How thick does aquarium glass need to be?
A side pane is a plate held on three edges with the top free, pushed by water that presses harder the deeper it goes. The bending stress in such a plate is σ = β·p·H²/t² — β a coefficient for the pane's shape, p the pressure at the bottom, H the water depth, t the thickness — so the thickness you need is t = H × √(β·p ÷ σallow), with the allowable stress the glass's tensile strength (19.2 MPa for float glass) divided by a safety factor, 3.8 by convention for a tank with nothing but silicone holding it. For a standard 55 gallon — 48 in long, 21 in of water — the pressure at the bottom is 5.23 kPa, the shape ratio 2.29 gives β = 0.337, and the front pane comes out at 10 mm — 1/2 in glass. That is the number every online calculator prints for a 55, and none of them show you how. The chart runs every standard tank, and — the column that matters — what they are actually built with.
| Tank | Front at SF 3.8 | Bottom | Typically built with | SF as built, bare | with a center brace |
|---|---|---|---|---|---|
| 5.5 gallon | 2.9 mm · 1/8 in | 3.5 mm | 3 mm | 4 | 8.1 |
| 10 gallon | 3.9 mm · 3/16 in | 4.8 mm | 3 mm | 2.2 | 4.5 |
| 15 gallon | 4.2 mm · 3/16 in | 5.8 mm | 3 mm | 1.9 | 3.9 |
| 20 gallon long | 4.4 mm · 3/16 in | 6 mm | 3 mm | 1.8 | 3 |
| 20 gallon high | 5.8 mm · 1/4 in | 6.7 mm | 5 mm | 2.8 | 5.8 |
| 29 gallon | 7.2 mm · 3/8 in | 7.4 mm | 6 mm | 2.6 | 5.3 |
| 37 gallon | 8.9 mm · 3/8 in | 8.2 mm | 8 mm | 3.1 | 6.1 |
| 40 gallon breeder | 6.6 mm · 3/8 in | 10 mm | 5 mm | 2.2 | 3.9 |
| 55 gallon | 10 mm · 1/2 in | 9.1 mm | 8 mm | 2.4 | 4.4 |
| 65 gallon | 10.7 mm · 1/2 in | 12.3 mm | 10 mm | 3.3 | 6.8 |
| 75 gallon | 10 mm · 1/2 in | 12.1 mm | 8 mm | 2.4 | 4.4 |
| 90 gallon | 11.9 mm · 1/2 in | 13 mm | 10 mm | 2.7 | 5.4 |
| 120 gallon | 11.9 mm · 1/2 in | 16.4 mm | 10 mm | 2.7 | 5.4 |
| 125 gallon | 10.4 mm · 1/2 in | 12.7 mm | 8 mm | 2.2 | 2.9 |
| 150 gallon | 15.7 mm · 5/8 in | 14.6 mm | 12 mm | 2.2 | 3.6 |
| 180 gallon | 13.5 mm · 5/8 in | 18.1 mm | 12 mm | 3 | 4.4 |
| 210 gallon | 16.5 mm · 3/4 in | 19.5 mm | 15 mm | 3.2 | 5.3 |
Float glass, filled to the rim. "Typically built with" is the trade's usual pane for that height (the weight page's table; the trade often goes thinner still). Red: safety factor under 2; amber: under 3 — where framed commercial tanks live, with the frame and brace doing the rest. DIY and rimless: build to 3.8.
Thickness goes as H1.5: the pressure grows with depth and the plate's span grows with it, so doubling the height of a tank at the same proportions needs 2.83× the glass. Doubling the length at the same height needs 1.08×, because a longer pane only moves β from 0.16 to 0.32 on its way to a ceiling of 0.37 — a pane three times longer than it is tall behaves like an infinitely long one. So a 6 ft 125 gallon at 21 in high wants the same glass as a 4 ft 55 at 21 in, and a 24 in high 120 wants a size up from both. If you are choosing between a taller tank and a longer one and the glass budget matters, this is the whole answer.
Why does my 55 gallon have ¼ in glass? Safety factors and frames
Because a 55 with ¼ in (6.35 mm) glass and nothing else has a safety factor of 1.5, not 3.8 — and it is fine, because it is not a bare pane. The plastic frame stiffens the top edge and the center brace splits the 48 in span into two 24 in panes, which changes the shape ratio from 2.29 to 1.14 and β from 0.337 to 0.189: the same glass is now at a safety factor of 2.8, or, put the other way, the braced tank needs only 7.5 mm to reach 3.8 where the bare one needs 10. That is the entire gap between the charts' "float minimum" and the glass on the shelf, and both are right for what they describe. The 3.8 convention exists for a rimless tank, a DIY build, or a tank whose frame is old and brittle — cases where the glass is on its own. Under 2 the margin against a scratch, a point load on a stand that isn't flat, or a sixty-degree summer is thin; the trade gets away with it on a frame, a brace and tempered bottoms.
