How this calculator works
The calculation treats the longest glass pane as a vertical strip that spans the tank length and is supported only at its two ends. Water pressure grows linearly from zero at the surface to its maximum at the bottom, so the strip carries a triangular load. Beam theory gives the peak bending moment, and the rectangular glass section converts that moment into bending stress. Setting the stress to the glass strength divided by the safety factor gives the minimum thickness. The result is deliberately conservative because real tanks often have a frame or bracing that helps, and because a glass panel is stiffer than a narrow strip. The pane list and weights use the rounded standard thickness, which is the thickness you would actually buy.
Formula
bottom pressure p = water density x gravity x water height. Peak moment per unit width M = p x span^2 / (9 x sqrt 3), where the span is the longer of the tank length and width, for a simply supported strip with a triangular load. Bending stress = 6M / t^2, so minimum thickness t = sqrt(6M x safety factor / glass strength). The recommended thickness is the next standard size above t.
Worked example
A tank 60 cm long, 30 cm wide and 40 cm deep has a bottom pressure of 1,000 x 9.80665 x 0.40 = 3,923 Pa. The peak moment per metre of width is 3,923 x 0.60^2 / (9 x sqrt 3) = 90.6 newton metres. With a 20 MPa working stress and a safety factor of 2, the minimum thickness is sqrt(6 x 90.6 x 2 / 20,000,000) = 0.00737 m, or 7.4 mm. That rounds up to 8 mm standard glass. The tank holds 72 litres of water.
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Frequently asked questions
Can I use this for a rimless tank?
The model is closest to a rimless or simply supported tank, which is why it is conservative. A framed tank with a bottom trim and corner braces is stronger, but a very large rimless tank still needs a designed brace or a thicker pane. For anything beyond a small tank, confirm the thickness with the glass supplier or a structural engineer.
Why does doubling the safety factor not double the thickness?
Bending stress depends on thickness squared, so the required thickness is proportional to the square root of the safety factor. Doubling the safety factor increases the thickness by about 41 percent, not 100 percent.
Does the tank width matter for the glass?
The long front and back panes span the tank length, so length drives the bending. Width changes the volume, the end panes and the bottom pane, and it therefore changes the glass weight, but it does not change the front pane thickness in this model.
What about tempered or laminated glass?
Tempered glass has a much higher strength, and laminated glass behaves differently after cracking. Do not substitute the strength of tempered glass into this model without the manufacturer's design values, because the failure mode and the safe working stress are different.