Aquarium glass thickness: where the mm come from

A 150 cm tank needs noticeably thicker glass than a 60 cm one at the same height. Here is what the safety factor actually covers.

Reference · Reference · updated 11 November 2026

What loads the glass

Glass holds back a column of water, and pressure rises with depth: lowest at the surface, highest at the bottom. So a pane bows most at the bottom and in the middle of its span, not at the corners, where the neighbouring panes and the silicone seam hold it in place.

Bowing is the main enemy. Glass is brittle and works badly in tension, and failure starts at a microdefect on the edge or at a scratch. The quality of the cut edge and of the joint therefore matters as much as thickness.

Why length beats height

A pane behaves like a beam clamped along its edges. Its stress and deflection depend on the length of the span more strongly than on the depth of the water: the bending moment grows roughly with the square of the length.

The practical consequence is blunt. Double the length at the same height and the moment goes up about four times. That is why a 60 cm and a 150 cm tank of equal height need different glass, while a small increase in height does far less than an increase in length.

Where the 3.8 factor comes from

The bending strength of glass is not one number. It varies with the grade, with microdefects, with the condition of the edge, with temperature. The calculator works out a design stress, divides the allowable stress by 3.8 and rounds the result up to thicknesses you can buy: 5, 6, 8, 10, 12, 15 and 19 mm.

The factor covers more than the scatter in strength. It also covers how tanks actually live: a stand slightly out of level, an uneven pad under the bottom, dynamic loads from pumps and fish, local heating from a lamp or a heater.

Rims and braces

A rim around the top perimeter and cross braces take part of the load off the long walls. With them, the same tank passes at a thinner glass, which is why large volumes are rarely glued without any reinforcement at all.

• Rim: a strip of glass around the top edge. It barely gets in the way of maintenance and adds stiffness.
• Braces: cross ribs between the front and back walls. More effective, but they block light and get in the way of a net, a hose and equipment.

What the formula leaves out

• the quality of a given sheet and of the batch it came from;
• the thickness and quality of the silicone seam;
• temperature swings and local heating;
• impacts, the most common way a tank fails;
• geometry errors: tilt, twist, an uneven floor.

Practical conclusions

• Run the numbers on water depth, not on tank height. A 5 cm substrate layer plus 1 to 2 cm of freeboard is 6 to 7 cm that the load calculation never sees.
• Make the bottom thicker than the walls. It bends across its whole area, not only along the edges.
• In doubt, go one step thicker. The difference in price is smaller than the cost of a new floor.
• Do not save on edge finishing. A polished edge cuts the risk of a chip, and a chip is where cracks begin.

Calculator

The calculator takes your dimensions and works out glass, weight, stand and doses.