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How Much Water Does a Fish Tank Really Hold?
Less than the label says. A tank sold as a 55 gallon holds about 44 US gallons once it is set up with 2 in of sand and a 1 in gap below the rim, which is 20% under the label. The label names a size class; the water is what fits inside the glass, under the rim and around the substrate.
This page is the method behind every “real water” figure on TankRatio: the sizes chart, the volume calculator and each tank guide. Everything below is computed from the makers' published sizes, and the figures that are estimates are marked.
Where the missing gallons go (55 gallon example)
Aqueon lists its 55 gallon at 48¼ × 12¾ × 21 in outside. Taking the deductions one at a time:
| Step | What changes | Volume left | Lost |
|---|---|---|---|
| Outside volume | Outside L × W × H ÷ 231 | 55.9 gal | – |
| Side glass | Take 5/16 in off each end and side | 52.5 gal | −3.4 |
| Bottom glass | Take 5/16 in off the height | 51.7 gal | −0.8 |
| Gap below the rim | Water stops 1 in below the top edge | 49.2 gal | −2.5 |
| 2 in of sand | Sand fills the bottom 2 in; water fills what is left | 44.2 gal | −5.0 |
The glass and the rim gap together cost more than the sand does. Skipping the sand gives a bare-bottom tank holding 49.2 gallons (186.3 litres).
The formula
- Inside length and width = outside size − 2 × glass thickness.
- Water depth = outside height − bottom glass − gap below the rim − substrate depth.
- Cubic inches = inside length × inside width × water depth.
- US gallons = cubic inches ÷ 231. Litres = cubic inches × 0.016387064.
The US gallon is defined as exactly 231 cubic inches (NIST Handbook 44, Appendix C) and 1 inch as exactly 2.54 cm, so the conversions add no error of their own. The same 55 gallon worked through by hand: 47.6 in inside length × 12.1 in inside width × 17.7 in water depth = 10,214 in³ ÷ 231 = 44.2 gallons. Three more hand-checked tanks are in the volume calculator's worked examples.
Which inputs are published and which are estimated
| Input | Value we use | Status |
|---|---|---|
| Outside length, width, height | Aqueon's chart; Marineland's dealer listing where Aqueon has none | Published by the maker |
| Empty and full weight | As printed by each maker | Published by the maker |
| Glass thickness | By size, table below | Estimate. Makers don't publish it for standard tanks. |
| Gap below the rim | 1 in | Convention. Water level in real tanks varies; with a lid or a trim you may run it lower. |
| Substrate depth | 2 in on the size pages | Assumption. Planted tanks often use more, bare-bottom tanks none. |
| Gallon and inch definitions | 231 in³ per US gallon; 2.54 cm per in | Exact (NIST) |
Glass thickness estimates by tank size (thicker glass for longer, taller tanks):
| Glass | Tank sizes |
|---|---|
| 1/8 in | 5½ gallon |
| 3/16 in | 10, 15, 20 tall, 20 long gallon |
| 1/4 in | 29, 30, 33 long, 38, 40 breeder, 40 long, 40 high, 50 gallon |
| 5/16 in | 45, 55, 60 breeder, 65 gallon |
| 3/8 in | 75 gallon |
| 1/2 in | 90, 110, 120, 125, 150, 180, 210 gallon |
How wrong can it be?
On the 55 gallon above, changing one input at a time:
- Glass 1⁄16 in thicker on every side: 0.7 gallons less water.
- One more inch of sand (3 in instead of 2): 2.5 gallons less.
- Water 2 in below the rim instead of 1 in: 2.5 gallons less.
- No sand at all: 5.0 gallons more.
The glass is the only estimated input, and it moves the answer by under a gallon. The sand and the rim gap are your choices, and each shifts it by about 2.5 gallons per inch. The label, by comparison, is off by 11. If your tank is custom, rimless or has a thick bottom pane, measure it yourself: outside size, the glass at the top edge, and the water level you actually run.
Why the label is a class, not a measurement
Makers name sizes after a nominal volume. Aqueon's 55 gallon is 55.9 gallons if you fill the whole outer box, and Marineland's full weights are usually the label gallons × 8.34 lb plus the empty tank. Neither counts glass, trim or rim gap. The sizes chart shows the label next to outside volume and real water for all 25 standard sizes.
Download the data
The sizes behind the chart are available as a spreadsheet: aquarium-tank-sizes.csv (nominal and exact sizes, weights, outside volume, estimated glass and real water). Free to reuse with credit; see the terms.
Checked 2 Nov 2026. Published by Sumeet Joseph Desilva. Spot an error? [email protected].