Aquarium Glass Thickness and Volume Loss

Every aquarium calculator uses the dimensions you enter — but which dimensions? Exterior dimensions, printed on the box, include the glass panels themselves. Interior dimensions, which determine actual water capacity, are smaller by the glass thickness on every side. On a small 10-gallon tank with 6 mm glass, this difference is less than half a gallon. On a 180-gallon tank with 12 mm glass, the volume loss exceeds 10 gallons. This page provides the formula, a complete reference table for common tank sizes and glass thicknesses, and practical guidance on measuring your own glass thickness accurately.

How Glass Thickness Reduces Interior Volume

Glass panels reduce two dimensions — length and width — each by twice the glass thickness, because there is one panel on each side. Height is reduced by the bottom glass only (one panel, not two), since the top is open or has a light plastic frame rather than a structural glass panel. The interior dimensions are therefore exterior length and width minus twice the glass thickness, and exterior height minus one glass thickness. 

Interior Length = Exterior Length − (2 × glass thickness)
Interior Width  = Exterior Width  − (2 × glass thickness)
Interior Height = Exterior Height − (1 × glass thickness)
Exterior Volume   = Ext. L × Ext. W × Ext. H ÷ 231  (US gallons)
Interior Volume   = Int. L × Int. W × Int. H ÷ 231  (US gallons)
Volume Loss (gal) = Exterior Volume − Interior Volume
Volume Loss (%)   = (Volume Loss ÷ Exterior Volume) × 100

Note: this page calculates volume loss from glass thickness only. For additional volume loss from substrate, equipment, and fill height, use the displacement calculator.

Volume Loss by Tank Size and Glass Thickness

The table below shows exterior dimensions for 8 common US standard tank sizes, paired with four glass thicknesses (3 mm, 6 mm, 10 mm, 12 mm). For each combination: interior dimensions, interior volume, and volume lost compared to the exterior calculation. Glass thickness in mm converted to inches: 3 mm = 0.118 in, 6 mm = 0.236 in, 10 mm = 0.394 in, 12 mm = 0.472 in.

