Aquarium glass thickness chart: float, low-iron, and tempered specs by tank size

The fundamental glass thickness formula

Aquarium glass thickness is set by a single engineering question: how much does the panel deflect (bow outward) under hydrostatic pressure, and is that deflection within the safe limit for the glass and silicone seam?

The industry-standard formula derives from plate bending mechanics for a panel supported on four edges:

t = SQRT( (Beta x p x b^4) / (Sigma_a x SF) )

Where:

• t is glass thickness in inches (the unknown we are solving for)
• p is hydrostatic pressure at the bottom of the panel, in PSI. Water exerts 0.0361 PSI per inch of depth. So a 24-inch tall tank has 0.866 PSI at the base.
• b is the shorter dimension of the panel in inches (typically the height for a wider-than-tall tank, or the length for a tall-and-narrow tank)
• Beta is a coefficient that depends on the panel aspect ratio (length/height). Ranges from 0.16 for square panels to 0.74 for very long panels.
• Sigma_a is the allowable stress in the glass. For annealed soda-lime float glass, 1000 PSI. For tempered, 4000 PSI. For low-iron, the same as standard float (low-iron is an optical property, not a strength property).
• SF is the safety factor. 3.8 is the long-standing industry minimum for aquarium use; some builders use 5.0 for premium tanks.

You do not have to calculate this by hand. The chart below pre-solves it for common tank sizes at standard safety factors.

Recommended glass thickness by tank size

Thicknesses below assume annealed float glass at SF=3.8, which is the hobby standard. Use these as MINIMUMS. Going one step thicker is always safer and rarely costs much more.

"Braced" means the design requires a center brace or full eurobrace on top. Without bracing, that panel is too tall for the listed thickness.

Float, low-iron, and tempered: what is the difference

Float glass. Standard soda-lime glass produced by the float process (molten glass on a bath of molten tin). 99% of tank manufacturing uses float. Has a slight green tint from iron oxide impurities, visible from edge-on or in thick panels.

Low-iron glass. Float glass with reduced iron content (under 0.01% Fe versus ~0.10% for standard). Produced by Pilkington as Optiwhite, Guardian as Ultraclear, or AGC as Krystal Klear. Optical clarity is significantly better - no green tint, especially noticeable on rimless tanks viewed from the side. Same structural strength as standard float. Cost premium of 30-60% per square foot.

Tempered glass. Float or low-iron glass that has been heat-treated to ~1150 F and rapidly cooled, putting the outer surfaces in compression and the inner core in tension. The compression makes tempered ~4x stronger than annealed. Critical caveat: tempered glass cannot be cut, drilled, or ground after manufacture. Any attempt causes it to shatter.

What this means for aquarium use:

• Most off-the-shelf framed tanks (Aqueon, Marineland Perfecto, Top Fin, Aqua Culture) use TEMPERED BOTTOMS only. The sides are annealed.
• Rimless tanks (UNS, Waterbox, Crystal Aquatics, Innovative Marine, Mr. Aqua) use ANNEALED low-iron only. Tempering interferes with the rimless construction process (cannot grind tempered edges for the bevel).
• Custom tanks built by Glasscages, Miracles Aquariums, or Reef Savvy use mostly annealed glass with optional tempered upgrade.

If you want to drill a tank for a bulkhead, the panel you drill MUST be annealed. Drilling tempered glass shatters it into thousands of pieces.

How to tell if your tank glass is tempered

Three methods, in order of reliability:

1. The polarized sunglasses test. Hold a pair of polarized sunglasses against the glass and look at a bright white screen (laptop, TV, window with bright sky behind). Tempered glass shows visible stress patterns: rainbow whorls, dark spots, or repeating geometric patterns. Annealed glass shows nothing.
2. Manufacturer documentation. Aqueon, Marineland, and Top Fin publish which panels are tempered for each tank size. If you have the original specification card or receipt, check.
3. The etched stamp. Tempered glass is required by US safety code to bear an etched stamp identifying it as tempered. The stamp is typically in one corner of the panel and shows the manufacturer name plus "tempered" or the letter T. Hard to see through the silicone seam and behind decorations but worth checking.

