In Traveller, a "ton" of starship size is defined as 14 cubic meters, the volume of a ton (1000kg) of liquid hydrogen. This is bit abstract for most Traveller players, who have no quick way to compare this "Traveller Ton" to everyday experience. This essay will remedy that.
[Editor's note: The "Traveller Ton" is routinely called the "displacement ton" in some Traveller materials and in conversation between Traveller fans. This is not the same as the displacement ton discussed below.]
Visualizing a Traveller Ton
A Traveller Ton is 14 cubic meters, which is about 500 cubic feet; the standard 10x10x10 foot (3x3x3 meter) cube used in most FRP game mapping is about 2 Tons Traveller.
Capacity
 Onecar garage: 4 tons
 Twocar garage: 8 tons
 20' (6m) "bobtail" truck: 4 tons
 50' (15m) "bigrig" truck semitrailer: 10 tons
 40' American TL6 (late steamera) railroad boxcar: 8 tons
 50' American TL7 railroad boxcar: 10 tons
 80' American TL8 "Hicube" railroad car: 1618 tons
Examples
The standard Type A Free Trader has a cargo capacity of 82 Tons Traveller. This translates into 20 fullsize American ground cars, or 2024 bobtail truck loads, or10 "bigrig" truck loads, or 810 typical American railroad boxcars.
The standard Type R Subsidized Merchant (capacity 200 Tons Trav) holds 50 fullsize cars/bobtail truck loads, or 20 "bigrig" loads, or 2025 boxcar loads.
Starshiptosurface ship comparisons
How big is a Traveller starship compared to a "wet" surface ship? This is complicated by the use of several different tonnage measurements for surfaceship measurements.
1. Civilian Ship Tonnage
1.1 Gross Tonnage:
The actual volume of the ship's interior, in "tons" of 100 cubic feet (about 1/5 of a Traveller Ton). This is the closest methodology to Traveller tonnage, and converts easily at a rate of 5 Tons Gross = 1 Ton Traveller. Examples are:
 WW2 Liberty Ship (TL6): 12000 Tons Gross = 2400 Tons Traveller
 RMS Titanic (TL5): 45000 Tons Gross = 9000 Tons Traveller
 RMS Queen Mary (TL6): 82000 Tons Gross = 16000 Tons Traveller
 Supertanker (TL8): 200000 Tons Gross = 40000 Tons Traveller
Going the other way:
 Type A Free Trader: 200 Tons Traveller = 1000 Tons Gross
 Type AL Stretched Free Trader: 300 Tons Traveller = 1500 Tons Gross
 Type R Subsidized Merchant: 400 Tons Traveller = 2000 Tons Gross
 Type RL Stretched Subsidized Merchant: 600 Tons Traveller = 3000 Tons Gross
 Type TI Frontier Transport: 2000 Tons Traveller = 10000 Tons Gross
 Type AHL Superfreigher: 6000 Tons Traveller = 30000 Tons Gross
The Commercial Efficiency Ratio (CER)
is a relative cost/benefit ratio indicating how well a ship will
perform in commercial service. The higher the CER, the more profitable
the ship will be to operate.
To calculate the CER for a ship: Total the Net Tonnage ("revenue space") of the ship in tons, counting each passenger stateroom as 4 tons, each passenger low berth as 1/2 ton, and cargo tonnage as straight tonnage. (Do not count crew staterooms, sickbay/ emergency low berths, or vehicle bays unless the vehicles are part of the cargo; Net Tonnage is only the part of the ship that can be used to carry passengers and cargo.) Multiply this net tonnage by the Jump number, then divide by the ship's cost in MCr. CER = NetTonnage * JumpNumber / MCr

1.2 Net Tonnage
The "revenue space" of the ship, i.e. the volume of the cargo holds and passenger accomodations only, measured in "tons" of 100 cubic feet. This is a bit harder to calculate; in Traveller terms, net tonnage counts only the cargo hold, passenger staterooms, and passenger low berths. Net tonnage is used in Traveller primarily to calculate the Commercial Efficiency Ratio (CER), a measure of a ship's relative profitability in service. Examples (all from the Traveller end) are:
 Type A Free Trader: 82ton hold, 6 passenger staterooms, 20passenger
low berths.
82 + (6x4) + (20x1/2) = 116 Traveller Tons Net = 580 Tons Net  Type AL Stretched Free Trader:
135ton hold, no passenger accommodations.
135 = 135 Traveller Tons Net = 675 Tons Net  Type R Subsidized Merchant: 200ton hold, 8 passenger staterooms, 10
passenger low berths.
200 + (8x4) + (10x1/2) = 237 Traveller Tons Net = 1185 Tons Net  Type RL Stretched Subsidized Merchant:
390ton hold, 8 passenger staterooms, 10 passenger low berths.
390 + (8x4) + (20x1/2) = 427 Traveller Tons Net = 2135 Tons Net  Type TI Frontier Transport: 1114ton hold, no passenger accomodations.
1114 = 1114 Traveller Tons Net = 5570 Tons Net  Type AHL Superfreigher: 4072ton
hold, 24 passenger staterooms, no low berths.
4072 + (24x4) = 4168 Traveller Tons Net = 20840 Tons Net
1.3 Deadweight Tonnage
This is the actual weight of the ship's fullload cargo capacity, the maximum weight of cargo the ship can carry. As this is a weight measurement and Traveller tonnage is a volume measurement, there is no way to convert between the two.
