In the ending of Inception (2010), there is some doubt that Cobb is in the real world. But if he is not in the real world then at which level of the dream he is in?
Starting at the presumed 'real world' as the first level of the story, I think the final scene it's the fifth level (limbo) because at the fourth level they save Fischer, but I need clarification.
None of the different means you can use to guess whether Cobb is in a dream or not at the end of the movie are completely reliable.
The spinning top has huge issues - the fact that it was Mal's totem, that it appears to act in a simplistic way that perhaps other people could dream, hence invalidating its usefulness - and of course the fact that we are never even shown what the top does at the end of the movie.
I am unconvinced by how completely watertight the wedding ring theory (referenced in @Eoin's answer), as it seems possible for Cobb to dream the wedding ring when he thinks he is in a dream and not otherwise. Yes it is an interesting theory, but whilst it is certainly a deliberate action from the writers/director - the meaning is not completely clear. Cobb clearly seems to treat the spinning top as his totem (flawed or not), as he uses it when flustered by his dream in Mombassa. So yes, it could be a clue, it could even be his true or second totem - but it is still not completely reliable in my opinion.
So is Cobb in a dream at the end? I don't think we can tell. Unlike the other answer, I like the ambiguity in the movie. If pushed to give an answer I would say that the fact that he is such an experienced shared dreamer, it seems likely that he would eventually work out if he is in a dream or not - so I guess he is likely to be in the real world, and the wedding ring therefore might be a true clue to that.
So to answer the original question, if this is not the real world, what level is this? Well if the top level of the story is not the real world, and is in fact another dream, then 'who knows' is the only real answer. The question has no real meaning as there are possibly untold levels of dreaming we have no knowledge of.
Although the ending of inception was meant to be debatable and ultimately was left up to the audience, there are several factors indicating that Cobb is indeed dreaming in the end of the film. In fact if you read my whole theory you shall understand that Cobb is actually on the third dream level and you will understand why.
Reason 1. Cobb told Saito and Ariadne how his totem works therefore he can never truly tell if he is inside one of their dreams. In the end of the movie the totem DID fall down, but it was a dream. (This is because the totem was corrupted; the whole purpose of a totem is that it's functions are known only to you and no one else. When you are in someone else's dream and they know how your totem works they can use it against you, therefore making it impossible to tell if you are dreaming.)
Reason 2. When Cobb met his children at the end of the film after he spun the top he asked his son what he was building and his son replied "A house on a cliff." This is A HUGE factor because we all know that throughout the movie elements of the subconscious work their way through to the dream; ie, Mal who was a projection. The relevance of the "house on a cliff" is that Saito lived in a house on a cliff in Limbo. If you remember when Cobb washed up on shore to the beach in Limbo he looked up and saw a house on a cliff. The son saying this is not a coincidence because there is no way he would have known that Saito lived in a house on a cliff. Therefore Cobb is inside Saito's dream hence the elements of the subconscious working its way to the surface.
Reason 3. The reason the screen to suddenly cut to black and possibly the reason for the top to take so long to fall was due to the end of the third dream. The dream was collapsing and it in fact ended. This is because of the song Je ne regrette rien. It was played in the dreams to indicate when the dream was ending. When the song ended so did the dream. The song was 2 minutes and 28 seconds long. The movie Inception was EXACTLY 2 hours and 28 minutes to the exact second (check if you want). This means that the ending was a dream but it was the end of the dream.
Reason 4. Why did Saito and Cobb end up in the third dream level? Well everytime someone dies in a dream within a dream they awake up one dream level. When Cobb and Saito shot themselves they died in Limbo and awoke in the third dream level which is one level higher than Limbo. However why did they wake up on the plane? Simple. It is not the plane and they did not "WAKE UP". Going back, you must keep in mind that Cobb and Saito missed the series of kicks that would allow them to leave the dream. So with everyone gone and Cobb and Saito still in Limbo, only EMPTY DREAM SPACE REMAINS, waiting for the first mind to make contact with it. Then Saito killed himself followed by Cobb. Saito was first and so his subconscious populated and inhabited the third dream level. The reason he "wakes up on a plane" is because he believed himself to be on a plane and so his subconscious built the plane from his memory along with the projections of his team. When Cobb entered right after he was entering a dream that belonged to Saito and therefore because it looked like reality he couldn't tell if it was a dream or not. Also his totem is "broken" so there is no telling whether he is dreaming or not.
Reason 5. After Saito and Cobb leave the plane and enter the airport they never once talk to the other "members of their team". This 1 dimensional characteristic was commonly used before from the projections in the various dream levels.
