ScapeMath

Aquarium volume & water weight calculator

Measure the inside of the glass, pick your tank shape, and get the volume in litres, US gallons and UK gallons, plus how much the water will weigh on your stand.

cm
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Real water volume (optional)
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°C
Glass volume
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Also
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Water volume
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Water weight
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How it works

Volume is the footprint area times the height. One litre is 1,000 cm³. One US gallon is exactly 231 cubic inches (3.785411784 L) and one UK gallon is 4.54609 L, both by legal definition.

Rectangular: V = L × W × H Cylinder: V = π × (D/2)² × H Bow-front: V = (L × d_end + A_segment) × H A_segment = R²/2 × (θ − sin θ), R = (L²/4 + s²) / 2s, θ = 2·asin(L / 2R) s = d_centre − d_end (how far the front bows out)

A bow-front is a rectangle (the back length times the depth at the ends) plus a circular segment: the curved slice between a straight line across the front and the arc of the glass. We treat the bow as an arc of a circle, which is how bent-glass and acrylic bow-fronts are usually formed. If yours is flatter in the middle, the true volume will be slightly smaller.

Water weight uses the density of pure water from the UNESCO standard (997.0 kg/m³ at 25 °C) or of 35 ppt seawater (1,023.3 kg/m³ at 25 °C) from the same EOS-80 equation used on our salinity calculator.

Worked examples

A 60 × 30 × 36 cm tank

60 × 30 × 36 = 64,800 cm³ = 64.8 L (17.1 US gal). Filled with fresh water at 25 °C it weighs 64.8 × 0.99705 = 64.6 kg, before glass, substrate and rock.

Why a "20 gallon long" holds 18.7 gallons

A standard 20-long is sold as 30 × 12 × 12 in. 30 × 12 × 12 = 4,320 in³, and 4,320 ÷ 231 = 18.7 US gal even if you fill it to the brim. Nominal sizes are rounded, and the dimensions are usually outside measurements, so the real figure is lower still.

A bow-front with a 91 cm back

Ends 38 cm deep, centre 45.7 cm, height 53 cm. The bow is s = 7.7 cm, so R = (91²/4 + 7.7²) / (2 × 7.7) = 138.3 cm and the segment adds 469.8 cm² to the 91 × 38 = 3,458 cm² rectangle. (3,458 + 469.8) × 53 = 208.2 L (55.0 US gal).

Things that change the real number

References

  1. NIST Handbook 44, Appendix C: General tables of units of measurement (US gallon = 231 in³; inch = 2.54 cm)
  2. UK Weights and Measures Act 1985, Schedule 1 (gallon = 4.54609 L)
  3. Millero, F.J. & Poisson, A. (1981). International one-atmosphere equation of state of seawater. Deep-Sea Research 28A: 625–629
  4. Weisstein, E.W. Circular Segment. MathWorld, Wolfram Research

Formulas on this page are checked by automated unit tests against the reference values above. Last reviewed October 29, 2026. Found an error? Tell us.

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