How many 6.5in driver equals one 8in driver

I guess it depends on what you mean by “equals.”

alright smart guy, two 6.5" drivers are equal in area to one _____ dia.?

I like math problems. The answer is 9.19" so I would say a 9" woofer rounded down or a 9.25" woofer rounded up would be closest.

A local dealer/builder utilizes single or twin 18” woofers in his custom speaker builds.  These woofers are custom-made for him to be very fast low-excursion drivers.  He claims that his twin 18” models go down to 35 hz.  Truly deep bass is not one of his priorities and he hates subwoofers.

My own speakers don’t go deep either as they are also low-excursion; they utilize twin 12” alnico magnet, paper-coned, pleated paper surround drivers designed to match very fast compression/horn midrange drivers.

Size isn’t all thst matters.

Identical Survace Area ≠ Identical Sonic Performance 

Difference in excursions, materials, execution, etc make a difference.

Size matters when you’re trying to move air.  I have 4 8 inch woofers in each of my Arendal 1723 towers.  Upgraded from the S model with 6.5 inch woofers.  I didn’t use a formula it just made a difference for music and movies.