How to determine Army Serial Numbers 
How to determine Army Serial Numbers
The following came from Army Regulation 615-30, dated 12 Feb 1942.
The system enabled one who knew the numbering system to tell from a serial number the component and corps area (later Service command and army area) from which a man had entered the Federal service, whether he had enlisted, or had been drafted through the Selective Service, and whether he had been a National Guardsman in his home state. Officer serial numbers always started with the letter O.
According to Army Regulation 615-30, dated 12 Feb 1942, Corps area commanders are charged with the assignment of Army serial numbers to all enlisted men within their respective areas, including exempted stations and commands, and oversea base commands.
For the Regular Army - Enlisted men to whom an Army serial number has never been assigned: Army serial numbers from the blocks shown below will be assigned to men enlisted in the Regular Army on or after July 1, 1940. The second digit indicates the corps area of enlistment.
1st Corps Area - 11,000,000 to 11,999,999
2nd Corps Area - 12,000,000 to 12,999,999
3rd Corps Area - 13,000,000 to 13,999,999
4th Corps Area - 14,000,000 to 14,999,999
5th Corps Area - 15,000,000 to 15,999,999
6th Corps Area - 16,000,000 to 16,999,999
7th Corps Area - 17,000,000 to 17,999,999
8th Corps Area - 18,000,000 to 18,999,999
9th Corps Area - 19,000,000 to 19,999,999
Hawaiian Department - 10,100,000 to 10,199,999
Panama Canal Department - 10,200,000 to 10,299,999
Philippine Department - 10,300,000 to 10,399,999
Puerto Rican Department - 10,400,000 to 10,499,999
For the National Guard - Army serial numbers from the following blocks will be assigned to members of National Guard units upon induction into Federal service, regardless of whether or not the man formerly had an Army, Navy, or other service number. The third digit indicates corps area of induction.
1st Corps Area - 20,100,000 to 20,199,999
2nd Corps Area - 20,200,000 to 20,299,999
3rd Corps Area - 20,300,000 to 20,399,999
4th Corps Area - 20,400,000 to 20,499,999
5th Corps Area - 20,500,000 to 20,599,999
6th Corps Area - 20,600,000 to 20,699,999
7th Corps Area - 20,700,000 to 20,799,999
8th Corps Area - 20,800,000 to 20,899,999
9th Corps Area - 20,900,000 to 20,999,999
Hawaiian Department - 20,010,000 to 20,019,999
Puerto Rican Department - 20,020,000 to 20,029,999
The serial number assigned at induction into Federal service will be retained thereafter upon enlistment or reenlistment for the Regular Army or Army of the United States.
For Men inducted into the Army - Army serial numbers from the following blocks will be used for trainees inducted under the Selective Training and Service Act of 1940, regardless of whether the man formerly had an Army, Navy, or Other service number, and will be retained thereafter upon enlistment or reenlistment for the Regular Army. The second digit indicates the corps area of induction.
1st Corps Area - 31,000,000 to 31,999,999
2nd Corps Area - 32,000,000 to 32,999,999
3rd Corps Area - 33,000,000 to 33,999,999
4th Corps Area - 34,000,000 to 34,999,999
5th Corps Area - 35,000,000 to 35,999,999
6th Corps Area - 36,000,000 to 36,999,999
7th Corps Area - 37,000,000 to 37,999,999
8th Corps Area - 38,000,000 to 38,999,999
9th Corps Area - 39,000,000 to 39,999,999
Hawaiian Department - 30,100,000 to 30,199,999
Panama Canal Department - 30,200,000 to 30,299,999
Philippine Department - 30,300,000 to 30,399,999
Puerto Rican Department - 30,400,000 to 30,499,999
Consecutive numbers - Consecutive numbers will not be assigned to men of the same surname. Canceled numbers will not be held available for reassignment.
The Army serial number assigned to an enlisted man is part of his official designation and, except as indicated will ordinarily appear in every military record in which his name appears, including every letter, telegram, order, report, pay roll, etc., at least once, preferably where the name first occurs.
Special care must be taken that the correct Army serial number is used for the reason that certain records are filed and certain accounts are paid by the Army serial number and not by name.
Corps Areas
1st Corps Area – ME, VT, NH, MA, CT, RI
2nd Corps Area – NY, NJ, DE
3rd Corps Area – PA, MD, VA, DC
4th Corps Area – NC, SC, GA, FL, AL, MS, TN
5th Corps Area – OH, IN, KY, WV
6th Corps Area – IL, MI, WI
7th Corps Area – MO, KS, NB, CO, IA, MN, ND, SD, WY
8th Corps Area – AR, LA, TX, OK, NM
9th Corps Area - WA, OR, CA, ID, MT, NV, UT, AK