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Comment by Domenic_S | Hacker News Reader

ZIPs are almost always fine at a state granularity, and pretty good for cities. If the form is street/city/state/zip, you can autofill State correctly from ZIP 99.9% of the time.

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Do you have data supporting the 99.9%? I suspect that's high; there are buildings within five blocks of where I'm sitting that have a zipcode that crosses a state line, and I don't think it's the only state-crossing zip in my area (Washington, DC, for what it's worth). In any event, even if it's accurate, it doesn't take that much traffic for a .1% error rate to start to add up to non-trivial number of users, especially because that .1% is going to have a noticeably-jarring experience, as opposed to the alternative (collecting all fields) which everyone is already completely accustomed to.

Well, this list shows 293 ZIPs that cross state lines [0], this FAQ says there are ~43,000 ZIPs in use [1] so that puts it closer to 99.3%.

But think about it, if you're taking an address to ship something, getting the ZIP right is what matters. If your package has 45201 on it, it doesn't matter if it says Cincinnati, OH or Cincinnati, KY.

[0] http://web.archive.org/web/20110112211559/http://maps.huge.i...

[1] http://www.carrierroutes.com/ZIPCodes.html

But if you're collecting a billing address (probably more common, since many places bill but don't ship, e.g., for digital goods, but not that many ship without billing), and are trying to expedite filling out the form by guessing the city and state from the zip, I doubt the payment processing would succeed if you got the city or state wrong. All depends on the situation, I guess.

Actually, should work as long as the zip code is right.

AVS[1] doesn't even look at the city/state. It only checks the ZIP (5 or 9 digit) and the numbers in the street address.

[1] http://en.wikipedia.org/wiki/Address_Verification_System

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