Extract from the Regulations for the Measurement of Tonnage recommended by the International Tonnage Commission assembled at Constantinople, In 1873.
 A - Rule I : General Principles:

1. The gross tonnage or total capacity of ships comprises the exact measurement of all spaces (without any exception). below the upper deck, as well as of all permanently covered and closed - in spaces on the deck.
   
   N.B.: By permanently covered and closed - in spaces on the upper deck are to understood all those which are separated off by decks or coverings, or fixed partitions and therefore represent an increase of capacity which might be used for the stowage of merchandise, or for the berthing and accommodation of the passengers or of the officers and crew.
   
   Thus, any one or more openings, either in the deck or coverings, or in the partitions, or a break in the deck, or the absence of a portion of the partition, will not prevent such spaces being comprised in the gross tonnage, if they can be easily closed - in after admeasurement, and thus better fitted for the transport of goods and passengers.
   
   But the spaces under awning decks without other connection with the body of the ship than the props necessary for supporting them, which are not spaces "separated off" and are permanently exposed to the weather and the sea, will not be comprised in the gross tonnage, although they may serve to shelter the ship's crew the deck passengers and even merchandise known as “ deck loads ”.
    
2. "Deck loads" are not comprised in the measurement.
    
3. Closed spaces for the use or possible use of passengers will not be deducted from the gross tonnage.
    
4. The determination of deduction for coal spaces may be effected either by the rules of the European Danube Commission of 1871 or by the exact measurement of fixed bunkers.

B - RULE 2 - For Laden Ships:

  When ships have their cargo on board, or when for any other reason their tonnage cannot be ascertained by means of Rule 1, proceed in the following manner :

Measure the length on the upper deck from the outside of the outer plank at the stem to the aftside of the stern-post, deducting there from the distance between the aftside of the stem-post and the rabbet of the stern-post at the point where the counter-plank crosses it.

Measure also the greatest breadth of the ship to the outside of the outer planking or wales.

Then, having first marked on the outside of the ship, on both sides thereof, the height of the upper deck at the ship's sides, girt the ship at the greatest breadth in a direction perpendicular to the keel from the height so marked on the outside of the ship, on the one side, to the height so marked on the other side by passing a chain under the keel; to half the girth thus taken add half the main breadth; square the sum, multiply the result by the length of the ship taken as aforesaid; then multiply this product by the factor 0. 17(seventeen hundredths) in the case of ships built of wood, and by the factor 0. 18 (eighteen hundredths) in the case of ships built of iron. The product will give approximately the cubical contents of the ship, and the general tonnage can be ascertained by dividing by 100 or by 2.83, according to the measurements taken in English feet or in meters.

    If there be a break, a poop, or other permanent covered and closed-in spaces (as defined in the general principles) on the upper deck, the tonnage of such spaces shall be ascertained by multiplying together the mean length, breadth and depth of such spaces and dividing the product by 100 or 2.83, according to the measurements taken in English feet or meters, and the quotient so obtained shall be deemed to be the tonnage of such space, and shall be added to the other tonnage in order to determine the gross tonnage or total capacity of the ship.

C - Deductions:

To be made from the Gross Tonnage in order to ascertain the Net Tonnage:

   To find from the gross tonnage of vessel as above set forth the official, or net registered tonnage, either for sailing vessels or for steam ships, the following mode of operations must be resorted to :

(1) Sailing Vessels :

    For sailing vessels deduct : the spaces exclusively and entirely occupied by the crew and the ship's officers, those taken up by the cookhouse and latrines exclusively used by the ship's officers and crew whether they be situated above or below the upper deck; the covered and closed in spaces, if there 'be any situated on the upper deck, and used for working the helm, the capstan, the anchor gear, and for keeping the charts, signals and other instruments of navigation.

Each of the spaces deducted as above may be limited according to the requirements and customs of each country, but the deductions must never exceed in the aggregate 5 percent of the gross tonnage.

    The measurement of these spaces is to be effected according to the rules set forth the measurement of covered and closed-in spaces on the upper deck, for result, obtained by deducting the total of such allowances from the gross tonnage, represents the net or tonnage of sailing vessels.

(2) Steam Ships :

 For vessels propelled by steam or any other mechanical power, deduct:

A - The same spaces as for sailing vessels (Art. 12) with the limitation to 5 percent of the gross tonnage.

