Whether the glass on a tank is thick enough for the water height, using the standard bending stress method.
| Water height | Minimum at this length | Common choice |
|---|---|---|
| 12 in, 30 cm | 4.4 mm | 6 mm |
| 18 in, 46 cm | 8.0 mm | 8 to 10 mm |
| 24 in, 61 cm | 12.3 mm | 12 mm |
| 30 in, 76 cm | 17.2 mm | 15 to 19 mm |
Water height drives this, not tank length or volume. Pressure rises with depth, so a long shallow tank needs thinner glass than a short deep one. The panel aspect ratio then adjusts for how much the edges help.
t = √(β × H³ × 1e-5 ÷ B), β = 0.3371, H = 533 mm, B = 19.2 ÷ 3.8 = 5.05 N/mm² → 10.06 mm
A tank wall is a plate loaded by water pressure that rises with depth. The standard method sets the maximum bending stress against an allowable stress and solves for thickness: t = √(β × H³ × 10⁻⁵ ÷ B), with H the water height in millimetres and B the allowable stress in N/mm².
The 10⁻⁵ term folds in water density and gravity. For H in millimetres the pressure at the bottom is 1000 × 9.81 × H/1000 pascals, which is 9.81×10⁻⁶×H N/mm², rounded to 10⁻⁵×H in the published form.
B comes from the tensile strength of annealed float glass under sustained load, taken as 19.2 MPa, divided by a safety factor. The usual factor is 3.8, giving an allowable stress of 5.05 N/mm². That large factor exists because glass under constant load fails by slow crack growth, so a panel that survives filling can still fail months later.
The coefficient β accounts for panel shape. A tall narrow panel is better supported by its edges than a long one of the same height, so β rises from 0.085 at a length to height ratio of 0.5 up to 0.37 at a ratio of 3, beyond which extra length stops mattering.
Height is what drives this, not volume and not length. Pressure depends only on depth. A six foot long tank 18 inches deep needs less glass than a two foot tank 30 inches deep.
Deflection is calculated separately using a Young's modulus of 69 GPa. A few millimetres of bow at the centre of a large panel is normal. Bow that increases over months is not, and neither is any visible chip or scratch on a loaded edge.
Tempered glass is stronger but cannot be drilled after tempering, which is why most drilled reef tanks use annealed glass. Never drill a tempered panel. The bottom panel of many production tanks is tempered and the sides are not.
This calculation covers flat panels with a bottom brace and a top eurobrace. Rimless tanks, unbraced spans, and anything you plan to sit under deserve a professional review rather than a web page.
Sources: Aquarium World NZ, glass thickness · MonsterFishKeepers, glass thickness method