Will have been/would have been

When do you say “will have ben” instead of “would have been”? I’m not quite sure.

AFAIK, [color=blue]would have been[/color] is a past hypothetical conditional (I’m sure some grammar guru will correct me if that’s not quite the right term). My grandmother’s no longer with us, but if she [color=blue]had been [/color] alive last November, she [color=blue]would have been [/color]104 years old. That’s just the past version of the following: if she [color=blue]were [/color]still alive today, she [color=blue]would be [/color]104.5 years old.

[color=red]Will have been[/color] is future perfect: on October 9th (i.e., next October), my girlfriend and I [color=red]will have been [/color]together 1.5 years. That’s simply the future version of the present perfect, as in “It’s our first anniversary! Can you believe we [color=red]have been[/color] together a whole year?”