Tank (nominal) Ext. L×W×H (in) Glass (mm) Int. L×W×H (in) Ext. Vol (gal) Int. Vol (gal) Vol Loss (gal) Vol Loss (%)
10 gal — 20.0 × 10.0 × 12.0 in exterior (Ext. Vol 10.39 gal)
10 gal 20.0 × 10.0 × 12.0 3 19.76 × 9.76 × 11.88 10.39 9.91 0.49 4.7
10 gal 20.0 × 10.0 × 12.0 6 19.53 × 9.53 × 11.76 10.39 9.37 1.02 9.8
10 gal 20.0 × 10.0 × 12.0 10 19.21 × 9.21 × 11.61 10.39 8.90 1.49 14.4
10 gal 20.0 × 10.0 × 12.0 12 19.06 × 9.06 × 11.53 10.39 8.63 1.76 16.9
20 gal High — 24.0 × 12.0 × 16.0 in exterior (Ext. Vol 19.95 gal)
20 gal High 24.0 × 12.0 × 16.0 3 23.76 × 11.76 × 15.88 19.95 19.21 0.72 3.6
20 gal High 24.0 × 12.0 × 16.0 6 23.53 × 11.53 × 15.76 19.95 18.51 1.42 7.1
20 gal High 24.0 × 12.0 × 16.0 10 23.21 × 11.21 × 15.61 19.95 17.60 2.33 11.7
20 gal High 24.0 × 12.0 × 16.0 12 23.06 × 11.06 × 15.53 19.95 17.15 2.78 13.9
20 gal Long — 30.0 × 12.0 × 12.0 in exterior (Ext. Vol 18.70 gal)
20 gal Long 30.0 × 12.0 × 12.0 3 29.76 × 11.76 × 11.88 18.70 17.95 0.75 4.0
20 gal Long 30.0 × 12.0 × 12.0 6 29.53 × 11.53 × 11.76 18.70 17.28 1.42 7.6
20 gal Long 30.0 × 12.0 × 12.0 10 29.21 × 11.21 × 11.61 18.70 16.43 2.27 12.1
20 gal Long 30.0 × 12.0 × 12.0 12 29.06 × 11.06 × 11.53 18.70 16.02 2.68 14.3
29 gal — 30.0 × 12.0 × 18.0 in exterior (Ext. Vol 28.05 gal)
29 gal 30.0 × 12.0 × 18.0 3 29.76 × 11.76 × 17.88 28.05 26.93 1.12 4.0
29 gal 30.0 × 12.0 × 18.0 6 29.53 × 11.53 × 17.76 28.05 25.95 2.10 7.5
29 gal 30.0 × 12.0 × 18.0 10 29.21 × 11.21 × 17.61 28.05 24.68 3.37 12.0
29 gal 30.0 × 12.0 × 18.0 12 29.06 × 11.06 × 17.53 28.05 24.06 4.00 14.2
40 Breeder — 36.0 × 18.0 × 17.0 in exterior (Ext. Vol 47.69 gal)
40 Breeder 36.0 × 18.0 × 17.0 3 35.76 × 17.76 × 16.88 47.69 46.31 0.96 2.0
40 Breeder 36.0 × 18.0 × 17.0 6 35.53 × 17.53 × 16.76 47.69 45.20 2.07 4.4
40 Breeder 36.0 × 18.0 × 17.0 10 35.21 × 17.21 × 16.61 47.69 43.41 3.86 8.2
40 Breeder 36.0 × 18.0 × 17.0 12 35.06 × 17.06 × 16.53 47.69 42.53 4.74 10.0
55 gal — 48.0 × 13.0 × 21.0 in exterior (Ext. Vol 56.73 gal)
55 gal 48.0 × 13.0 × 21.0 3 47.76 × 12.76 × 20.88 56.73 53.35 0.68 1.3
55 gal 48.0 × 13.0 × 21.0 6 47.53 × 12.53 × 20.76 56.73 52.02 2.01 3.7
55 gal 48.0 × 13.0 × 21.0 10 47.21 × 12.21 × 20.61 56.73 49.88 4.15 7.7
55 gal 48.0 × 13.0 × 21.0 12 47.06 × 12.06 × 20.53 56.73 48.82 5.21 9.6
75 gal — 48.0 × 18.0 × 21.0 in exterior (Ext. Vol 78.55 gal)
75 gal 48.0 × 18.0 × 21.0 3 47.76 × 17.76 × 20.88 78.55 76.50 2.05 2.6
75 gal 48.0 × 18.0 × 21.0 6 47.53 × 17.53 × 20.76 78.55 74.64 3.91 5.0
75 gal 48.0 × 18.0 × 21.0 10 47.21 × 17.21 × 20.61 78.55 71.49 7.06 9.0
75 gal 48.0 × 18.0 × 21.0 12 47.06 × 17.06 × 20.53 78.55 69.93 8.62 11.0
125 gal — 72.0 × 18.0 × 22.0 in exterior (Ext. Vol 123.43 gal)
125 gal 72.0 × 18.0 × 22.0 3 71.76 × 17.76 × 21.88 123.43 120.50 2.32 1.9
125 gal 72.0 × 18.0 × 22.0 6 71.53 × 17.53 × 21.76 123.43 117.57 5.25 4.3
125 gal 72.0 × 18.0 × 22.0 10 71.21 × 17.21 × 21.61 123.43 112.68 10.14 8.2
125 gal 72.0 × 18.0 × 22.0 12 71.06 × 17.06 × 21.53 123.43 110.27 12.55 10.1
Exterior volume uses box dimensions ÷ 231. Interior volume applies glass reduction: length and width each reduced by 2 × glass thickness; height reduced by 1 × glass thickness (open top). Typical framed tanks use 6 mm below 29 gal, 10 mm from 40–75 gal, and 12 mm on 90 gal and above. Rimless tanks typically use 10–12 mm even on smaller sizes.

