Fill in, fill out and fill up all start with fill, but the small word changes the job. Fill in and fill out can both mean to complete a form, and fill up means to make something completely full.
What Do They Mean?
You'd expect in and out to be opposites. Not here. Both can mean "complete a form", and nobody has fixed it yet.
Up is different. It means "to the top".
Fill In
Please fill in the blanks with the correct words.
Fill in has three common meanings:
- Complete a form or a gap: fill in the form, fill in the blanks.
- Tell someone what they missed: Sam missed the lesson, so Mia filled him in.
- Do someone's job for a while: Leo filled in for the goalkeeper. (Here the verb has a third word, for.)
Fill Out
Can you fill out this form before you go in?
Fill out has two common meanings:
- Complete a form: fill out an application, fill out a survey.
- Grow bigger and fuller: The puppy is filling out now that he's growing.
Fill Up
The bus filled up at the second stop.
Fill up means to make something completely full, or to become full:
- Make full: Fill up your water bottle before the match.
- Become full: The hall filled up with parents.
- Eat until you are full: Don't fill up on bread before dinner.
Which One for Forms?
Both are fine. People in the United States usually say fill out a form. People in Britain and Australia usually say fill in a form. Some writers use fill in for one box and fill out for the whole form, but that is a habit, not a rule.
One thing is almost the same everywhere: people say fill in the blanks, even where they say fill out a form.
🗣️ Natural Examples
Please fill in the blanks with the correct words.
Can you fill out this form before you go in?
I forgot to fill my water bottle up.
Mia was away, so Leo filled in for her in the school play.
Sam missed the lesson, so Mia filled him in.
The bus filled up at the second stop.
My puppy is filling out now that he's growing.
✂️ Can You Separate It?
Yes, mostly. Fill in, fill out and fill up are all separable. You can put the thing before or after the small word. Only fill in for stays together.
With a noun, both places are fine.
✅ Fill in the form.
✅ Fill the form in.
With it, them, him, her, me, the small word must go after.
✅ Fill it in.
❌ Fill in it.
✅ Fill it out.
❌ Fill out it.
✅ Fill it up.
❌ Fill up it.
Filling someone in works the same way.
✅ Fill me in.
❌ Fill in me.
Fill in for is different. It's a three-word verb, and it stays together.
✅ Leo filled in for the goalkeeper.
❌ Leo filled for the goalkeeper in.
🛠️ How to Use It
- Forms and gaps: fill in / fill out + the form. Please fill in the blanks.
- Telling someone: fill + person + in (+ on the news). Fill me in on what I missed.
- Taking a place: fill in for + person. She filled in for the teacher.
- Full: fill up + container. Fill up the tank. Or with with: The room filled up with balloons.
- Past tense: filled in, filled out, filled up.
⚠️ Common Mistakes
Fill up is not for forms.
❌ Please fill up this form.
✅ Please fill in (or fill out) this form.
Don't put the small word before a pronoun.
❌ I'll fill in it now.
✅ I'll fill it in now.
Fill someone in, not fill in someone.
❌ Can you fill in me on the news?
✅ Can you fill me in on the news?
Fill up, not fill in, for a container.
❌ Fill in the water bottle.
✅ Fill up the water bottle.
💬 In a Conversation
Sam: Do I fill in this form or fill it out?
Mia: Yes.
Sam: That is not an answer.
Mia: It's correct. Both work. But please don't fill it up.
Sam: With what?
Mia: Doodles.
💡 The OddLingo Trick
Think about a glass and a form.
- A form is empty until you fill it in or fill it out. Either small word works, because the goal is "nothing left blank".
- A glass is empty until you fill it up. Only up works, because the goal is "all the way to the top".
In and out are twins when it comes to forms. Up is the one that means "full".
👯 Similar Expressions
- complete: the formal word for finishing a form. Please complete the form.
- stand in for or cover for: other ways to say that someone takes your place.
- The opposite of fill up is often run out: you fill the bottle up, and sooner or later the water runs out.