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This is regarding a validation process in an app. User has downloaded the app, within it they get asked to validate their account through a text message. So they have to check their messages, then enter a code in the app.

When you get a text message sent to the phone number you've already entered to validate your account, I know iCloud uses 4 digits and some uses 6. In my mind 4 is better since it's easier to remember as a chunk than 6 digits. I can easily remember 4432 but harder to remember 443 267 as I jump between the messaging app and the app that validates. I get there's a security issue, but since they've entered the number themselves and received the text the security should be high even with 4 digits.

So, here's the question: which will result in more conversion and does anyone have any articles regarding this?

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As is common when dealing with user experience and security, this is a balancing act.

For the user experience side of it, I'd recommend you read at least the introduction of the Short-term memory article on Wikipedia. Particularly the part that mentions the average capacity is 3-5 items. The upper end of that range might be a sufficient length while still enabling the user to retain it in their short-term memory.

I wouldn't overthink it though—I don't think your conversion should be affected much. It seems unlikely that 2 additional digits in your verification text will cause a significant number of users to abandon everything, and if it does, they likely had additional reasons (i.e. 2 digits is not a likely dealbreaker).

However, the folks at Information Security surely have already provided their recommendations on the security implications at play, and you should seek those out before making your decision.

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  • +1 pretty much how I would have answered the question, but you beat me to it :D – Michael Lai Mar 31 at 23:07
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Most 2FA/MFA apps (Microsoft and Google ones) use 6.

I'm imagining this is because people remember 7 items (+/- 2 https://lawsofux.com/millers-law.html) best.

Six is the same as a phone number which again most people can remember, but on both the apps I mentioned the numbers are split into two sets of three digits.

For convenience, both MS and Google 2FA apps also offer a copy / paste and some mobile apps I've seen seem to be able to read your SMS as it arrives and auto populate fields so this is a consideration.

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  • +1 good point but the 7+/-2 rule is a little bit less straight forward than that... – Michael Lai Mar 31 at 23:08
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    You shouldn't use phone number as generalized rule for how many digits people can remember. For example my phone number has 10 digits but I remember it very well through the repeated use. – locationunknown Apr 1 at 5:11

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