# Round-End aquarium volume calculator

> Enter the length, width, and height of a round-end tank in inches or centimeters to get its water volume in US gallons and liters, plus freshwater and saltwater weight. a 24 × 12 × 16 in tank holds 17.81 US gal (67.41 L) — the worked example below uses the same shape, so the numbers shown here always match the calculator.

Use the interactive calculator on the page: https://petguidecalc.com/aquarium/round-end-aquarium-volume-calculator/. Enter the length, width, and height in cm or inches, then optionally set the fill height and substrate depth for the actual water-holding volume.

## How much water does my round-end tank hold?

Volume is the rectangular middle plus the two semicircular ends; the calculator converts cubic inches to US gallons (÷231) and to liters (÷61.024). a 24 × 12 × 16 in tank holds 17.81 US gal (67.41 L) at full fill.

## Common Round-End volumes at a glance

| Tank size | US gallons | Liters |
|---|---|---|
| 9.4 × 4.7 × 6.3 in (≈ 24 × 12 × 16 cm) | 1.08 gal | 4.07 L |
| 14.2 × 7.1 × 9.4 in (≈ 36 × 18 × 24 cm) | 3.66 gal | 13.86 L |
| 18.9 × 9.4 × 12.6 in (≈ 48 × 24 × 32 cm) | 8.66 gal | 32.77 L |

Dimensions are shown inches-first with approximate metric equivalents; all volumes at 100% fill — use fill height and substrate corrections for the real water-holding volume.

## How the volume is calculated

The volume of a round-end tank is V = ((L − W) × W × H) + (π × (W/2)² × H) in cubic inches, divided by 231 to convert to US gallons (÷ 61.024 for liters).

Most tanks are not filled to the rim, and substrate takes up real space below the water line. Use the fill-height and substrate-depth switches: each subtracts footprint × depth from the full volume, so the result reflects the water you actually hold.

Worked example from the calculator: a 24 × 12 × 16 in round-end tank at full fill returns 17.81 US gal (67.41 L). Change any input and the result updates from the same shared formula.

## Common questions about round-end tanks

**Q: Is the Round-End formula the same as a regular rectangle?**

No — the rounded ends replace two square corners with half-discs, so the footprint is the rectangular middle plus the two semicircular ends. The box formula would overstate the volume.

**Q: Why is my round-end tank's real volume less than the label?**

Most tank labels round to a nominal size, and the glass walls, trim, and headroom below the rim each reduce the water-holding volume below the geometric figure. Use the inside dimensions, subtract substrate depth, and set the fill height to your real water level for the honest number.

**Q: Should I subtract substrate from the water volume?**

Yes, in a planted tank. Sand or gravel occupies real volume below the water line, so subtract its depth × the tank footprint from the full volume. The substrate calculator computes that offset for you.

**Q: How accurate is the fill height?**

Most aquariums are filled to within 0.5–1 in (≈ 1–2 cm) of the rim, which on a 16 in (≈ 40 cm) tall tank is 95–100% of the box height. Set the fill height to the actual water level — an inch or so below the rim is typical.

**Q: Do I need to convert centimeters to inches first?**

No — toggle the unit switch and the calculator converts each dimension internally before applying the same formula. The result is the same regardless of which unit you type in.

## References

Volume and weight follow the standards below; geometric formulas are standard mathematics (CRC Handbook); the shape-accuracy note explains why each shape gets its own formula.

- [Guide for the Use of the International System of Units (SI) — Special Publication 811 (2008 Edition)](https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication811e2008.pdf) — Thompson A, Taylor BN, NIST Special Publication 811, 2008. Defines the liter–cubic-meter relationship and the US gallon ≈ 3.785412 L used for volume conversion on this page.
- [NIST Handbook 44 — Appendix C: General Tables of Units of Measurement (2024 Edition)](https://www.nist.gov/document/nist-hb-44-2024-appendix-c-general-tables-units-measurement) — National Institute of Standards and Technology, NIST Handbook 44, 2024. Confirms 1 US gallon = 231 cubic inches exactly (statutory definition) — the conversion anchor for the cubic-inch result.
- [ITS-90 Density of Water Formulation for Volumetric Standards Calibration](https://pmc.ncbi.nlm.nih.gov/articles/PMC4909168/) — Jones FE, Harris GL, Journal of Research of the National Institute of Standards and Technology, 1992. Freshwater density ρ = 0.99820 g/cm³ at 16.67 °C (62 °F) via the ITS-90 formulation — basis of the ≈8.34 lb/US gal freshwater weight shown here.
- [International one-atmosphere equation of state of seawater](https://www.sciencedirect.com/science/article/pii/0198014981901229) — Millero FJ, Poisson A, Deep-Sea Research Part A. Oceanographic Research Papers, 1981. Standard seawater density at 25 °C, S = 35: ρ ≈ 1.024 g/cm³, specific gravity ≈ 1.025 relative to pure water at 25 °C; adopted by UNESCO/IOC/ICES/SCOR/IAPSO and superseded by TEOS-10 (2010). Basis of the saltwater weight shown here.
- [The Practical Salinity Scale 1978: Conversion of Existing Data](https://www.vliz.be/imisdocs/publications/29449.pdf) — Lewis EL, Perkin RG, Limnology and Oceanography, 1981. Defines the Practical Salinity Scale 1978 (PSS-78), valid for salinity 2–42 — background for salinity-based density references across the aquarium calculators.
- [Assessment of 2D ultrasound fluid volume estimation accuracy in different shaped objects: an in vitro study](https://doi.org/10.1177/0284185119854198) — Kodikara I, Abeysekara I, Gamage D, Ilayperuma I, Acta Radiol, 2020. Methodology background: shape geometry measurably affects how accurately a formula-based volume estimate matches real volume, which is why each tank shape here uses its own formula rather than a single box approximation.

_Educational resource, not veterinary advice. PetGuideCalc — https://petguidecalc.com_