It works for post office boxes, ZIP+4 is sufficient. The zip code is for that post office and the +4 is the box number.
What I’ve seen most commonly is that a Zip+4 encompasses one block on one side of the street. For more dense locations (like apartments or office complexes), it is typically even a smaller area.
I put my nine-digit zipcode into this URL given by hende in post #5:
There are only twelve apartments in my building. That website returns just four of those apartments. They are the four apartments on my floor of the building.
Old thread, yes, I know…
I just checked my neighbor’s address and mine (on the USPS website). We both have the same zip+4. That’s at least one counter example to one zip+4=one building.
The wiki page on the subject suggests that it essentially just breaks zip codes down into smaller areas, not necessarily individual buildings.
ETA, and I have to admit I’m pretty surprised that this website, melissa . com, that I’ve never heard of and appears to be a throwaway site like all the other address look up pages, is still up and running 7 years later.
The Canadian Postal code system was developed later than the zip code, and used the lessons from it. First it uses alternating letters and numbers, so it’s easy to tell if a pair of characters are transposed (and less likely to transpose). It was designed so a large building or one side of a block (or blocks) had a single code. machines could sort to the code level. Basically the final hand-sort would be fairly trivial.
As I understand it. Plus-4 zips were developed a bit after this, based on the same idea; but they were already numbers-only for the main zip code, so why change? But again, it allows machine sorting to the point where the final hand-sort is easy; so it reduces the area to a single large building (or part of a large building, I see) or a block of a street. I assume the Canadian concept of one side of a block for a code was because postmen would go up one side of the street, down the other more efficiently than crisscrossing. Not sure if the plus-4 copies this idea?
I entered my house zip+4 into ZIP Code, City, Phone, Area Code Lookup Tool | Melissa Lookups, the site mentioned upthread, and it returned all houses on my block and on my side of the street.
My 9-digit zip translates to the 18 houses on both sides of the street at the end of my cul-de-sac. The next 9-digit zip in the sequence attaches to the next 9 houses down, again on both sides of the street.
But the numbers don’t really follow a geographical sequence. The 9 digits before mine are for houses two streets over, and the 9 digits after my neighbors attach to a single unit in a larger condominium a couple of streets in the other direction.
Mine was limited to 3 houses next to each other on the same side of the street.
I never knew how to find the +4 code, until I found that website. Sometimes my mail is addressed to zip+4 but I had no way of knowing if the sender was right. Did the post office ever inform people of this? Or was it just an internal code until recently? It was only probably 10 years ago that I first learned about them, and like I said, had no way of verifying mine. By the way, mine is apparently limited to my (linear) block. 4 houses total.
In certainly cases, a Zip+4 can indicate a particular individual, usually if they got a lot of mail.
Years ago, I got a NYC Zip+4 directory and saw that Isaac Asimov and Walter Cronkite had their own. It also listed their addresses, including their apartment. I suspect the USPS doesn’t do that any more.
Not exactly. I have a PO box with a number 6155, my 9 digit Zip ends 0155.
I guess more accurate to say the box number is part of the +4. my landlord’s box is 83 and the +4 is 0083.
The USPS zip code finder is probably most reliable way to find the +4 part, as well as verifying that an address is correct. For example, if you type in 123 Main St, it might correct it to 123 E Main St or 123 Main Street Ct.
https://tools.usps.com/zip-code-lookup.htm?byaddress
No, ZIP+4 values change (A) at an intersection, or (B) whenever the hundreds digit changes, or (C) different sides of street.
For single family residences, 301 Main Street through 399 Main Street (odd numbers) have the same zip+4. This presumes there is no intersection between 301 and 399 Main Street.
Apartment buildings may have a different zip+4 for different floors.
Sometimes, your mail may have imprinted an 11 digit code. The last two digits are the last two digits of your house number.
Since this is Straight Dope, the proper answer is apparently part of the PO box number is part of the +4
This is called Delivery Point Barcode. While all 11 digits (plus a check digit) are rarely printed in Arabic numerals, they are encoded in most postal barcodes. Basically, they provide a distinct, unique number for each delivery dropoff address.
The “last two digits of your house number” is generally true, but there are exceptions for large buildings, quantity mail recipients, etc.
This is good to know. I’ve bookmarked the site.
Out of curiosity, how did the Post Office inform people of their new +4 code in 1983 when it began?
Not surprising at all. Consider that Social Security numbers are 9 digits, too, and that’s enough for every person in the country to have their own. And there are fewer residences than people (and probably fewer addresses, even counting non-residential addresses).
I’ve seen a lot of cases where the 2 numbers for a PO box match the last 2 digits for the Zip code. For example, my PO box number is XXYYY with a Zip code of ZZZXX-0YYY. And I’ve seen cases where the PO box is XYYY and the Zip is ZZZZX-0YYY. Maybe that’s the case with yours (Zip=XXXX6-0155?). My assumption would be that the zero is used to standardize things for post offices that have more than 999 PO boxes.