Trade Documentation
K Number
Also known as: K-number, mail entry number, 3509 file number
A K number is the control number CBP assigns to a detained mail parcel. It appears in Box 3 of a Notice to Addressee of Arrival of Mail Shipment (CBP Form 3509) and takes the form K26-0000 — the letter, the fiscal year, then a sequence.
It identifies your parcel to the facility holding it. Every message about that parcel, from you or from your broker, should carry it, and a broker cannot do much without it: it is the reference CBP uses to connect an entry to the physical package sitting on a shelf.
It is not a tracking number and will not appear on USPS tracking. Nor is it an entry number — the entry gets its own, assigned when the filing is transmitted. The K number belongs to the detention, not to the clearance.
Give us the K number and the date on the notice and we can start; give us a tracking number and we usually cannot. It is the difference between a parcel CBP can identify on a shelf and a parcel that exists only in the postal system's records.
It is also the field people most often mistype, because it is short, printed small, and looks like a reference nobody would need twice. The year prefix is a fiscal year rather than a calendar one, which is why a notice received in October can carry the following year's digits and look wrong.
Photograph the whole notice rather than transcribing the number. Box 3 and the date of notice in one image saves a round trip, and the remarks in Box 12 usually explain more than the ticked box in Box 11 does.
Where it matters for compliance
- The K number appears in Box 3 of CBP Form 3509 and identifies the shipment to the facility holding it.
- It is not an entry number. The entry number is assigned when the filing is transmitted to CBP.
- It does not appear on USPS tracking, and USPS cannot look it up.
- The date of notice, printed on the same form, is what the response period runs from in practice. The notice states the period; 19 CFR 132.25 sets a reasonable time not exceeding 30 days for quota mail, and the same outside limit is what CBP applies here.
Related terms
Reviewed by a licensed customs broker. Definitions are general information, not legal or customs advice for a particular shipment. Duty rates, tariff programs and agency requirements change frequently.