I have a textbox where the user can enter a number. In the past, an empty value would indicate that they didn't know the value. Now, the client wants to add an extra step where the user needs to specify that they don't know the value, otherwise the textbox should be marked as mandatory. They think this would increase the chance of people filling in the textbox.

I'm trying to limit space to make sure the form isn't too large. I'm looking for a bunch of different ideas. I thought of adding a checkbox beside the textbox (I'm using Bootstrap), which would set the textbox as disabled.

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But it doesn't look intuitive. I don't want a bunch of people calling the helpdesk.

  • 3
    There are a couple of good answers here that both mention the one thing I was going to point out: The language around the check box. There are several ways to deal with the checkbox itself, as others have outlined below, but your text needs to be a little more human/friendly to help the user understand what's expected from them. Commented Aug 12, 2021 at 7:59
  • Why is this a value which the user doesn't know? What are they filling out exactly and what is the relative importance of this number?
    – MonkeyZeus
    Commented Aug 12, 2021 at 12:23
  • 1
    The spelling error of "Unknown" is killing me.. see, I couldn't do it myself even to describe the misspelling!
    – slebetman
    Commented Aug 13, 2021 at 8:04

3 Answers 3


In a previous project we went for something just like you thought of, just a bit more visually clean:

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Why it's good:

  • Law of proximity (you clearly see the connection of form field & checkbox)
  • Clear visual feedback (change happens upon checking the box, grayed out form field is clearly disabled now)

Now I don't know if this is enough to increase the amount of people filling it in. Maybe there could be a little info button like (i) that opens a popup, which explains the benefit of filling this in.
In our customer's case, users would only get warranty if they filled this in, otherwise it's only for personal tracking. So the value was already very clearly given.

  • 4
    +1. It's also worth considering if what should happen in the system when the user checks the box and already has a number filled — presumably you're discarding that input, maybe you want to prompt the user so they're clear the number won't be counted. If the number is always required, just not now (you're letting them proceed filling the form but you're going to want it before they can complete their goal), it may also be good to highlight that to the user up-front (e.g. using language like "Skip for now" or "I will fill later") Commented Aug 12, 2021 at 9:03
  • This will work well with all the accessibility requirement that I have.
    – the_lotus
    Commented Aug 12, 2021 at 13:17
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    Minor: I think you should not gray the title of the field ("Serial number") when graying the rest...
    – Pablo H
    Commented Aug 12, 2021 at 14:19
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    @PabloH : Indeed. Having the checkbox checked, when I start typing into the field, I'd expect the checkbox to clear itself automatically.
    – vsz
    Commented Aug 13, 2021 at 17:18

if the need is to be specific, make them choose.

Instead of a checkbox, try radio buttons. These are used for mutually exclusive choices.

If the default is that most users know the value, you can have that preselected, so there's nothing else to do but enter a number.

enter image description here

The example above is just using some sample text, but this way there is no guessing about what the checkbox would do.

  • 1
    Although it works fine when users enter a number, the visual feedback for radiobutton selected, but no value in the textfield is already giving me headaches. Which is exactly OP's most important case. Commented Aug 12, 2021 at 8:05

The field should use whatever pattern you've established for a required field since there is a required answer (i.e. you know the number and enter or you don't know the number). I'd go with a checkbox that makes the field inactive, or a toggle. I don't know if I'd put the checkbox in the field as that's a pretty uncommon pattern.

I can't really comprehend why the client thinks this will make more people enter a value. If I saw this I would assume that information is something that is needed, but not necessarily now.

  • It did work for us, at least once, to make a field mandatory but with a None option. A lot of users go with the only fill in what turns red approach, so if they can leave a field blank and not even think about it they will, but once they have to think about it -- even to check None -- they are more likely to fill in the actual data. (Our users are internal, so it is actually their job to be filling in this data correctly, but...) Commented Aug 13, 2021 at 21:19

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