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Gross Register Ton to m³ Converter

Enter a value and pick a direction to convert between GRT and m³.

How it is calculated

A register ton is a volume of 100 cubic feet, which is exactly 2.8316846592 m³ because 1 cubic foot is 0.028316846592 m³. GRT to cubic metres: m³ = GRT × 2.8316846592. Cubic metres to GRT: GRT = m³ ÷ 2.8316846592. The direction selector (grt2m3 or m32grt) chooses the formula; with no selection GRT to m³ is used. The value must be zero or greater, and the answer is rounded to four decimals. The tool is a pure unit conversion: it does not measure a ship, and it does not convert between GRT and modern Gross Tonnage (GT), whose figures come from a different formula and cannot be derived by simple scaling.

Worked example

An older register lists a vessel at 1,500 GRT and you want the enclosed volume in cubic metres. Step 1: one GRT is 2.8316846592 m³. Step 2: multiply 1,500 × 2.8316846592 = 4,247.5269888. Step 3: rounded to four decimals the result is 4,247.5270 m³. In the other direction, a 10,000 m³ enclosed volume is 10,000 ÷ 2.8316846592 = 3,531.4667 GRT.

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Common mistakes

  • Treating GRT as weight. A register ton measures enclosed volume (100 cubic feet), so 1,500 GRT is not 1,500 tonnes of cargo or displacement.
  • Mixing up GRT, net register tonnage (NRT), deadweight tonnage and modern Gross Tonnage (GT). They measure different things, and only GRT converts with this factor.
  • Using the 2.83 shortcut for large vessels. At thousands of tons, a rounded factor drifts noticeably from the exact figure.
  • Picking the wrong direction. A ship's m³ volume entered with GRT to m³ selected comes out nearly three times too large.
  • Comparing GRT figures with a modern GT value as if they were the same unit.

About this calculator

Gross register tonnage (GRT) is a historical unit of ship volume — not weight — representing the total enclosed internal space of a vessel, where one register ton equals 100 cubic feet, or 2.8316846592 cubic meters. This calculator converts between GRT and cubic meters in either direction, letting you translate older ship documentation and registries into the metric volumes used in modern naval architecture and port paperwork.

GRT was the standard measure used for port fees, canal tolls, and registry classification for most of the 20th century before being formally superseded by the dimensionless Gross Tonnage (GT) system under the 1969 International Convention on Tonnage Measurement of Ships. Maritime historians, ship surveyors, and enthusiasts researching older vessels — many of which are still documented in GRT in historical records, insurance archives, and classic ship registries — rely on this conversion to compare old and new tonnage figures.

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Comments (1)

  • Editorial team

    Gross register tonnage belongs to older ship registers. Newer ships are measured in gross tonnage under the 1969 tonnage convention, which came into force in 1982, so a modern vessel will not have a register-ton figure and an old one cannot be turned into gross tonnage by scaling. Which document or register did you read your GRT from, and how old is the vessel?

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