Fill in / Fill out / Fill up
Regional usageFill in or fill out a form; fill up a container or space. In formal Singapore and British-style instructions, fill in or complete the form is clearest.
Fill in
Use when
You enter the required information in a form, field or blank; this is the usual British and formal Singapore construction.
Example
“Please fill in the declaration form and submit it through the portal.”
Fill out
Use when
You complete a form by supplying the required information; this is standard English and is especially common in North American usage.
Example
“Applicants may fill out the online form before their appointment.”
Fill up
Use when
You make a container, space or capacity full.
Example
“Fill up the water tank before the equipment test.”
Quick check
fill up the form, fill in the water tank, treating fill out as universally incorrect
Usage note
Fill out is standard and should not be labelled wrong merely because fill in is more common in British English. Complete the form avoids the regional preference altogether.