"Habrá salido" can mean "she has probably left"
The Spanish future perfect can mark completion before a future reference point: Para las ocho habrá salido means "By eight she will have left." In another context, No contesta. Habrá salido ya means "She is not answering. She has probably left already." The surrounding sentence determines whether it is a prediction or a conjecture.
The Real Academia Española's own grammar gives essentially the same example with a different verb: "Me habré confundido al anotar el número", which it glosses as roughly "I must have gotten the number wrong when I wrote it down," not any statement about a future mistake. That gap between the literal English translation and the actual meaning is a translation trap. The Spanish simple future also has a conjectural use; identifying that shared use does not rank either use by frequency.
Building it: haber's own future, plus a participle that never changes
The formation is two pieces. First, conjugate haber in the simple future: habré, habrás, habrá, habremos, habréis, habrán. That stem, habr-, is one of the twelve irregular future stems that swap in for the full infinitive before Spanish verbs add these endings, the same stem-dropping pattern that turns tener into tendré or poder into podré. Second, add the main verb's past participle, and leave it alone: habré hablado, habrás comido, habrá vivido. The participle never changes for gender or number here, no matter who the subject is or what the main verb's participle would do as a plain adjective; habrá terminado is exactly the same whether the subject is él or ella.
Regular participles follow one of two patterns: -ar verbs take -ado (hablado, terminado), -er and -ir verbs take -ido (comido, vivido). Everything below is about the verbs that don't follow either pattern.
Fourteen verbs whose participle isn't just -ado or -ido
These examples of Spanish participles, the ones the Real Academia Española calls participios fuertes or irregulares, replace the expected -ado/-ido ending with a stressed-root form instead: abierto (abrir), absuelto (absolver), cubierto (cubrir), dicho (decir), disuelto (disolver), escrito (escribir), hecho (hacer), muerto (morir), puesto (poner), resuelto (resolver), roto (romper), satisfecho (satisfacer), visto (ver), and vuelto (volver). Every one of these plugs straight into the future perfect the same way a regular participle would: habrá dicho, habremos hecho, habrán vuelto.
A smaller handful of verbs have two competing participles that both stay in real use, one regular and one irregular: imprimir gives both imprimido and impreso, freír gives both freído and frito, and proveer gives both proveído and provisto. Per RAE, either form works in a compound tense (hemos imprimido veinte ejemplares, habían impreso las copias are both attested), but the split isn't random: the DPD entries for imprimir and freír allow both forms in compound tenses and passive constructions. They describe a general preference for impreso and frito, not a rule assigning forms to future versus past time. Reached for as a plain adjective instead of part of a verb phrase, impreso is preferred and frito is used almost exclusively: libros impresos and huevos fritos.
The genuinely future use: marking which of two future events finishes first
The straightforward, future-facing use of this tense is anteriority: one action will be complete before a second, later point that is itself still ahead of now. "Para cuando llegues a casa, ya habré terminado de cocinar la cena" ("By the time you get home, I'll have already finished cooking dinner") sets up two future moments, your arrival and the finished dinner, and places the second one before the first. "Habré leído todo el libro para el lunes" ("I'll have read the whole book by Monday") makes the same point with a deadline in place of an event.
Planning and deadlines provide clear examples of the future-facing use. A para cuando clause or a date can establish the later reference point. The RAE glossary describes both anteriority and conjecture, without ranking their overall frequency in everyday conversation.
The conjecture use: guessing about the past, not the future
The conjecture use points backward. RAE describes the compound future of conjecture as roughly equivalent to a present perfect plus a modal marker of probability: "Habrá estado enfermo" works out to about "Probablemente ha estado enfermo" ("He's probably been sick"). The tense itself carries the hedge; no separate word for "probably" is required, though Spanish speakers can and do add one anyway for emphasis.
More examples read naturally in everyday contexts: "No contesta el teléfono. Habrá salido ya" ("He's not picking up. He must have already left"), or "Habrán llegado tarde por el tráfico" ("They probably arrived late because of the traffic"). The Spanish simple future can express a conjecture about the present. In this compound-future example, it targets something already finished, not something unfolding this very moment. Serán las tres guesses about the present; habrán llegado guesses about the past.
Habrá llegado vs. ha llegado vs. probablemente ha llegado
Three ways of saying roughly the same thing carry different weights of certainty. Ha llegado ("she's arrived") states it as settled fact. Habrá llegado ("she's probably arrived") carries the uncertainty inside the verb form itself, no extra word needed. Probablemente ha llegado says the identical thing by bolting an adverb onto the plain present perfect, with no tense change at all. All three are grammatical, but only the middle one lets the tense itself do the hedging, which is why it's worth learning as its own form, not just as a stylistic variant of the present perfect.
This compound-tense structure, an auxiliary's own tense plus an unchanging participle, isn't unique to Spanish. German Perfekt also combines an auxiliary and a participle: a present-tense haben or sein plus an unchanging Partizip II, though the two languages' compound tenses don't line up use-for-use beyond that shared architecture. A selection of Spanish tenses, including where this compound future sits relative to the simple forms, is laid out on the site's tense overview.