fill out or fill in?

[Zoe2008]

Please advise us the serial no. for us to fill out / fill in the form and return to you.

[Soup]

Fill in.

[stuartnz]

Please advise us the serial no. for us to fill out / fill in the form and return to you.

(not a professional teacher) This is one of those choices that mark the differences between variants of English. In NZ English, either one would be perfectly acceptable, but my instinctive response was "fill out". It's little more than an educated guess, but I'd say that "fill out" would have a 60/40 lead over "fill in" in NZ English.

[riverkid]

Please advise us the serial no. for us to fill out / fill in the form and return to you.

fill out the form

fill in the blanks [in the form]

but I agree that both are used.

[stuartnz]

fill out the form

fill in the blanks [in the form]

Perfect! That is exactly how it would be used here. It's always "fill in the blanks", but it would most likely be "fill out the form".

[Daruma]

Is "fill out the blanks" unacceptable?

[Anglika]

Is "fill out the blanks" unacceptable?

Yes - blanks need to be filled in.

[Stilo]

Please advise us the serial no. for us to fill out / fill in the form and return to you.

Hi Zoe,
In this instance ,I would say 'Fill in' as someone is supplying information to enable you to 'fill in' the blanks.
If in any doubt, I would use the word 'complete' ie supply the information to enable me/us to complete the form.

Regards

Stilo