Correct way to verify square footage?

[Bienes Raices]

I'm making an offer & trying to verify what's in the MLS. The house has a detached garage, so that makes it easy to exclude garage. When I measured the individual rooms in the house and added together, it came up short of what the MLS listed as square footage for the Living area. Am I supposed to be measuring outside the perimeter of the house instead, or is the MLS/appraiser site wrong? Thanks.

[Jon Holdman]

It usually measured on the outside, not inside. If its rectangular with no garage, measure the outside width and depth and multiple to get square footage. If there's a garage, measure that, too, and subtract it off. If the shape is more complex, you'll have to measure each piece of wall, break it into rectangles, compute the size of each one and add or subtract as appropriate.