There is a wedding ring theory which leads to the conclusion that he is indeed back in the real world. A quick Google will turn up a number of explanations of that theory.
On the otherhand, Christopher Nolan refuses to answer the question, indeed here he seems to go so far as to insist there is no answer. Personally I think that's nonsense: either the wedding ring theory is true, or the person in charge of consistency screwed up in a way that happened to perfectly support the theory.
The other thing I believe, is that not having a clear answer to the question is not some wonderful way of getting people to question the nature of reality, rather it's just a cheap trick to avoid a clichéd ending, which is done in such an obvious way that it only leaves people annoyed.
I know that sounds harsh, I did truly enjoy the film, but I think the ambiguity at the end didn't improve the message of the film.
I know that we can all agree that the ending is up in the air, but were he dreaming in the end, what dream level he is in, can be one of at least two possibilities.
1st level: Although we are introduced to the spinning totem early on in the film and further enlightened to its significance throughout, we only see it spin out once, in the hotel room before Cobb and Arthur catch the helicopter. The next time Cobb spins the top, in the restroom in Mombasa, it doesn't fall, Cobb accidentally knocks it off of the sink. So some think from his sleep in Yusef's basement on is all dream and the entire scheme never takes place.
Limbo: Four levels down, in limbo, Cobb confronts his projection of Mal. This limbo is the limbo of the shared dreamers' subconscious. It is filled with what Cobb and Mal had left behind. Cobb describes this to Ariadne that he and Mal washed up on the shores of their own subconscious. When he washes upon the shores at the end, he is washing up on the shores of his and Saido's subconscious. If he and Saido kill themselves and it doesn't wake them up, but keeps them in Limbo, it is Cobb returning to his own Limbo, the otherwise unconstructed dream space that he's filled with his and Mal's house and projections of his children as he remembers them. He also brings in this image of the plane and Saido is there as well, and he follows his own path still within this vast detailed mixed-reality of Cobb's subconscious.
There is no real conclusion, but those are two possible levels that the conclusion may leave him.
Mal was right!
I know I am coming at this late:
According to Cob, he and Mal spent roughly 50 years building that city by the ocean. Assuming they were using a strong sedative that give you a 1:20 time dilation factor, that would mean the next level up would be roughly 2.5 years, and then 45 days, then 2.28 days, then about 3 hours. This would imply that they were five levels down. Given the variables and unknowns perhaps we could assume they were somewhere between 3 and 5 levels down. If they were 1 level down they would have been asleep on the floor of their house for years. Therefore, they were more than 1 level down.
When Cob & Mal laid their heads down on the train tracks to leave their huge city, they went up one level and awoke in their "house." At this point they are either two or three levels down. For sure they are were not in reality.
Therefore, when Mal committed suicide, THEY WERE NOT IN REALITY. They were at least one level down. As a result, Mal is one or more levels up waiting for Cob. It is possible she is waiting in reality, but more likely she is still asleep at level one. In any case, soon....very soon she will disconnect the machine and the flow of (real or imagined) sedative and he will wake up.
The good news here is that even though Cob gave up caring about dream vs reality in order to be with his kids, he will eventually wake up to find Mal & his kids on the next level up.
The bad news here is that since he is so lost (and using Mal's totem), after he wakes up and discovers Mal is not dead, will he be able to stop from killing himself and Mal again and again...
Could there be a possibility that Cobb's mentor, Mal's father, purposely trapped him within the maze as revenge for his part in Mal's suicide?
I can only offer my own personal observations on this theory, and if it's true then maybe the story isn't about one man incepting another, but about his own personal inception. In thematics, it might make sense: just as Mal was made to believe that her reality was an illusion, Cobb was made to believe that the illusion was his reality.
It could also be argued that he is still dreaming because of the thematic space, in which Christopher Nolan occupies Cobb in. His character is usually associated with large castles or buildings that overlook a lonely sea, but in the 'real world' he is mostly associated with vast, winding, decaying metropolises. So if the end of the film is reality, then how is his setting that of a rural, seaside home.
Answering directly, I believe that Cobb isn't in any specific level of dream, he's simply lost in the vast maze of his own consciousness. He's simply trapped within his own world, lost to reality after what he reaches what he thinks is his goal.
But it could also be argued that he was in the real world because of the children's ages. If you stick around for the credits, you can see that two sets of children were used to act out James and Phillipa. But it's simply a technicality, the film achieves a lot more depth if Cobb is stuck in the dream. If he had been 'incepted'.
