Learn Hardcore Portuguese (Portugal): Agora estou totalmente calmo em casa.

estar is used for:
- temporary states or conditions
  - Estou calmo. = I am calm (right now, at this moment)
- locations
  - Estou em casa. = I am at home
ser is used for:
- permanent characteristics
  - Sou calmo. = I am a calm person (by nature)
- identity, origin, professions, etc.

In Agora estou totalmente calmo em casa, the calmness is understood as a current, possibly temporary state, so estar is correct.

Why is there no eu at the beginning? Can I say Eu estou totalmente calmo em casa?

Portuguese often drops subject pronouns because the verb ending already shows who the subject is.

Estou totalmente calmo em casa.
The ending -ou in estou already tells us it’s eu (I).

Using eu is not wrong, just usually unnecessary in neutral statements. You’d use eu:

for emphasis or contrast:
- Eu estou totalmente calmo em casa, mas tu estás nervoso.
to avoid ambiguity if the context isn’t clear.

So both are correct:

Estou totalmente calmo em casa. (more natural, neutral)
Eu estou totalmente calmo em casa. (with a bit of extra emphasis on eu)

Could I change the word order? For example: Agora estou em casa totalmente calmo or Estou totalmente calmo agora em casa?

Yes, Portuguese word order is flexible, but some options sound more natural than others.

All of these are grammatically correct:

Agora estou totalmente calmo em casa. (original; very natural)
Agora estou em casa, totalmente calmo. (slight pause; emphasizes the calmness as an added detail)
Estou totalmente calmo em casa agora. (neutral, also common)
Estou em casa totalmente calmo agora. (understandable; stress feels a bit heavier on totalmente calmo)

The most natural ones in everyday speech would probably be:

Agora estou totalmente calmo em casa.
Agora estou em casa, totalmente calmo.
Estou totalmente calmo em casa agora.

Very unusual orders are technically possible but may sound marked or poetic.

Why is it calmo and not calma? When would it change?

Calmo is an adjective and must agree with the subject in gender and number.

calmo – masculine singular
calma – feminine singular
calmos – masculine (or mixed) plural
calmas – feminine plural

So:

A man: Agora estou totalmente calmo em casa.
A woman: Agora estou totalmente calma em casa.
Group of men or mixed group: Agora estamos totalmente calmos em casa.
Group of women: Agora estamos totalmente calmas em casa.

The sentence as given assumes the speaker is male (or grammatically masculine).

Why is it em casa and not na casa? What’s the difference?

Both exist, but they don’t mean exactly the same thing.

em casa = at home (as an idea of home, your place of residence)
- Estou em casa. = I’m at home.
na casa = in the house / at the house (a specific house, with an article)
- Estou na casa do João. = I’m at João’s house.
- Estou na casa amarela. = I’m in the yellow house.

So Agora estou totalmente calmo em casa means Now I’m totally calm at home, referring to your own home (or “home” in general), not just any physical house.

Can agora go at the end, like Estou totalmente calmo em casa agora? Does that change the meaning?

Yes, agora can move, and the basic meaning stays the same:

Agora estou totalmente calmo em casa.
Estou totalmente calmo em casa agora.

Both mean Now I’m totally calm at home.

Subtle nuance:

At the beginning (Agora): slightly more emphasis on the change in time or situation:
- As if contrasting with before: Before I wasn’t; now I am.
At the end (agora): can sound a bit more like extra information you’re adding at the end, but it’s still very natural.

In everyday speech, both orders are common.

Is totalmente necessary? Can I say muito calmo or completamente calmo instead?

Totalmente is not necessary; it just intensifies calmo.

Possible options (all correct):

Estou calmo em casa. – I’m calm at home.
Estou muito calmo em casa. – I’m very calm at home.
(most common in speech)
Estou completamente calmo em casa. – I’m completely calm at home.
Estou totalmente calmo em casa. – I’m totally/fully calm at home.

Nuances:

muito – very; most frequent, very natural in conversation.
completamente – completely; slightly stronger, still very common.
totalmente – totally; also strong, feels a bit more “absolute”, sometimes a bit more formal or emphatic depending on context.

So yes, you can replace or omit totalmente depending on how strong you want the calmness to sound.

Is calmo the only natural word here, or could I say tranquilo instead?

You can also say tranquilo, and it’s very common:

Agora estou totalmente tranquilo em casa.

In European Portuguese:

calmo – calm, not agitated
tranquilo – calm/relaxed, peaceful

In many everyday contexts, they overlap a lot. A few nuances:

calmo often focuses on not being nervous or agitated.
tranquilo can suggest a relaxed, peaceful mood, sometimes more “chilled”.

Both are natural in this sentence; choice depends on the exact feel you want.

Is this sentence the same in Brazilian Portuguese, or would Brazilians say it differently?

The sentence Agora estou totalmente calmo em casa is perfectly correct and understandable in Brazilian Portuguese as well.

Differences would mostly be in:

Pronunciation (European vs Brazilian accent)
Common choices of intensifiers/adjectives in speech

In Brazil, you might also hear very natural variants like:

Agora estou bem calmo em casa.
Agora estou bem tranquilo em casa.
Agora estou super calmo em casa. (more colloquial)

But grammatically and structurally, your sentence works in both varieties.