What you will be doing — and the future that means "probably"
To say what you will be doing at a future moment, combine the future of estar with the gerund — a perfect mirror of English.
Mañana a esta hora estaré volando a Madrid. (Tomorrow at this time I will be flying to Madrid.)
No llames a las tres: estaremos comiendo. (Don't call at three: we will be eating.)
Future of estar: estaré, estarás, estará, estaremos, estaréis, estarán + -ando/-iendo.
The simple future adds -é, -ás, -á, -emos, -éis, -án to the infinitive. The twelve irregular stems (tendr-, har-, dir-, podr-...) apply here too.
El año que viene viajaré más. (Next year I will travel more.)
Tendremos más tiempo en julio. (We will have more time in July.)
Remember Spanish also loves ir a + infinitive for plans — the simple future sounds slightly more formal or distant.
Here is a use English simply doesn't have: Spanish uses the future tense to guess about the present.
¿Dónde estará Juan? (I wonder where Juan is / Where can Juan be?)
Serán las ocho. (It must be about eight o'clock.)
Estará durmiendo. (He is probably sleeping.)
No "probablemente" needed — the future tense alone carries the "probably". This is everywhere in conversation.
Combine both ideas: estará + gerundio = "he is probably ...ing right now".
—¿Por qué no contesta? —Estará conduciendo. (Why isn't she answering? — She's probably driving.)
Los niños estarán jugando en el parque. (The kids are probably playing in the park.)
Context tells you whether estará trabajando means "will be working" (tomorrow) or "is probably working" (right now).
Anchor your futures with the right markers.
mañana a esta hora · el año que viene (next year) · la semana que viene
dentro de dos horas (in two hours) · para entonces (by then)
El lunes que viene a las diez estaré haciendo el examen.
"Que viene" (literally "that comes") is the everyday way to say next: el mes que viene.
Traps for English speakers
These are the errors English speakers make most often.