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Inspired by Why do credit card forms ask for Visa, Mastercard, etc.? -

Why is it standard for web sites in the United States to ask for ZIP code and state?

  • Asking for the state is redundant. The ZIP code already provides an unambiguous identification of the address.

  • Asking for the state is inconvenient. It's usually a drop-down with 50 entries, and is often not caught by the browser's autofill feature (at least not in Chrome).

  • True, the state is basic required information when shipping with the USPS. Also, it may be relevant for tax and other regulations.

    However, it would be easy to fill in automatically based on the ZIP code the user entered, e.g. using USPS' API, or plain old Google Maps.

Why does everybody and their dog still force you to enter a state?

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4  
Although I don't live in the US, I believe that state is required to apply the correct level of sales tax(?) Obviously, if a ZIP code straddles two states, you couldn't determine the correct tax. –  Brendon Jan 31 at 8:05
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@Brendon that's a good point - I didn't think of the case where a ZIP code could span across two states. –  Pekka 웃 Jan 31 at 13:56
    
Aren't there addresses without zip codes, too? –  MSalters Jan 31 at 15:09
    
@MSalters not in the United States - at least as far as my research shows –  Pekka 웃 Jan 31 at 20:12
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@PatomaS: There are entire countries without post codes (e.g. Ireland). –  MSalters Feb 5 at 9:25

6 Answers 6

ZIP codes are not unambiguously assigned to states. ZIP codes are assigned strictly on the basis of how efficiently the mail can be delivered. Some codes span multiple states, often when a small border town straddles two states. While a parcel with the correct ZIP code will be sent to the proper address, the customer wants their state to be recognized.

See http://www.unitedstateszipcodes.org/ for more details.

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Plus, many people don't know the ZIP code. They may not be entering their own address. Even if they know the rest of the address, the ZIP code can be appended. We just assume ZIP codes are localized to a particular area. –  Matt Feb 20 at 0:24

Asking for the state is redundant

Exactly, it's redundant:

redundant (adj.) Duplicating or able to duplicate the function of another component of a system, providing back-up in the event the other component fails.

If you get the zip-code wrong, the letter can still be delivered if the street and state are correct.

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After going through the trouble of reinventing the wheel by writing a 7K+ line T-SQL script that creates the ISO 3166-1 and ISO 3166-2 Country and CountrySubdivison tables with their respective data, what I can say is that companies don't like to pay for ISO databases, lol.

Not only that, but I've yet to work at a place that had a database that even stored zipcodes with their related states in a table. Country table, Country Subdivision table, US State table, Address table, yes; Zipcode table, no.

So to answer your question, I think the simplest answer is that those companies are just plain cheap and lazy. It's easier for them to create a small US States table with the state's name and abbreviation for use in drop downs, rather than doing things the correct, more efficient, and better user experience way by using an ISO database or some other service they could subscribe to that would provide up-to-date data.

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It's a cross-check on the zip code, to look up the city+state and see if they correspond to the zip code and vice versa. If they match, great, the user entered it wrong. If they don't match, the user got something wrong.. but you don't know which one.

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Even though there are some zip codes that span across states (such as 65733), I doubt there is a single instance where address+ZIP are not sufficient to uniquely determine the state. Unless of course someone created such address specifically with the intent to cause a confusion...

A better approach from UX perspective is to have the first option in "state" dropdown to be auto-detect, letting majority of the users to skip entering this information.

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From a UX perspective - absolutely. From a development / performance issue - perhaps not. –  Mayo Sep 22 at 13:30

I don't know how pervasive it is in the US, but in Canada, it's not uncommon for people to not know their postal code. They pretty much always know their province though.

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