The tonnage is a measure of the size of the ships. The tonnage usually used for merchant ship losses is the gross register tonnage (GRT) which is the enclosed volume of the ship (which would have been measured in cubic feet, and then converted to a weight in tons using the density of seawater of 1 ton for 35 cubic feet).

A rough conversion is possible. Allied merchant shipping losses during the Battle of the Atlantic were about 6,000 ships and a total tonnage of about 21 million. This gives an average tonnage of about 3,500 per ship. This does include some quite small ships, and excluding them might give an average tonnage of about 5,000. Japanese merchant losses averaged a tonnage of about 4,200 excluding ships of under 500 tons. So Lexington's total would be about 70 ships sunk.

For warships, the displacement is used rather than the tonnage. This is a measure of the weight of the ship; the standard displacement assumes that the ship is fully loaded (ammunition, stores, etc.) and crewed except for fuel and reserve boiler water. It's harder to give a useful average displacement, since warships vary from small minesweepers of perhaps a few 100 tons to battleships of over 30,000 tons.

Some extras:

• Tonnage varied depending on the type of tons used: long, short, or metric. (The long ton and metric ton are close to each other; the short ton is about 10% smaller. Long tons are the traditional non-metric unit for tonnage.)

• Since WW2, the measurement of tonnage has become standardised. Before then, there were various methods to measure it. Also, today, GRT is not used, just gross tonnage (GT). Tonnage is also officially a dimensionless number, so the above question of units doesn't apply any more.

• Net tonnage measures the cargo carrying capacity. While GT is the total enclosed volume of the ship, net tonnage is the volume of the cargo carrying spaces. (GRT is usual when giving wartime shipping losses.)