Why water column matters more than tank volume

A 75-gallon tank with 21-inch water depth and a 75-gallon tank with 17-inch water depth (e.g. shallower 75-gallon "long" variant) have very different glass requirements. The taller water column generates more hydrostatic pressure at the base, requiring thicker glass.

Hydrostatic pressure scales linearly with depth: 0.0361 PSI per inch. A 21-inch tank has 0.758 PSI at the bottom. A 30-inch tank has 1.083 PSI - 43% more pressure. This is why a 240-gallon tank with 30-inch water column needs 3/4 inch glass while a 240-gallon shallower "wide and shallow" reef tank with 18-inch water column may need only 1/2 inch.

When sizing replacement glass for a custom build, the height of the water column is the most important number to give your glass shop, more important than total volume.

Bracing and its effect on glass thickness

A top brace (eurobrace, center brace, or full perimeter rim) reduces the effective span of the front and back panels. With a center brace, the front panel is no longer a single 48 x 21 inch panel - it is two 23 x 21 inch panels structurally. Smaller panel = less deflection = thinner glass acceptable.

• No brace, no rim (true rimless). Full panel deflection. Use the "rimless recommended" column in the chart above.
• Plastic rim top only (Aqueon-style). Minimal bracing benefit, mostly cosmetic. Treat as rimless for thickness purposes.
• Center brace (single beam across top). Cuts effective panel span in half. Allows one step thinner glass than rimless minimum.
• Eurobrace (perimeter glass strip around top, 2-3 inches wide). Stiffens all four top edges. Allows one step thinner glass than rimless minimum, sometimes two on smaller tanks.
• Full glass top. Maximum bracing. Some builders use this on shallow frag tanks where viewing from above matters.

For DIY builds, plan bracing into your design BEFORE ordering glass. The savings on glass cost often pay for the brace material.

Bottom panel sizing

The bottom panel sees less hydrostatic stress (water column above does not bow the bottom outward, gravity holds it in place) but it must support point loads from rocks and equipment. The bottom panel is usually:

• The same thickness as the side panels on stocked builds
• One step thicker on heavy-aquascape builds (rockwork over 1 lb/gal)
• Required to be tempered if the stand contacts only the perimeter rim (not fully supported)
• Required to be annealed if you intend to drill it

The bottom panel is also the panel most likely to break under impact (dropped tools, magnetic cleaners). Building one step thicker on the bottom is cheap insurance.

Silicone seam thickness

Aquarium silicone bead width should be approximately equal to the glass thickness. A 3/8 inch glass tank should have a 3/8 inch silicone bead at each seam. Thicker bead is not stronger - it actually carries less load because more silicone means more compliance.

• 1/4 inch glass: 1/4 inch bead
• 3/8 inch glass: 3/8 inch bead
• 1/2 inch glass: 1/2 inch bead
• 3/4 inch glass: 1/2 to 5/8 inch bead (beyond 5/8 inch, additional silicone is cosmetic, not structural)

When to upgrade from chart minimum

• Heavy rockwork build. Reef aquascape over 1 lb per gallon adds point loads. Step up one thickness.
• Tank in a finished living space. Failure consequences are catastrophic; safety factor 5+ is justified. Step up one thickness.
• Tank over a finished floor above an occupied room below. Step up one thickness, plus add flood matting.
• Tank in a region with seismic risk. California, Pacific Northwest, parts of Alaska. Tank on raised seismic restraints, glass one step thicker.
• Tank with high-impact livestock. Large cichlids, oscar, fahaka puffer, arowana. Impact resistance matters more than chart minimum.
• Custom tank over 6 feet long. Long panels deflect more than the simple formula predicts. Step up one thickness.

Glass cost reference

Glass pricing at typical 2026 US glass shops, per square foot, cut to size:

• 1/4 inch float: $5-8 / sq ft
• 3/8 inch float: $8-14 / sq ft
• 1/2 inch float: $14-22 / sq ft
• 3/4 inch float: $30-45 / sq ft
• Low-iron premium: add 40-60% to above
• Tempered: add $50-100 setup fee plus 20-30% per square foot
• Polished edges (required for visible top edge): $3-6 per linear foot

For a 75-gallon rimless build (1/2 inch low-iron, all six panels polished): expect $400-650 in glass alone before silicone and labor.

FAQ

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