A guess, not a future
The futuro simple does not only describe what happens later. Very often it expresses a guess about the present, something you consider likely without knowing it for sure. English has a small version of this (That'll be the postman, He'll be about thirty), but it usually says must be or probably is. In Spanish the future is the standard way to guess.
Typical uses
Guessing time, age, place and state
The verb agrees with the time as in the present: Son las tres → Serán las tres, but Es la una → Será la una.
Serán las tres.
It must be about three.
Tendrá unos cincuenta años.
He must be about fifty.
Estará enfermo.
He must be ill.
Wondering
In questions the future expresses puzzlement: you wonder about something instead of asking someone. English says I wonder… or who on earth….
¿Dónde estará María?
Where can María be?
¿Quién llamará a estas horas?
Who can be calling at this hour?
¿Por qué no contestará?
Why isn't he answering, I wonder?
Future or guess?
The same form can mean both. Context decides: a time marker such as mañana points to a real future, speculating about right now points to a guess.
One form, two readings
Future
Mañana lloverá.
It will rain tomorrow.
Mañana estará en casa.
Tomorrow he will be at home.
Guess about now
Ahora estará en casa.
He must be at home now.
Tendrá hambre.
He must be hungry.
Alternatives with signal words
Instead of the future you can also express a guess with a signal word. After a lo mejor comes the indicative, after quizás and tal vez either indicative or subjunctive, depending on how sure you are.
A lo mejor está en casa.
Maybe he's at home.
Probablemente tiene cuarenta años.
He's probably forty.