Which Glass Thickness Applies to My Tank?

Standard framed aquariums from major US manufacturers (Aqueon, Marineland) follow a consistent pattern — tanks up to 29 gallons typically use 4–6 mm glass (¼ inch); tanks from 40 to 75 gallons use 8–10 mm glass (⅜ inch); tanks 90 gallons and larger use 10–12 mm glass (½ inch). These are the same tanks listed in the aquarium volume by size reference. Rimless tanks — popular for planted aquariums and nano reefs — use thicker glass for structural rigidity without a frame: typically 10 mm on sizes up to 30 gallons and 12 mm or more on larger rimless builds.

If you do not know your tank’s glass thickness, the most reliable method is direct measurement with a digital vernier caliper at the top edge of a side panel. Alternatively, a strong refrigerator magnet and a glass thickness gauge (a tool used by auto glass professionals) can measure through the full panel without access to the edge. For acrylic tanks, panels are typically 6–12 mm depending on size — acrylic is lighter than glass and can be somewhat thinner for equivalent strength, but the same volume-loss calculation applies. See how to measure aquarium volume for full measurement technique.

Glass Thickness and Volume Loss — Questions

How much volume does glass thickness actually remove?
It depends on both the glass thickness and the tank size. On a 10-gallon tank with 6 mm glass, interior volume is approximately 9.4 gallons — a loss of about 1 gallon or 10%. On a 75-gallon tank with the same 6 mm glass, interior volume is approximately 74.6 gallons — a loss of about 3.9 gallons or 5%. The percentage loss decreases as tank size increases because the glass represents a smaller fraction of each dimension. On large tanks with 12 mm glass, volume loss can reach 8–12%.
Do aquarium calculators use interior or exterior dimensions?
It depends on what you enter. All our shape calculators use the dimensions you provide — enter interior dimensions for actual water volume, exterior dimensions for box volume. The aquarium volume by size table uses exterior dimensions as published by manufacturers. To get the true interior volume from that table, subtract volume loss using the appropriate glass thickness row from the table above.
Does the bottom glass panel reduce height?
Yes, by one panel thickness. The bottom glass sits below the water column — water rests on top of it. So interior height = exterior height minus one glass thickness. The top of most tanks is open or has a plastic trim frame, which does not affect water depth. This is why height is reduced by 1× glass thickness while length and width are each reduced by 2× glass thickness (a panel on each side).
My tank is acrylic — does this table apply?
The formula applies, but typical acrylic panel thicknesses differ slightly from glass. Acrylic tanks in the 10–55 gallon range commonly use 6–9 mm panels. For tanks 75 gallons and above, acrylic is typically 10–12 mm. You can use the same calculation: measure your acrylic panel thickness with a caliper and apply the same interior dimension formula. The volume loss will be similar to the 6 mm or 10 mm rows depending on your panel thickness.
How do I measure my aquarium glass thickness?
The most accurate method is a digital vernier caliper measured at the top edge of a side panel before filling. For a filled tank, a glass thickness gauge — available from auto glass suppliers — uses magnets to measure through the full panel without draining. As a rough estimate, you can also measure the exterior dimension and compare it to the known nominal dimension: the difference divided by two (for length/width) gives approximate panel thickness.
Why does my aquarium hold less than its stated gallon rating?
Three main reasons beyond glass thickness: first, tanks are filled 1–2 inches below the rim rather than brimful; second, substrate (gravel, sand) displaces several gallons of water volume; third, equipment such as internal filters, heaters, and rocks displace additional water. Combined with glass thickness, these factors typically reduce usable water volume to 80–88% of the nominal label. Use the displacement calculator to estimate the combined effect on your specific tank.

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