Quick answer: gallons = length × width × water height (inches) ÷ 231. Then subtract what the substrate takes up. A "55 gallon" tank (48 × 13 × 21 in) filled to 19 inches with 2 inches of gravel holds 45.9 gallons — and that, not the label, is the figure every medication dose should be based on.
Why your tank holds less than its name
Aquariums are sold by nominal size, calculated from the outside dimensions right to the rim. Three things eat into it before a fish ever swims:
- Fill level. Nobody fills to the rim. An inch or two of freeboard on a standard 55 removes roughly 3 to 5 gallons.
- Substrate. A gravel or sand bed displaces its entire volume. Two inches across a 48 × 13 in footprint is 1,248 in³ — about 5.4 gallons gone.
- Hardscape and equipment. Rocks, driftwood, heaters and intakes all take space. In an aquascaped tank this can be another 5 to 10%.
Stacked up, a nominal 55-gallon tank commonly runs 45 to 48 gallons of real water — 15% or more below the label. Dose that tank as if it were 55 and every treatment lands roughly 20% too strong, which is the difference between a therapeutic dose and a dangerous one for shrimp, snails and scaleless fish.
Standard aquarium sizes
| Nominal size | Dimensions (in) | Brim-full gal | Filled weight |
|---|---|---|---|
| 5 gal | 16 × 8 × 10 | 5.5 | ≈ 62 lb |
| 10 gal | 20 × 10 × 12 | 10.4 | ≈ 111 lb |
| 20 gal high | 24 × 12 × 16 | 19.9 | ≈ 225 lb |
| 20 gal long | 30 × 12 × 12 | 18.7 | ≈ 225 lb |
| 29 gal | 30 × 12 × 18 | 28.1 | ≈ 330 lb |
| 40 gal breeder | 36 × 18 × 16 | 44.9 | ≈ 458 lb |
| 55 gal | 48 × 13 × 21 | 56.7 | ≈ 625 lb |
| 75 gal | 48 × 18 × 21 | 78.5 | ≈ 850 lb |
| 90 gal | 48 × 18 × 24 | 89.8 | ≈ 1,050 lb |
| 125 gal | 72 × 18 × 21 | 117.8 | ≈ 1,400 lb |
Filled weights include glass, water and a typical substrate bed. Note the 55: it is unusually narrow at 13 inches, which is why it looks big but is awkward for aquascaping — and why the 40 breeder, with its 18-inch depth, is a favourite despite being nominally smaller.
What you do with the real number
Dosing medication and fertiliser
Treatments are specified per gallon. Copper-based medications and many dewormers have a narrow safety margin, and invertebrates are especially sensitive. Using a real 46 gallons instead of a nominal 55 changes a dose by 16% — worth getting right before you medicate a tank you have spent a year building.
Water changes
A "25% water change" on a real 46-gallon tank is 11.5 gallons, not the 13.75 you would draw if you trusted the label. Consistency matters more than precision here, but knowing the true figure makes matched temperature and dechlorinator dosing far easier.
Filter and heater sizing
Filters are rated by turnover — most freshwater setups aim for four to six tank volumes per hour, planted tanks often less, and marine systems considerably more. Heaters run around 3 to 5 watts per gallon depending on how cold the room gets. Both specs start from actual water volume.
Common mistakes & pro tips
- Using the nominal size for dosing. The single most consequential error in this hobby's arithmetic.
- Measuring the outside of the glass. On a rimmed tank with 1/2-inch glass you lose an inch across the width. Measure inside pane to inside pane. The same inside-dimension rule governs any box-shaped vessel — see the rectangular tank volume calculator for non-aquarium uses.
- Measuring tank height instead of water height. Use the distance from the substrate surface to the waterline.
- Forgetting the sump. A marine system's total volume includes the display, the sump and the plumbing. Dose to the whole system, not just the display.
- Pro tip — measure it by filling. Fill with a known container and count. It settles every argument, catches every wrong assumption, and takes ten minutes once.
- Pro tip — bowfronts. The curved front adds roughly 5 to 8% over a plain rectangle of the same footprint. This calculator's bowfront mode adds the bow segment for you.
Frequently asked questions
How do I calculate my fish tank's volume in gallons?
Multiply length × width × height in inches, then divide by 231 to get US gallons. A 30 × 12 × 12 in tank works out to 4,320 ÷ 231 ≈ 18.7 gallons — which the trade sells as a 20-gallon long. Then subtract substrate and decor to get the water you actually have.
Why does my 55 gallon tank hold less than 55 gallons?
Three reasons stack up. The nominal size is measured to the rim, but you fill an inch or two below it. Substrate takes up real space — 2 inches of gravel in a standard 55 removes about 5 gallons. And rocks, wood and equipment displace more. A stocked 55-gallon tank typically holds 45 to 48 gallons of actual water.
How much water does substrate displace?
Substrate displaces its full bed volume, since water only occupies the gaps between grains and those are already counted in the bed. For a 48 × 13 in footprint with 2 inches of gravel that is 1,248 in³ ≈ 5.4 gallons. This calculator subtracts the substrate bed automatically when you enter its depth.
Why does real water volume matter for fish keeping?
Because every dose and every calculation scales with it. Medications are prescribed per gallon, and overdosing in a tank you believed was 20% larger can be fatal to invertebrates and scaleless fish. Filter turnover ratings, water change percentages and stocking guidance all depend on the same number.
How much does a full aquarium weigh?
Roughly 10 lb per gallon once you include glass, substrate and rock — water alone is 8.34 lb per gallon. A 55-gallon tank comes in around 600 lb fully set up, and a 125-gallon around 1,400 lb. Place large tanks against a load-bearing wall and perpendicular to the floor joists.