I see that some site forms use the ZIP+4 method of an input, while others just use ZIP. How often do people actually know their ZIP+4? Also, what companies really benefit from this or require it? Just debating the best route for a typical ZIP input.

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    For a Swede like me, this make absolutely no sense. We already have trouble filling out american forms with ZIP and Region. So my answer would be - disregard the ZIP and ZIP+4 and make it more international. If it's online, it could be accessed frome everywhere... Aug 29, 2012 at 19:23
  • @BennySkogberg - Sure, it can be accessed from everywhere, but there are plenty of websites (especially ecommerce sites) that are US only (or Sweden only, or Europe only, or anything like that). Lots of small companies want to sell online but don't want to deal with international shipping. Sorry, but it's true. Aug 29, 2012 at 20:00
  • Didn't even know this exists. If it makes things even more precise in the US, pre-filling the city from the ZIP code could get easier (had a question on this topic ux.stackexchange.com/questions/18553/…) Aug 29, 2012 at 21:15
  • @BennySkogberg - The issue with just making it a open field that handles all standards creates an interface that's harder to use for everyone. What do you label it, how many characters should it allow, & how do you validate it when all these things are different. A better option might be to select country first (or detect and allow override) and place the appropriate fields/labels/etc into the form so it is tailored to the user. Now everyone has a focused form rather than a generic one that says "Mailing Code" for Zip and "Region" for state and I have to guess at what the designer meant. Aug 30, 2012 at 17:45

2 Answers 2


The best route is to allow both. If someone knows it and wants to enter it, it doesn't hurt you at all.

Zip+4 makes it easier to have things sorted when shipped out. For some companies that are shipping lots of items, this can save on shipping costs if they agree to presort the things they are shipping. This typically only really happens with things like magazines or other bulk mailers (ads, catalogs, bills, etc.).

  • I think most just look up the zip+4 code themselves, rather than asking a user to provide it. Most people don't know their own zip+4 code.
    – Brian
    Aug 29, 2012 at 19:27
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    Yes--in general it's good to adhere to the en.wikipedia.org/wiki/Robustness_Principle Aug 29, 2012 at 19:28
  • @Brian - That's fine, but there's no reason you can't allow people to enter it if they know it. Aug 29, 2012 at 19:49
  • +1. I agree, and I think Zip+4 also might help resolve discrepancies if you make a typo in entering your address.
    – devuxer
    Aug 29, 2012 at 20:00

I did a quick search on how to find your Zip+4 code and there seems to be no definite way to find it (and the USPS page for it is broken). Most people seem to revert to looking at their mail to determine it (As per Yahoo answers). However with regards to if the Zip+4 code is really needed, I think this excerpt from Wikipedia explains it best :

In 1983, the U.S. Postal Service began using an expanded ZIP code system that it called ZIP+4, often called "plus-four codes", "add-on codes", or "add ons". A ZIP+4 code uses the basic five-digit code plus four additional digits to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery. But initial attempts to promote universal use of the new format met with public resistance, and today the plus-four code is not required. In general, mail is read by a multiline optical character reader (MLOCR) that almost instantly determines the correct ZIP+4 code from the address

Quoting from this discussion on a forum (Note: the responses are not supported by solid facts)

The zip code gets it to the right post office. With the zip+4, it can be sorted to which letter carrier will deliver it. It can even be sorted into the right order.

If your handwriting is good, the zip+4 won't get your mail there any faster. They have software capable of reading the address instead. But if your handwriting sucks, it has to be shunted off into a pile for an actual human being to read, and they may take a while to get around to it.

When zip codes were first introduced, a lot more stuff was done by hand, and the zip code and zip+4 made a bigger difference. People can sort numbers very quickly, and looking up actual street addresses and figuring out the carrier route was a lot of work.

These days, it's unlikely to take more than a day or two longer without the zip+4. In fact, it's usually the same, with our without the zip+4; the software has gotten amazingly good at reading the addresses without human help.

So considering that most of the web based inputs would printed if used for shipping or delivery notifications, you would not need a Zip+4 code as a normal zip code would suffice

  • usps.com/zip4 - Doesn't look broken to me. And I just searched for "find zip + 4" in google and it came up as the first result. Aug 29, 2012 at 19:48
  • I appreciate your research, but I disagree with your conclusion. As a user, I feel better when sites allow me to enter my Zip+4 because it provides an extra level of error checking. If I made a typo entering the address, the Zip+4 acts as an extra check to make sure my important package arrives at the right place at the right time. Even if I'm completely wrong about this, and the Zip+4 is just ignored, I still feel better as a user, and I think that alone makes it worth having an optional +4 when entering a zip code.
    – devuxer
    Aug 29, 2012 at 19:58

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