Nano Aquarium Volume Guide

Nano aquariums — tanks under 10 gallons — are more varied in shape and proportion than standard sizes. A 5-gallon tank might be a tall column, a shallow tray, a rimless cube, or a traditional framed rectangle, each holding a different amount of water despite the same nominal label. This guide lists the most common nano tank footprints and profiles with their calculated geometric volumes, realistic usable water volumes, and freshwater weights — so you can dose accurately and plan your setup from the start.

What counts as a nano aquarium? The hobby uses “nano” loosely. This guide defines nano as any tank with a nominal volume under 10 gallons (38 litres). Tanks under 2.5 gallons are sometimes called pico tanks. The 10-gallon standard size is included at the bottom of this table as a comparison point because it is the most common beginner tank and directly above the nano range. International hobbyists working in metric units can cross-reference these volumes on aquarium volume in litres.

Nano Tank Size Reference Table

Calc. Volume = (L × W × H) ÷ 231. Actual Water = calculated × 0.87 (glass thickness, 1-inch fill gap, minor displacement). FW Weight = actual water × 8.34 lb/gal.

Size / Type Exterior L × W × H (in) Calc. Volume (gal) Actual Water (gal) Actual Water (L) FW Weight (lb)
0.5 gal — Shallow Pico (UNS 16S) 6.29 × 6.29 × 3.14 0.54 0.47 1.8 3.9
1 gal — Cube (UNS 16C) 6.29 × 6.29 × 6.29 1.08 0.94 3.6 7.8
1.5 gal — Tall Pico (UNS 16T) 6.29 × 6.29 × 9.44 1.62 1.41 5.3 11.8
2.1 gal — Cube (UNS 20C / 25S) 7.87 × 7.87 × 7.87 2.11 1.84 7.0 15.3
2.5 gal — Standard Framed 12 × 6 × 8 2.49 2.17 8.2 18.1
2.9 gal — Nano Rectangle (UNS 3N) 12.2 × 7.48 × 7.48 2.95 2.57 9.7 21.4
3.1 gal — Tall Nano (UNS 20T) 7.87 × 7.87 × 11.81 3.17 2.76 10.4 23.0
3.7 gal — Tall Rectangle (UNS 3T) 12.2 × 7.48 × 9.44 3.73 3.25 12.3 27.1
4.1 gal — Cube (UNS 25C) 9.84 × 9.84 × 9.84 4.12 3.58 13.6 29.9
4.6 gal — Nano Rectangle (UNS 5N) 14.17 × 8.66 × 8.66 4.60 4.00 15.1 33.4
5.0 gal — Standard Framed 16 × 8 × 10 5.54 4.82 18.2 40.2
5.5 gal — Standard Framed (Aqueon) 16.19 × 8.38 × 10.5 6.17 5.37 20.3 44.8
5.8 gal — Tall Rimless (UNS 5T) 14.17 × 8.66 × 11.02 5.85 5.09 19.3 42.5
5.9 gal — Shallow Rimless (UNS 45S / 25T) 17.71 × 11.02 × 7.08 5.98 5.20 19.7 43.4
6.3 gal — Long Rimless (UNS 60L) 23.62 × 7.87 × 7.87 6.33 5.51 20.9 46.0
7.1 gal — Cube Rimless (UNS 30C) 11.81 × 11.81 × 11.81 7.13 6.20 23.5 51.7
10 gal — Standard Framed (comparison) 20 × 10 × 12 10.39 9.04 34.2 75.4
All dimensions are exterior. Actual water volume uses a factor of 0.87 to account for glass thickness (typically 5 mm on rimless nano tanks), a 1-inch fill gap, and minor displacement. Use the rectangular calculator for your specific interior measurements, or the aquarium volume unit converter to express any nano volume in litres or cubic centimetres.

Why Nano Volume Calculations Are Trickier

On a standard 55-gallon tank, the ¼-inch glass panels reduce each dimension by only about 0.5 inches, which is less than a 2% reduction in interior volume. On a nano tank, the same 5 mm glass wall reduces a 6.29-inch exterior dimension to a 5.90-inch interior dimension — a 6% reduction in just one axis. Applied across all three axes, this compounds to a 15–20% reduction in true interior volume compared to the exterior geometric calculation. This is why the “nominal” label on small tanks often matches poorly against any single formula. For the underlying mathematics, see aquarium volume formulas. For a deeper look at how panel thickness affects usable capacity, see glass thickness and volume loss.

Nano tanks also come in a much wider variety of profiles than standard sizes — cube, tall, shallow, long, and rimless formats are all common under 10 gallons, whereas standard framed tanks above 20 gallons are almost exclusively rectangles with conventional proportions. A 5-gallon standard framed tank (16 × 8 × 10 in) and a 5-gallon UNS tall rimless (14.17 × 8.66 × 11.02 in) have very similar nominal ratings but different footprints, water heights, and surface areas — all of which matter for equipment sizing, lighting, and stocking. For the most accurate volume on any nano shape, use the matching shape calculator on the shape identification guide, or browse the full aquarium volume by size reference for standard tanks 5 gallons and above.

Nano Aquarium Volume Questions

What is considered a nano aquarium?
In the hobby, “nano” generally means any tank under 10 gallons (approximately 38 litres). Some hobbyists use “pico” for tanks under 2.5 gallons. There is no strict industry standard — Bulk Reef Supply, for example, categorises tanks up to 30 gallons as nano for reef purposes, but the volume-calculation challenges described on this page are most pronounced below 10 gallons.
Why does my 5-gallon tank seem to hold less than 5 gallons?
Two main reasons. First, the “5-gallon” label is based on nominal exterior or rounded dimensions, not interior measurements. Second, glass thickness on a small tank represents a larger fraction of each dimension than on a large tank — 5 mm glass on a 6-inch dimension removes nearly 4% of width from each side. With typical substrate and a fill line 1 inch below the rim, a 5-gallon standard framed tank realistically holds about 3.8–4.2 usable gallons of water.
How do I calculate the exact volume of my nano cube?
Measure inside the glass for all three dimensions. Multiply length × width × height (in inches) and divide by 231 for US gallons, or multiply the cubic centimetre result (centimetres × centimetres × centimetres) by 0.001 for litres. Use the rectangular aquarium volume calculator to do this instantly. For unusual shapes use the shape identification guide to find the right formula.
Do rimless nano tanks hold more water than framed tanks of the same label volume?
Marginally yes, because rimless tanks use thinner glass (5 mm is common on small rimless tanks vs 6–9 mm on framed tanks) and have no plastic trim frame that reduces usable depth. However the difference is typically 0.2–0.5 gallons on a 5-gallon tank — small enough that the nominal label remains a reasonable starting point for dosing.
What is the actual water volume of a 2.5-gallon standard tank?
A standard 2.5-gallon framed tank (typically 12 × 6 × 8 in exterior) has a geometric volume of about 2.49 gallons from exterior dimensions. Interior volume accounting for glass is closer to 2.1–2.2 gallons brimful. With a 1-inch fill gap and a thin substrate layer, usable water is typically 1.8–2.0 gallons. Always measure interior dimensions for dosing accuracy.
Is a 10-gallon tank a nano tank?
The 10-gallon standard tank sits on the boundary. This guide includes it as a comparison point. For volume reference data on 10 gallons and above — including 15, 20, 29, 40, and 55-gallon standard sizes — see the aquarium volume by size reference table.

Related Tools and References

To calculate the volume of any nano shape from your own measurements, use the free aquarium volume calculator.