The glass thickness formula
For the front, back, and end panes this page uses the method Warren Stilwell published in Aquarium World in 2001 (read the article):
t = √(β × γ × H³ × SF ÷ 19.2)
t is the thickness in millimeters, H is the water depth in millimeters (1 inch = 25.4 mm), γ is the weight of water, 9.81 × 10-6 newtons per cubic millimeter (times 1.025 for salt water), 19.2 N/mm2 is the strength figure for float glass, and SF is the safety factor. With SF 3.8 the allowed bending stress is 19.2 ÷ 3.8 = 5.05 N/mm2. β depends on how long the pane is compared with the water depth:
| Pane length ÷ water depth | 0.5 | 0.666 | 1.0 | 1.5 | 2.0 | 2.5 | 3.0 or more |
|---|---|---|---|---|---|---|---|
| β (wall panes) | 0.085 | 0.1156 | 0.16 | 0.26 | 0.32 | 0.35 | 0.37 |
Between rows the calculator interpolates in a straight line. Below 0.5 it uses 0.085, and above 3 it uses 0.37. Worked example: a front pane 48 inches long holding 20 inches of water. The ratio is 48 ÷ 20 = 2.4, so β = 0.344. H = 20 × 25.4 = 508 mm. t = √(0.344 × 9.81 × 10-6 × 508³ × 3.8 ÷ 19.2) = 9.4 mm.
Bottom pane
If the tank will sit on a flat, level stand with a soft pad under the whole bottom, the same article says the bottom can match the thickest wall pane, because the stand carries the load. The calculator shows that figure first.
If the bottom rests only on its edges, for example on a frame-only stand, it is a plate under uniform pressure. For that case this page uses the simply supported rectangular plate from Roark's Formulas for Stress and Strain: t = √(β × γ × H × b² × SF ÷ 19.2), where b is the shorter side (usually the width) and β comes from the long side divided by the short side: 1.0 is 0.2874, 1.4 is 0.4530, 2.0 is 0.6102, 3.0 is 0.7134, 5.0 is 0.7476, and above 5 it uses 0.75.
This is a planning estimate, not an engineering approval. It assumes clean, undamaged annealed float glass held on all four edges, with the water no higher than the pane. Scratches, chips, a stand that is not level, and poor silicone joints cause failures that no formula covers. Factory tanks are designed and tested by their makers; this page does not rate or replace any factory tank.
Example results
All at safety factor 3.8 in fresh water. The water box is the inside length, width, and water depth, and the gallons are that box divided by 231 cubic inches per US gallon.
| Water box | US gallons | Front and back | Ends | Bottom on edges only |
|---|---|---|---|---|
| 20 x 10 x 12 in | 10.4 | 3.9 mm, 3/16 in | 2.8 mm, 1/8 in | 4.8 mm, 1/4 in |
| 30 x 12 x 12 in | 18.7 | 4.4 mm, 3/16 in | 3.0 mm, 5/32 in | 6.0 mm, 5/16 in |
| 36 x 18 x 18 in | 50.5 | 7.7 mm, 3/8 in | 5.4 mm, 1/4 in | 10.6 mm, 1/2 in |
| 48 x 13 x 20 in | 54.0 | 9.4 mm, 1/2 in | 5.4 mm, 1/4 in | 8.9 mm, 3/8 in |
| 48 x 18 x 20 in | 74.8 | 9.4 mm, 1/2 in | 6.1 mm, 5/16 in | 11.8 mm, 1/2 in |
| 48 x 24 x 24 in | 119.7 | 11.9 mm, 1/2 in | 8.4 mm, 3/8 in | 16.4 mm, 3/4 in |
| 72 x 18 x 22 in | 123.4 | 11.2 mm, 1/2 in | 6.8 mm, 5/16 in | 13.0 mm, 5/8 in |
Two patterns stand out. Depth drives thickness: the 48 x 24 x 24 inch box needs more front glass than the 72 inch long box only 22 inches deep. And a bottom supported only at its edges often needs the thickest glass in the tank, though not always: in the 48 x 13 x 20 inch box the front pane needs more. That is one reason a full, level, padded stand matters.
Standard glass sizes and their real minimum
A pane sold as 1/4 inch is not always 6.35 mm. The ASTM C1036 flat glass standard lets each nominal size vary within a range. The calculator only picks a size if the bottom of that range still meets the result. The sizes it considers are the ones in this table, from 1/8 inch to 1 inch.
| Nominal size | Metric designation | ASTM C1036 minimum thickness |
|---|---|---|
| 1/8 in | 3 mm | 2.92 mm |
| 5/32 in | 4 mm | 3.78 mm |
| 3/16 in | 5 mm | 4.57 mm |
| 1/4 in | 6 mm | 5.56 mm |
| 5/16 in | 8 mm | 7.42 mm |
| 3/8 in | 10 mm | 9.02 mm |
| 1/2 in | 12 mm | 11.91 mm |
| 5/8 in | 16 mm | 15.09 mm |
| 3/4 in | 19 mm | 18.26 mm |
| 7/8 in | 22 mm | 21.44 mm |
| 1 in | 25 mm | 24.61 mm |
Before ordering, confirm the actual thickness with your glass supplier, ask for polished edges, and have the glass cut square. Once you know the tank's size, the aquarium volume calculator gives the gallons and the water weight your stand must carry.
Frequently asked questions
How thick is aquarium glass?
It depends mostly on water depth. With the 3.8 safety factor used on this page, a 30 inch front pane holding 12 inches of water needs about 4.4 mm, so 3/16 inch glass. A 48 inch front holding 20 inches needs about 9.4 mm, which rounds up to 1/2 inch glass.
Does a longer tank need thicker glass?
Somewhat. Length raises the bending factor until the pane is about three times longer than the water is deep, and then it stops rising. Depth matters far more: for a pane of the same shape, thickness grows with depth to the power 1.5, so doubling the depth needs about 2.8 times the glass.
Why can the bottom pane need thicker glass than the walls?
A bottom that rests only on its edges bends like a floor with no joists. It carries the full water pressure across its short span, so on wide tanks it often needs more glass than the walls. On a narrow tank the front pane can still come out thicker. A bottom that sits on a flat, level, padded stand is supported from below, so the method allows the same thickness as the thickest wall pane.
Can I use this for acrylic or tempered glass?
No. The numbers assume annealed float glass with a 19.2 N/mm2 strength figure. Acrylic bends much more and is sized differently. Tempered glass is stronger but cannot be cut or drilled after tempering, and it shatters completely when it fails.