks, and the break is sudden, so every aquarium calculation leans on a safety factor rather than a predicted failure load. The number 3.8 has been the hobby standard for decades, and it is the difference between a tank that survives a bump from a vacuum cleaner and one that does not.

Bracing. A center brace or a full eurobrace cuts deflection across the top edge and lets a builder drop one glass size. It also traps salt creep and blocks light, which is why rimless tanks pay for thicker glass instead. If you remove a brace from a manufactured tank, you have removed the reason its glass was thin.

The bottom. The bottom pane carries the full water column plus the point loads from rock. One size up is the norm: 3/8 inch sides get a 1/2 inch bottom, 1/2 inch sides get 5/8 inch. On a large tank the bottom is often tempered, which is 4 times stronger in bending but cannot be drilled after tempering.

The seam. Silicone is the weak link in most failures. A tank needs a 1/8 inch to 3/16 inch silicone gap, a clean bond surface and 24 hours of cure per millimeter of thickness before it sees water. A perfectly sized pane with a bad seam still fails.

The stand. Glass is strong in compression and weak in torsion. A stand that is out of level by 1/4 inch over 4 feet twists the tank, and the seam takes the load. Shim the stand, use a foam mat under a rimless tank, and check level after the tank is filled.

Acrylic and glass: how the thickness compares

Acrylic tanks are built from cast sheet, and the sheet has to be thicker than glass for the same span. The reason is stiffness: the elastic modulus of acrylic is roughly 400,000 psi against about 10 million psi for glass, a factor of 25. Acrylic is tougher and does not shatter, but it deflects far more under the same load, so thickness goes up to keep the bow in check.

As a rule of thumb, cast acrylic runs 1.25 to 1.5 times the glass thickness for the same water depth. A tank that calls for 1/2 inch glass usually ships with 5/8 inch acrylic, and a 3/4 inch glass tank ships with 1 inch acrylic. The weight saving is real: acrylic is about half the density of glass, so a 120 gallon acrylic tank can weigh 200 pounds less than the glass version.

The trade-offs run the other way on scratches and clarity over time. Acrylic scratches if you look at it wrong, though the scratches polish out. Glass resists scratching but chips at the edge, and a chipped edge is where a crack starts. For a tank over 200 gallons, the weight difference usually decides it, and the thickness chart follows from there.

What the calculator cannot tell you

This is a planning estimate, and it is worth being clear about the gaps.

No code covers it. US residential codes regulate the floor and the wall that hold a tank, not the tank. There is no inspection and no stamped drawing for a home aquarium, so a bad build fails quietly until it does not. The National Glass Association publishes guidance on flat glass strength and handling that applies to the material, but not to aquarium design.

Glass strength varies. The rupture strength of annealed float glass depends on edge quality, surface flaws and how the pane was handled. A chipped edge can cut strength by half. That scatter is exactly why the safety factor exists, and why a calculated 0.47 inch is bought as 1/2 inch, not 15/32 inch.

Manufacturing tolerance. Nominal 1/2 inch glass is often 11.5 to 12 mm, and a pane can be 0.5 mm under nominal. Buy from a supplier who states the actual thickness and who cuts and polishes edges for aquarium use.

The rest of the load. Water is the easy part. Sand weighs about 100 pounds per cubic foot, and rock can add 1 to 2 pounds per gallon. A 120 gallon tank with 2 inches of sand and 100 pounds of rock puts close to 1,100 pounds on the stand, and the stand spreads that over four small feet.

Use these figures as a starting point for a build, not as a substitute for a structural review on a large tank. Confirm the actual thickness with your glass supplier.