Does the bottom need to be thicker?
Usually not, and often it can be thinner. The bottom is a plate on all four edges under a uniform pressure — the full-depth pressure over the whole sheet — and its span is the tank's width, which is the short side. For the 55 that is 13 in against the front's 48 in, and the bottom works out at 9.1 mm to the front's 10. Wide tanks (a 120 or a 180 at 24 in) close the gap, and a bottom that sits on a stand with a foam sheet is supported across its face as well, which the formula doesn't credit. Commercial tanks temper the bottom — the one pane that meets the stand, the gravel scoop and the dropped rock — and that is worth more than a size up. The tool prints the bottom at the same safety factor as the sides; if you're buying glass, matching the front's thickness is the simple, safe call.
What does a center brace or euro-brace actually do?
It shortens the span. A single center brace turns one long pane into two half-length panes, which the tool models exactly — the 55's front goes from a 10 mm requirement to 7.5 mm — and a euro-brace (a strip of glass around the top perimeter) supports the top edge continuously, which does at least as much again and is why rimless tanks with a euro-brace can run a size thinner than the bare figure. The bracing does nothing for the ends, whose span is the width, and nothing for the bottom. Braces are the cheapest thickness there is: on a 6 ft tank a strip of 10 mm glass siliconed across the middle is worth about two sizes of front pane. Bracing also cuts deflection, which is what the silicone joints care about.
| Length ÷ height | β (stress) | α (deflection) |
|---|---|---|
| 0.5 | 0.085 | 0.003 |
| 0.666 | 0.1156 | 0.0064 |
| 1 | 0.16 | 0.0138 |
| 1.5 | 0.26 | 0.0188 |
| 2 | 0.32 | 0.0226 |
| 2.5 | 0.35 | 0.0251 |
| 3 | 0.37 | 0.0267 |
Plate coefficients for a side pane (three edges supported, top free, hydrostatic load), from Roark's Formulas for Stress and Strain via the AGA / Stilwell spreadsheet every calculator descends from; interpolated between rows. The bottom uses Roark's four-edge uniform-load table (0.2874 at square to 0.7476 at 5:1). This is the whole black box.
By stress alone acrylic needs less than glass — its tensile strength is three times higher, so the 55's front works out at 5.4 mm against 10. Nobody builds it that way, because acrylic's stiffness is 23× lower: at 12 mm the acrylic pane bows 2 mm to the glass's 0.09, the joints see it, and the tank looks pregnant. Acrylic is sized for deflection, which is why the hobby rule is about 1.5× the glass thickness, and why a big acrylic tank has thick rims and a full top brace. The tool's acrylic mode prints the stress-based figure and the deflection beside it; keep the deflection under about a third of the thickness, and expect to buy the size above that.
Tempered, laminated or low-iron?
Tempered glass is about four times as strong in bending and shatters into crumbs rather than shards — and it cannot be cut, drilled or ground after tempering, which is why commercial tanks temper the bottom and nothing else: you can't add a bulkhead to it, and a tempered side would have to be tempered to size. Low-iron ("starphire") glass is the same strength as float, just clearer; the formula doesn't change. Laminated glass is two sheets with a plastic interlayer; it holds together when it cracks, which matters for very large panes, but it flexes more than one sheet of the same total thickness and the calculators overrate it — treat it as its thicker leaf plus a little. And whatever the sheet, the edges are where it fails: polished or at least seamed edges, no chips, and silicone joints wide enough that the panes never touch.
My tank is bowing — is it going to fail?
Every long pane bows; the question is how much. At ¼ in the 55's front deflects about 0.58 mm at the middle when full — invisible without a straightedge, and the silicone doesn't notice. The hobby's working limit is a deflection under about 33 % of the pane's thickness; past that the joints are being peeled rather than sheared, which is how a tank that "held for years" lets go at a seam. Type your glass into the tool and it prints the deflection and the safety factor together. Bowing that is new, growing, or that shows as a line in the silicone is the one to act on; a brace fitted across the top is the fix and can be added to a running tank. And a tank's weakest point is rarely the glass: the stand and the floor that twist a flat pane into a loaded one are, which is why the weight page exists.