2. Military Ship Tonnage
Warships are always measured by "displacement tonnage", the actual weight of the ship, which translates directly into the hull volume in cubic meters below the waterline. This varies according to how fully the ship is loaded; fortunately, various armscontrol treaties of the battleship era (TL56) defined specific types of loading for tonnage determination. The most useful of these are:
 Full Load Displacement
 Fully loaded  crew, supplies, ammunition, fuel all topped off. This is the closest approximation to Traveller measurement. The volume of the ship below the waterline translates into Traveller tonnage on a 14to1 basis. (However, most of the ship is above the waterline.)
 Standard Displacement
 As Full Load, but no fuel. This is the default size measurement of a warship; if size is given in "tons" with no other qualifier, it is standard displacement tonnage.
 Submerged Displacement
 Used only for submarines, this is the weight of the sub at neutral buoyancy, which translates directly into hull volume in cubic meters. This translates directly into Traveller tons at a ratio of 14 tons submerged = 1 Ton Traveller.
Since displacement tonnage is a measure of weight and Traveller tonnage of volume, any conversion between the two will be approximate.
To convert Surface Ships from wetnavy to Traveller:
2.1 Always start with fullload displacement tonnage.
2.2 Divide the fullload tonnage by 14 to get the Traveller tonnage for the amount of the ship below the waterline.
2.3 Estimate the proportion of the ship below the waterline (about 1/3 for most ships, down to 1/2 for heavily armored battleships) and multiply by the reciprocal of this fraction.
2.4 A ruleofthumb for 2.3 is:
2.4.1 Unarmored ship (Destroyers, TL45 light cruisers, most TL79 construction)  5 tons fullload = 1 Ton Traveller (coincidentally, this is the same ratio as a civilian ship's Gross Tonnage).
2.4.2 Moderatelyarmored ship (TL45 armored cruisers, TL56 battlecruisers, most TL6 cruisers)  6 tons fullload = 1 Ton Traveller.
2.4.3 Heavilyarmored ship (TL46 battleships only)  7 tons fullload = 1 Ton Traveller.
Examples
(Unarmored)
Destroyer USS Fletcher (TL6): 2000 tons std, 3000 fullload = approx. 600 Tons Traveller
Destroyer USS Cole (TL9): 8400 tons fullload = approx 1700 Tons Traveller
Carrier USS Enterprise (TL7): 75000 tons std, 90000 fullload = approx. 18000 Tons Traveller
Light Carrier HMS Invincible (TL8): 16000 tons std, 20000 fullload = approx. 4000 Tons Traveller
Carrier USS Nimitz (TL89): 80000 tons std, 92000 fullload = approx. 18000 Tons Traveller
(Moderatelyarmored)
Battlecruiser HMS Hood (TL5): 42000 tons std, 45000 fullload = approx. 7500 Tons Traveller
Typical "Treaty Cruiser" (TL56): 10000 tons std, 13000 fullload = approx. 2000 Tons Traveller
Armored Cruiser KMS Graf Spee (TL6): 12000 tons std, 16000fullload = approx. 2600 Tons Traveller
Battlecruiser KMS Scharnhorst (TL6): 32000 tons std, 38000 fullload = approx. 6300 Tons Traveller
Carrier HMS Ark Royal (TL6): 22000 tons std, 28000 fullload = approx. 4500 Tons Traveller
Carrier USS Enterprise (TL6): 20000 tons std, 26000 fullload = approx. 4300 Tons Traveller
(Heavilyarmored)
Battleship USS Oregon (TL4): 10000 tons std, 12000 fullload = approx. 1700 Tons Traveller
Battleship HMS Majestic (TL4): 15000 tons std, 16000 fullload = approx. 2300 Tons Traveller
Battleship HMS Dreadnaught (TL5): 18000 tons std, 22000 fullload = approx. 3000 Tons Traveller
Battleship USS Arizona (TL5): 25000 tons std, 33000 fullload = approx. 5000 Tons Traveller
Battleship KMS Bismarck (TL6): 42000 tons std, 50000 fullload = approx. 7000 Tons Traveller
Battleship USS New Jersey (TL6): 45000 tons std, 58000 fullload = approx. 8000 Tons Traveller
Battleship HIJMS Yamato (TL6): 60000 tons std, 72000 fullload = approx. 10000 Tons Traveller
To convert Submarines to Traveller tonnage:
2.5 Start with submerged tonnage.
2.6 Divide the submerged tonnage by 14 to get the Traveller tonnage.
Examples
Type VII Uboat (TL6  Das Boot): 900 tons submerged = approx. 65 Tons Traveller
USS Nautilus (TL7): 4000 tons submerged = approx. 300 Tons Traveller
USS Los Angeles (TL8): 6900 tons submerged = approx. 500 Tons Traveller
Kursk (TL8): 18000 tons submerged = approx. 1300 Tons Traveller
"Red October" (TL8): 29000 tons submerged = approx. 2000 Tons Traveller
Notes
The use of 100 cubic feet as a "ton" for civilian shipping dates back to the 19th Century (TL4), when the British Empire was Earth's maritime superpower. (Longitude on Earth is still measured in degrees East or West from the Royal Observatory in Greenwich, a suburb of the British capital.) The "foot" of 30.5cm was a standard measurement of the British Empire of the time; the familiar metric system was just becoming established after being originated by the Brits' bitter rivals, the French.
Standard Displacement was defined by the Washington Treaty of 1922, a TL5 arms limitation agreement drawn up to prevent a bankrupting naval arms race after the end of the First World War. This treaty defined maximum sizes and weapons for various types of warship, limited each navy to a maximum total tonnage for "capital ships" (the largest combatants), and provided for replacements to be built at the end of a defined service life. The treaty spawned other treaties further limiting navies, but all such treaties were allowed to expire or were withdrawn during the buildup for the Second World War.