UK postcodes and US ZIP codes: how they work and how to write them

Royal Mail’s address file describes a postcode as two parts: the outward code (for example PO1), which identifies the area and district, and the inward code (for example 1AF), which narrows it down to a few addresses. The two parts are separated by a single space: PO1 1AF.

• The outward code is 2–4 characters; the inward code is always a digit followed by two letters.
• Write it in capitals on the last line of the address, on its own line.

A ZIP code is five digits. USPS explains that the first three identify the sectional center facility that sorts the mail. The longer ZIP+4 form is “five digits, a hyphen (or dash), and four digits”, for example 10001-0001, and narrows delivery down further.

• Put the city, the two-letter state abbreviation and the ZIP on the same line: NEW YORK NY 10001.
• Use a PO box or a street address, not both.
• The return address goes in the top left corner, on the same side as the delivery address.

UK addresses end with the post town in capitals and the postcode on the last line; US addresses end with city, state and ZIP on one line.

• US: the USPS ZIP Code lookup finds the ZIP or ZIP+4 for a street address.
• UK: Royal Mail’s Postcode Finder on royalmail.com gives the postcode for an address.

Every postcode on Postmapia has its own page with the post offices, parcel shops and lockers that carry it. UK examples:

• EC1A
• B32
• G1
• M1
• S1
• LS1
• EH1
• L1
• BS1
• CF10

US examples:

• 10007
• 90001
• 11201
• 60601
• 11419
• 77002
• 85003
• 19126
• 78201
• 10021

• UK: dropping the space or writing O for 0 in the inward code (it always starts with a digit).
• US: putting the ZIP on its own line or before the state.
• Both: using an old postcode after a move; check it officially.