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 shipâ€™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.