A service number is an identification code used to identify a person within a large group. Service numbers are most often associated with the military; however, they may be used in civilian term as well. Social Security Numbers may be seen as types of service numbers.
The term "serial number" is often seen as synonym of service number; however, a serial number more accurately describes manufacture and product codes, rather than personnel identification. In the Canadian military, a "Serial Number" referred to a unique number assigned each unit that mobilized for the Second World War.
In the First AIF soldiers were allotted numbers known as regimental numbers. These were allotted to NCOs and other ranks but not to officers or nurses, who had no numbers. Regimental numbers were rarely unique. Each battalion or corps had its own sequence, usually starting at 1, although some units were formed in the field and this did not occur. The result was that several dozen soldiers had the prestigious number 1, which was usually given to the Regimental Sergeant Major or the Regimental Quartermaster Sergeant. When soldiers were transferred from one unit to another, they often kept their number if it was not already held by someone else. Otherwise, they might be allotted a new number, or the letter A or B might be added to make the number unique with their unit again. Re-enlisted soldiers often used the additional letter R. In 1917, the AIF switched to a scheme whereby reinforcements were drawn from common pool instead of being supplied on a per-unit or corps basis. These were known as "general reinforcements" and they were allotted unique numbers in the range of 50000-80000. Despite the limitations of the scheme, in researching a soldier, it is handy to know the regimental number.
The problems inherent in this scheme were acknowledged and all members of the Second AIF were allocated a unique service number known as an Army number. The first letter represented the state of enlistment: N - New South Wales; V - Victoria; Q - Queensland; S - South Australia; W - Western Australia; T- Tasmania; D - Northern Territory. The serial numbers of female soldiers followed this with an F. AIF serial numbers then had an X. A low number indicated an early enlistment. General Sir Thomas Blamey was VX1. Soldiers transferring from the Militia often kept their old number with 100,000 added, while PMF officers had 200,000 added.
Canada began using "Regimental Numbers" during the First World War.
During the Second World War, units were allocated blocks of Regimental Numbers to issue out, usually in the 5 or 6 digit range, though extremely low numbers were also possible due to the blocks.
- X12345 - the X was an alphabetic character denoting the Military District the soldier was recruited in (A represented MD1, B MD2, etc.) Up until 1945, officers never received numbers and were identified by name and rank only.
The Social Insurance Number (SIN) replaced the regimental number in the 1960s.
- 123 456 789
The SIN was itself replaced by a Service Number in the 1990s.
- X12 345 678
The use of the SIN was granted by Revenue Canada to the CF for service numbers as a temporary measure, and was revoked in the 1990s. The new Service Number used a random alphabetic letter and 8 numbers in the same format as SINs to avoid changing service forms.
In Nazi Germany, the equivalent of a service number was known as a "membership number" which were issued by various Nazi groups based on when a person had initially joined. Nazi membership numbers were also preceded by the name of the organization to which the number applied. For instance, a person who was both a member of the SS and Nazi Party would state their numbers (as an example) "NSDAP #15337 und SS #4436". Membership numbers were considered extremely important in the Nazi system and to hold a low membership number was almost more important than what rank an individual presently had been granted.
The Wehrmacht did not use service numbers in the same sense as their western military counterparts. Soldiers were inducted in their home districts, and identity recorded in a master roster book. A position in the roster book served as a unique identifier; this number was recorded in the soldier's paybook, and was stamped on the soldier's "Erkennungsmarke" (identity disc, or "dogtag").
Current number of officers use a 5 digit number and are worn by all ranks below senior officers.
The same numbering pattern is used by Correctional Services and Fire Services.
Soldiers in the British Army are given an eight-digit number, e.g. 25232301. Prior to 1920 each regiment issued their own service numbers which were unique only within that regiment, so the same number could be issued many times in different regiments. When a serviceman moved, he would be given a new service number by his new regiment. Commissioned officers did not have service numbers until 1920. The modern system was introduced by Army Order 338 in August 1920. Numbers were then a maximum of seven digits, later groups of numbers up to eight digits were added.
- For Example
Until 1960, National Servicemen who voluntarily remained in the Armed Forces continued to use their National Service numbers. Until 2007 and the introduction of the Joint Personnel Administration (JPA) system, Army Officers were issued with a six-digit service number. Newly commissioned officers now receive an 8-digit service number, but 6-digit Officers' numbers issued prior to the introduction of JPA remained unchanged.
In the Royal Navy, prior to the introduction of JPA, service numbers were also of eight digits, but began and ended with a letter, depending initially on the depot where the sailor was recruited. The first letter designators were: P (Portsmouth), C (Chatham), and D (Devonport), with the final letter being a meaningless checksum.[clarification needed] Later, the designators were reassigned and were used to distinguish between men and women within the Royal Navy as well as to distinguish between Officers and Ratings. A service number beginning with D designated a Royal Navy male Rating, W a Royal Navy female Rating, C male Officers, and V female Officers. P designated a Royal Marines Other Rank, while N a Royal Marine Officer. Following the introduction of JPA, all newly issued Royal Navy service numbers became an 8-digit number format beginning with 3, with no distinction made between male, female, Ratings, Officers, and Royal Marines.
The following formats were used to denote U.S. military service numbers:
- 12-345-678: United States Army and U.S. Air Force enlisted service numbers
- 123-45-67: United States Navy enlisted service numbers
- 1234-340: United States Coast Guard enlisted service numbers
- 123456: United States Marine Corps enlisted service numbers
- 12345: Service number format for most U.S. military officers
Social Security Numbers are today used as the primary means to identify members of the U.S. military. The common format for a social security numbers is 123-45-6789.
Effective June 2011, the US military has introduced a plan to eliminate the use of Social Security Numbers on military and dependent ID cards, and replace them with a service number, in an effort to prevent identity theft against members of the armed services. All members are expected to have been granted the new service number by June 2015. 
- "Detailed Description of First World War Embarkation Roll". Retrieved 25 November 2009.
- Long, To Benghazi, p. 63
- History of the British Army Volume One, Henry William and Catherine Patricia Adams, Major Book Publications 1990, ISBN 1-872491-02-2
- Renumbering of the army in 1920