A 13-digit catalog identity assigned under the NATO codification system. Here's how to read one, digit by digit, using a real stock number — and what the number does and does not tell you.
A National Stock Number is a 13-digit identifier assigned to an item of supply — a thing the federal logistics system needs to order, stock, and issue consistently. When a federal activity registers a recurring need for an item, the item gets cataloged: it receives a stock number, an approved item name, a set of coded characteristics, and a list of reference part numbers that satisfy it. From that point on, every warehouse, requisition, and solicitation in the system can refer to the item by one unambiguous number instead of a tangle of manufacturer part numbers.
That is the entire job of the NSN. It does not describe a design the way a drawing does, it does not certify quality, and it does not restrict who may buy the item — a point we cover at length in the companion guide on buying NSN parts without a government contract. What it does do, extremely well, is give you a stable key into a large body of public cataloging data. Learn to read the number and you can extract a surprising amount of information before you ever open a lookup tool.
Take a stock number from our own shelves: NSN 3020-00-277-1119, a groove pulley. Written out with its conventional hyphens, the number splits into two functional pieces and one embedded country code:
The Federal Supply Class answers "what kind of thing is this?" Class 3020 is gears, pulleys, sprockets, and transmission chain. Likewise 5305 is screws, 5306 is bolts, 5331 is o-rings, and 4730 is hose, pipe, and tube fittings. The first two digits of the FSC are the broader Federal Supply Group — group 30 is mechanical power transmission equipment, group 53 is hardware and abrasives. The FSC is a filing label: it is why every serious NSN catalog, ours included, lets you browse by class.
The National Item Identification Number is the unique, permanent identity of the specific item. The hyphens are cosmetic; the NIIN is simply 002771119. This number never changes, even if the item is later refiled under a different FSC — which happens. Experienced buyers treat the NIIN as the durable identity and the FSC as the folder it currently sits in. If two records show different FSCs but the same NIIN, they are the same item.
Those first two NIIN digits identify the national codification bureau that cataloged the item. Codes 00 and 01 mean the United States; other NATO and partner nations each have their own code — 21 is Canada, 99 is the United Kingdom, 27 is Türkiye. So a stock number beginning 5310-99-... was cataloged by the UK bureau, and the item circulates through the shared NATO catalog. Note the NCB code records who cataloged the item, not where any given unit of it was manufactured — those are separate questions.
The cataloging record for 3020-00-277-1119 lists its governing reference: MS20219-4, a military-standard grooved pulley. That MS number is the design; the NSN is the supply-system name for the item built to it. Our catalog page shows both, along with the published unit price — $30.00 at this writing — so the decode and the price check happen in one step. Hold a stock number like this one and want it priced in quantity? Get a price on it below.
NSNs are assigned under the NATO codification system, a shared cataloging framework used by NATO members and dozens of sponsored partner nations. Each participating country runs a national codification bureau; in the United States, the function sits with the Defense Logistics Agency. When an item enters a member nation's supply system, that nation's bureau assigns the NIIN, stamps it with its NCB code, and publishes the record into the shared catalog so every other member can identify the same item the same way.
This is why the "N" in NSN is best read as NATO rather than merely National: a stock number assigned in Ankara or Ottawa is recognizable in a U.S. maintenance manual, and vice versa. It is also why the catalog data behind an NSN — approved item name, characteristics, reference part numbers, and the CAGE codes of the organizations behind those references — is fundamentally public information. The system only works if everyone can read it. For the practical mechanics of actually pulling those records, see our guide on how to look up an NSN.
From the number and its public record you get: the item's supply class and approved item name; the reference part numbers and specifications the item must satisfy; coded physical characteristics (thread size, material, plating, dimensions) for many hardware classes; and which nation cataloged it. For standard hardware this is usually enough to know exactly what you are buying and to cross it to an MS, NAS, or AN callout on a drawing.
An NSN does not tell you who manufactured the specific units in a given box, where they were made, what they cost, whether stock exists anywhere, or whether the item is current — NSNs get superseded and replaced as specs evolve. It also carries no purchasing permission or restriction: an NSN is not a credential, and buying against one requires no contract, CAGE code, or SAM registration. Origin, price, and availability are attributes of a particular seller's particular stock. Our own stock, for the record, is resale hardware with country of origin Türkiye (Turkey), sold with published pricing and source documentation on every order — distinct from our U.S.-produced custom CNC machining line.
