(Choosing an answer as a best answer, or at least as a thumbs up, helps those who answer questions.)
feel larger because of the height. But as the other agents explained, sq.ft. is length and width of any room,
never put height into it, you want the living space, surface area only...
As you measure the sq.ft. of a lot size, same there width by depth of the lot....
It is easiest when it is a one floor ranch or a symmetrical 2 story home, and you can actually measure the
outside foundation width and length and then multiply by 2 if a 2 story building, but often it becomes complicated when you have split level homes, or irregular floor plans.
Good luck to you.
Edith YourRealtor4Life! Working always in the very BEST interest of her clients
Your Chicago, Northern Illinois and North Shore Expert
The height of the ceilings should not be considered when measuring square footage.
As a general rule, you will arrive at square footage amounts by measuring the foundation of any above-grade FINISHED living space and then multiplying length by width. If there are multiple foundations, you will do the same for these and then add all the results together. If there are multiple floors, you will do the same things and then add all the results.
You could certainly use the field card as Heidi suggested below, but you might want to check the town's data...it is not always accurate.
Best of luck to you for a speedy sale!