There is an important clue in the final scene that is easy to miss, and David Kyle Johnson, author of Inception and Philosophy: Because It’s Never Just a Dream, believes that Christopher Nolan intentionally misdirects the audience’s attention in the final scene.
In short, Johnson’s view is the whole movie is a dream, Saito’s in particular.
Johnson gave a Google tech talk on the subject, and his slides (DOI:10.13140/RG.2.1.4290.5683) are available elsewhere. Johnson and his collaborators sank considerable time and thought into their analysis of Inception and its implications. My summary of Johnson’s justification for his position is below.
To start, subconscious elements work their way through dreams, e.g., the train barreling down the street in the rain dream, the random string of numbers in the hostage scene appears repeatedly in later scenes: the safe combination, the fake phone number, hotel room numbers. At the beginning of the film, Saito dreams of a mansion on a cliff.
Picking up on a big clue in the final scene may require turning on the closed-caption subtitles. Cobb asks his children what they’ve been doing, and they reply that they were building a house on a cliff.
Nolan leaves many clues that the film is a dream.
When someone commits suicide in limbo, the subject goes one layer up. For Saito, the next layer up was the snow fortress. But everyone was gone, so he filled the empty dream space with his own expectations, namely the airplane scene. Eames pickpockets the passport in the airplane without touching Robert, the way he did in other dreams.
But then consider what happened to Cobb and Mal who were experimenting with multi-level dreams after being struck by the train in Limbo. They also would have gone merely one level up, but still within a dream.
This interpretation makes a much better film. Consider:
If the entire film is a dream, these are not gaffes but strengths. The characters are flat because they’re projections, not because the writing is bad. The jumping around is not bad editing but because that’s how dreams run. Saito’s well-timed appearance and corny line become subtle clues that Cobb is dreaming.
For completeness, at least two clues suggest the move is not a dream.
The answer above repurposes content from an answer of mine on scifi.SE.
We might have finally gotten a direct answer that solves the ending of this movie:
After more than eight years of debate over the ending of Inception, Michael Caine has offered some clarity on the movie’s mysterious final moments.
While speaking at a Film 4 Summer Screen showing of Christopher Nolan’s acclaimed thriller, Caine — who played the father of Leonardo DiCaprio’s Dom Cobb — shared some insider knowledge on whether Cobb was awake or dreaming in Inception‘s closing sequence.
“When I got the script of Inception, I was a bit puzzled by it,” Caine said. “And I said to [Nolan], ‘I don’t understand where the dream is.’ I said, ‘When is it the dream and when is it reality?’ He said, ‘Well, when you’re in the scene, it’s reality.’ So get that — if I’m in it, it’s reality. If I’m not in it, it’s a dream.”
A totem is an item, in the real world, with a secret, unique property. Its purpose is that, when somebody targets you for extraction and tries to counterfeit that object in the process of counterfeiting the rest of the world, you will notice they have missed that detail and you are able to know that the world is not real. This idea was invented by Mol.
The first time this concept is introduced in the movie is when Saito rubs his cheek on the rug and instantly knows that, despite appearances, he is not really lying on his carpet in his apartment.
As far as the audience knows, the first time Cobb ever actually touches Mol's totem it is really just a projection in the dream realm.
Mol's totem is in the shape of a top which the audience is led to believe, through Cobb, is perfectly average in reality but will spin forever in a dream. This totem both:
One logical conclusion of all this is: If somebody else believes, incorrectly, that they know the special property of your totem and they try to use it, they are in danger of fooling themselves into a trap. The same way building familiar places can make you forget that you are in a dream.
This tells me that Cobb is actually incorrect about the special property of the totem he took from Mol, and it topples, not because Cobb is in the real world, but because he believes he is in the real world and has misunderstood how the totem works.
Considering all of this, Mol's totem was still reliable to her and it allowed her to accurately know that they were both still trapped in Cobb's dream.
Inception either ended at Limbo, in Reality, or Level 1, but I think the important thing to factor in is that, in the end, Cobb sees their faces and decides to hug his children which he never could while bringing them in from his subconscious as projections in other dreams. So I think it may be a reality.
Another thing is that he does not wait for the top to stop or keep on spinning, meaning he doesn't care which shows that he has given up all intentions of going back in(dreaming) with someone else which indicates to me that he is in limbo or he is in reality.
Two points for reality so I'm assuming reality.
Also, if Nolan had decided to extend the movie further and someone says he himself uses "the device" then we would have found out for sure whether he was in limbo or in reality because in limbo you can't go deeper. So if he can use the device dreams then he in reality, else...