These three identifiers get conflated constantly, and untangling them prevents most sourcing mistakes:
A part number — MS20219-4, NAS1523AA4Y, AN960-10, or an OEM catalog number — points at a defined configuration: a drawing or specification plus a dash number selecting size, material, or finish. Part numbers belong to whoever controls the design, which is why one physical item can carry several equivalent part numbers from different standards bodies and manufacturers. Our MS, NAS & AN guide unpacks the military-standard families in detail.
The NSN sits one level of abstraction up. It names the need — "a pulley meeting MS20219-4" — and collects under itself every reference part number that satisfies that need. The mapping runs many-to-many: one NSN may list several interchangeable part numbers, and one specification with many dash sizes generates many NSNs, one per size. That asymmetry is the root cause of most cross-referencing confusion, and our part number lookup exists to run the translation in both directions.
A CAGE (Commercial and Government Entity) code is a five-character ID for a company or government activity — a manufacturer, a distributor, a design authority. In cataloging data, CAGE codes tell you whose part number a reference is. A CAGE code never identifies a part; it identifies who stands behind one. (Ours is 201W8 — it identifies us, not anything on our shelves.)
You do not need to be a defense contractor to meet these numbers. They surface in illustrated parts catalogs for aircraft and legacy vehicles, in maintenance manuals for surplus and ex-military equipment, in export spares packages, and in bills of materials inherited from a government program. A machine shop filling a BOM, an MRO buyer chasing an airframe fastener, an exporter assembling a kit — all of them eventually hold a 13-digit number and need it to become a part in a bag with a price on it.
That last step is where the NSN world has traditionally been unfriendly: RFQ-only distributors, no published prices, quotes sized for primes. We run it differently. Our NSN catalog publishes unit pricing on 8,000-plus stock numbers, browsable by Federal Supply Class or through the part number cross-reference, no login and no contract required. If your part is an o-ring, the companion guide on AS568 o-ring sizes shows how dash numbers, mil-specs, and NSNs interlock in FSC 5331. And if the number you hold turns out to be obsolete or unstocked anywhere, that is a manufacturing problem rather than a distribution one — our custom CNC machining, produced in the U.S. through an ISO 9001:2015 certified production network, covers that case.
The first four digits are the Federal Supply Class (FSC), which categorizes what kind of item it is — 5305 is screws, 3020 is gears and pulleys, 5331 is o-rings. The last nine digits are the National Item Identification Number (NIIN), the unique identity of the specific item. The first two NIIN digits are the NATO codification bureau (NCB) code identifying which country cataloged the item; 00 and 01 mean the United States.
No. A part number identifies a design — a specific configuration defined by a manufacturer or a standard such as MS, NAS, or AN. An NSN identifies an item of supply in the federal catalog. One NSN can reference several interchangeable part numbers, and one specification can generate many NSNs, one per size or configuration. Our cross-reference lookup translates in both directions.
National codification bureaus operating under the NATO codification system. In the United States that function sits with the Defense Logistics Agency, which assigns NSNs when a federal activity registers a recurring need for an item. The first two digits of the NIIN record which country's bureau made the assignment.
No. An NSN is a cataloging identity, not a purchasing restriction. Most NSN-cataloged hardware is standard commercial or mil-spec hardware that anyone can buy — see our guide on buying NSN parts without a government contract. Restrictions, where they exist, come from the nature of the item, not from the presence of a stock number.
Yes. We publish unit pricing on more than 8,000 stock NSN pages in our catalog, searchable by stock number or by MS/NAS/AN/OEM part number, with no login or contract required. Stock hardware is country of origin Türkiye (Turkey) and ships with source documentation.
Why the number doesn't gate the buyer.
Free methods that actually work.
Dash numbers, mil-specs, and FSC 5331.
8,000+ stock numbers with published pricing.
MS, NAS, AN, and OEM cross-reference to NSN.
The military-standard families decoded.