B - The spaces occupied by the engines, boilers, coal bunkers, shafttrunks of screw steamers, and the spaces between decks and in the covered and closed-in erections on the upper deck surrounding the funnels, and required for the introduction of air and light into the engine-rooms and for the proper working of the engines themselves.
Such deductions cannot exceed 50 per cent of the gross tonnage.

 The measurement of the spaces allowed for both in sailing vessels and in steam ships  is to be effected according to the rules set forth in Articles 12 and 13 for sailing vessels.

Spaces for which allowances are made in steam ships only  are measured according to the following rules.

(3) Ships having coal-bunkers with movable partitions :

    In ships that do not have fixed bunkers but transverse bunkers with movable partitions, with or without lateral bunkers, measure the space occupied by the engine-rooms, and add to it, for screw steamers 75 percent, and for paddle steamers, 50 percent of such space.

By the space occupied by the engine-rooms is to be understood that occupied by the engine-room itself and by the boiler-room together with the spaces strictly required for their working, with the addition of the space taken up by the shaft-trunk in screw steamers and the spaces between decks which enclose the funnels and are necessary for the admission air and light into the engine-rooms.

These spaces are measured in the following manner :

Measure the mean depth of the space occupied by the engines and boilers from its crown to the ceiling at the limber strake, measure also three, or, if necessary, more than three breadth of the space at the middle of its depth, taking one of such measurements at each end and another at the middle of the length: take the mean of such breadths; measure also the mean length of the space between the foremost and aftermost bulkheads or limits of its length, excluding such parts, if any, as are not actual occupied by or required for the proper working of the engines and boilers.

Multiply together these three dimensions of length, breadth and depth, and the product will be the cubical contents of the space below the crown.

Then find the cubical contents of the space or spaces, if any, between the crown aforesaid and the uppermost or poop deck, as the case may be, which are framed in for the machinery or for the admission of light and air, by multiplying together the length, depth and breadth thereof.

Add such contents as well as those of the space occupied by the shafttrunks to the cubical contents of the space below the crown; divide the sum by 100 or by 2.83, according to the measures taken in feet or meters and the result shall be deemed to be the tonnage corresponding to the engine and boiler room which serves as basis for the deductions referred to.

If in any ship in which the space aforesaid is to be measured, the engines and boilers are fitted in separate compartments, the contents of shall be measured separately in like manner, according to the above rules, and the sum of their several results shall be deemed to be the tonnage of the engine-rooms which serves, as aforesaid, as basis for the total deductions.


(4) Ships with fixed coal-bunkers

     In ships with fixed coal-bunkers measure the mean length of the engine and boiler-room, including the coal-bunkers. Ascertain the area of three transverse sections of the ship (as set forth in the rules given in Art. 3 and 4 for the calculation of the gross tonnage) to the deck which covers the engines.

One of these three sections must pass through the middle of the aforesaid length, and the two others through the two extremities.

Add to the sum of the two extreme sections four times the middle one, and multiply the sum thus obtained by the third of the distance between the sections. This product divided by 100, if the measurements are taken in English feet, or by 2.83 if they are taken in meters, gives the tonnage of the space in question.

If the engines, boilers and bunkers are in separate compartments they are separately measured, as above set forth, and the results are added together.

In screw steamers the contents of the shaft-trunk are measured by ascertaining the mean length, breadth and height, and the product of the multiplication of these three dimensions divided by 100 or 2.83 according to the measurements taken in feet or in meters, gives the tonnage of such space.

The tonnage of the following spaces between decks, and in the covered and closed-in erections on the upper deck, is ascertained by the same method :

• The spaces framed-in round the funnels.
• The spaces required for the admission of light and air into the engine-rooms.
• The spaces, if any, necessary for the proper working of the engines.

     Instead of the measurement of fixed bunkers, the rules for bunkers with movable partitions as set forth in Art. 16 may be applied.

     In the case of tugs the allowances are not limited to 50 per cent of the gross tonnage; all the space occupied by machinery, boilers and coal-bunkers may be deducted.

Nevertheless, if such vessels are not excursively employed as tugs, the deductions in question cannot exceed 50 percent of the gross tonnage.