Which one is correct?
Please fill out / in / up the forms.
1 2
"Out" is most common. "In" is also used.

- A.
Fill out a form

Fill in a form

Complete a form
But not: Fill up a form
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optilang Fill out a form

Fill in a form

Complete a form

But not: Fill up a form
I see an odd thing here. While I would fill out a form, I would fill in the blanks of the form. Complete, of course, is good. I agree that we do not fill up a form.
Definitely DON'T use "fill up the form" if the form is not in a form of a glass or something that would contain matter. FILL OUT is correct line to use.
out of three options (phrases), fill out and fill up are most commonly used for completing forms by providing reuired information. fill in can also be used to complet the forms but mostly used to fill in the blanks, spaces and fields on the forms.

Here are some instances:

fill out

1. To complete (a form, for example) by providing required information: carefully filled out the job application.

fill up
vb (adverb)
1. (tr) to complete (a form, application, etc.)

2. to make or become completely full

fill in - write all the required information onto a form; "fill out this questionnaire, please!"; "make out a form"

To complete (something) by insertion or addition: fill in the blanks.

1. Informal To provide with information that is essential or newly acquired: I wasn't therewould you fill me in?
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Anonymousfill out
1. To complete (a form, for example) by providing required information: carefully filled out the job application.
fill up
vb (adverb)
1. (tr) to complete (a form, application, etc.)
2. to make or become completely full
fill in - write all the required information onto a form; "fill out this questionnaire, please!"; "make out a form"
Hi, Anon.
Do you have a source for this?

Thanks,

- A.
Fill in and fill out are the most widely used phrases to convey that meaning.

The use of fill up in that context is weird to me.

Fill up is mostly used when you want to gas up your vehicle.

I wonder if any native speakers use fill up in every day English for such purpose.

Having looked the corpus of contemporary American English, interestingly, it presents 225 examples for the combination of fill out a form and 6 examples for fill in a form and just one example for fill up a form.

All in all, fill up might be correct but it is not in widespread use.

Cheers
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