Guide
How thick should aquarium glass be?
By the Aquarium Calculator team
For a rimless glass aquarium filled to the top, a useful rule is 4–5 mm for tanks up to 30 cm (12 in) tall, 6–8 mm at 40 cm (16 in), 10 mm at 50 cm (20 in) and 12–15 mm at 60 cm (24 in). Height matters far more than volume, and braced or framed tanks can use thinner glass.
Why height matters more than volume
Water pressure depends only on depth. At the bottom of 50 cm of water the pressure on the glass is about 4.9 kilopascals, whether the tank holds 50 litres or 500. Pressure rises steadily from zero at the surface to its maximum at the base, so the total force on a pane grows with the square of its height, and the bending effect on the glass grows faster still.
To put it in perspective: the front pane of a 120 × 50 cm tank holds back roughly 1,500 newtons of force, about the weight of 150 kg pressing on it, concentrated towards the bottom edge. That's why a tall, narrow tank can need thicker glass than a long, shallow tank holding twice the water. Double the height of a tank and it needs roughly 2.8 times thicker glass.
How the thickness is calculated
Aquarium builders treat each upright pane as a flat plate held along its base and sides, with the top edge free. The thickness needed is t = √(β × γ × H³ × SF ÷ σ), where H is the water height, γ is the weight of water, σ is the strength of float glass (a modulus of rupture of 19.2 N/mm²) and SF is a safety factor, traditionally 3.8. The coefficient β depends on the pane's proportions: long, low panes bend more in the middle than square ones, so they need slightly thicker glass.
The safety factor of 3.8 allows for things you can't see: tiny flaws and scratches in the glass, a stand that isn't perfectly flat, and the long-term fatigue of glass under constant load. It means the glass is designed to carry nearly four times the stress the water actually puts on it. The glass thickness calculator applies this formula pane by pane and rounds up to a stock sheet size.
Glass thickness chart
These are the recommended thicknesses for rimless tanks filled to the top at the standard safety factor, from the calculator: 45 × 30 × 30 cm needs 4 mm all round; 60 × 30 × 36 cm needs 6 mm front and back with 4 mm ends; 80 × 35 × 40 cm needs 8 mm front and back with 5 mm ends; 90 × 45 × 45 cm needs 8 mm with 6 mm ends; 120 × 45 × 50 cm needs 10 mm with 6 mm ends; and 150 × 50 × 60 cm or 180 × 60 × 60 cm needs 15 mm front and back with 8–10 mm ends.
Notice how the 120 × 45 × 50 cm tank holds 270 litres on 10 mm glass, while the 150 × 50 × 60 cm tank, only 10 cm taller, jumps to 15 mm. Height is the dominant factor. Many builders use the thickest figure for every pane for a uniform appearance, and the base should always be at least as thick as the thickest upright pane.
Rimless, braced and framed tanks
The figures above are for rimless tanks with no support along the top edge, the most demanding case. Most mass-produced tanks have a plastic frame and often a centre brace, which stop the long panes bowing outward and let manufacturers use much thinner glass. A standard framed 55-gallon (48 × 13 × 21 in) typically uses about 6 mm glass, while a rimless tank of the same size would need about 10 mm.
That has an important consequence: never remove the frame or centre brace from a framed tank to make it look rimless. The brace is part of the structure, and without it the glass may be far too thin. For tall custom tanks, above about 60 cm (24 in), add a glass euro-brace around the top and have the design checked by an experienced glass supplier.
Glass types and other considerations
Standard annealed float glass is the usual material, and low-iron (ultra-clear) glass has practically the same strength, so the same thicknesses apply; it just looks clearer, without the green tint. Tempered glass is several times stronger but can't be cut or drilled after tempering and shatters completely if it fails, so it's only used where a manufacturer has designed for it. Acrylic is a different material altogether and needs its own calculation.
Thickness is only part of a safe tank. The silicone joints, the flatness of the stand, and the age of the tank all matter. Always use a level stand with a full-surface support or a foam levelling mat if the tank is designed for one, and inspect older tanks for peeling or cloudy silicone. Enter your dimensions in the glass thickness calculator for a recommendation for your exact build, then use the tank weight calculator to check what your stand must hold.
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