How to Read a National Stock Number
A National Stock Number (NSN) is a 13-digit identifier assigned to a specific item in the U.S. federal supply system. Every digit belongs to one of two fields, and reading those fields is the first step before a stock number can be searched, cross-referenced, or quoted.
The structure: FSC + NIIN
An NSN splits into two parts: a 4-digit Federal Supply Classification (FSC) followed by a 9-digit National Item Identification Number (NIIN). Written together and formatted with dashes, the two fields read as FSC-NN-NNN-NNNN — four digits, a dash, then the NIIN's own two-three-four grouping.
The Federal Supply Classification (FSC)
The first four digits place the item in one of the federal government's supply classes — a category covering a related group of items, not a single product line. FSC 5310, for example, covers nuts and washers; FSC 5905 covers fixed resistors. The FSC tells a buyer what kind of thing the item is before they've read anything else about it.
The National Item Identification Number (NIIN)
The remaining nine digits are the NIIN, which itself splits into two fields: the first two digits are the National Codification Bureau (NCB) code — identifying the country or NATO-system agency that originally assigned the number (items assigned by the United States are coded 00 or 01) — and the remaining seven digits are a sequential item identifier. Those seven digits carry no embedded meaning of their own: two NIINs that happen to share an NCB code are not otherwise related, and nothing about an item's size, material, or class can be read out of them directly.
NSN vs. part number
An NSN is not a manufacturer's part number, and the two shouldn't be read interchangeably. A single NSN identifies one item as defined by its federal record — technical characteristics, governing specification, and, in most cases, more than one qualified manufacturer's part number that meets that same record. The part number identifies one manufacturer's offering; the NSN identifies the item itself, independent of who built it.
A worked example
Stock number 5310-00-186-0965 breaks down as FSC 5310 (Nuts, Washers) and NIIN 00-186-0965 (NCB code 00 — a U.S.-assigned number — followed by its sequential item identifier). Reading the FSC alone already tells a buyer this is hardware from the nuts-and-washers class, before the NIIN is looked up at all.
FSC 5310 — Nuts, Washers
NIIN 00-186-0965 — NCB 00 (U.S.-assigned) + sequential item identifier
This particular stock number identifies a Military Standard key washer, designated MS19070-061 — see Reading Military Standard Nut and Washer Families for how that designation itself is built, or view the item's full federal record on its part page.