How to Calculate Tonnage

By Contributing Writer; Updated April 24, 2017

Tonnage is the measure of the overall size of a ship. Tonnage is a function of the volume of all enclosed spaces of a ship from keel to funnel and does not refer to the ship weight. Particularly, the gross tonnage (GT) is the common metrics to estimate harbor and registration fees for a ship. As an example, calculate the gross tonnage if the total volume of all the ship’s enclosed spaces is 5,000 cubic meters.

Calculate the logarithm (with base 10) of the ship volume using the log function on your calculator. In our example, log 5,000 = 3.7.

Multiply the result in Step 1 by 0.02 and add 0.2 to the product. In our example, it is 3.7 x 0.02 + 0.2 = 0.274.

Multiply the ship volume by the number from Step 2 to calculate the gross tonnage. In our example, tonnage = 0.274 x 5,000 = 1,370.