Square feet

If it is a condo, look in the offering document, which will have square footage and also list how it was calculated, which usually is from the interior of the exterior walls, and includes all dead wall space. If it is a co op, get a tape measure.

The only true measure of sqft is that which you can walk on. Pretend you are going to install carpet or hardwood. Go measure it. Every other metric is worthless.

Condo offering plans, on ACRiS, give fairly accurate measurements. The exception seems to be condos built this past decade which often include a % interest in the common elements, along with the actual square footage.

If a co-op, there is no "official" square footage. The offering plan, which is likely to be decades old and is not on ACRIS, will list how many of the corporation's shares "belong" to the apartment you lease from the corporation, but will list the size of the apartments only in terms of rooms and baths. The square footage you see in ads comes from building legend, the ads for previously-sold apartments in the same line, previous appraisals, best guesses, etc. Smart brokers don't list square footage at all, unless they want to get into arguments with every square-footage-obsessed browser who sees the place.

If a condo, the condo declaration will be on ACRIS, but not the offering plan. Doesn't matter, as both describe units the same way. There's a section titled "Dimensions of Units" that describes how the "approximate square footage" of the units was arrived at. There'll be other sections describing the Common Elements, the Residential Common Elements, the Commercial Common Elements, the Residential Limited Common Elements, and so on, as appropriate.

Then there'll be a schedule listing the approximate square footage of each unit and of any balconies or terraces. The approximate square footage does not include the unit's share of the Common Elements. On the same schedule is each unit's Percentage of Common Interest (PCI). The PCI determines how the building's running costs are allocated between the units. The PCI is determined by several factors, explained in detail, only one of which is the units' square footage.

You can always develop your own method, as many posters here have, but if that method is extremely detailed and precise it'd also be time-consuming if applied to every apartment you look at. After looking at enough floor plans you'll get a sense for overall size, key dimensions that matter to you, layout factors, etc.

Thanks much for your comments! Acris does not have a listing for this apartment. I measured it as 18% smaller than the declared size, which, to me, is a misrepresentation (regardless of the number of shares I get in the building).

http://www.streeteasy.com/nyc/sale/358249-coop-205-east-78th-street-upper-east-side-new-york

Whats the SF of 20C?

patient -- although this has been beaten to death in prior threads -- you are completely ignoring the fact that you can knock out interior non-loadbearing walls during a coop renovation and regain that square footage. Therefore "floor area" is NOT square footage of an apartment - it is "area circumscribed by your exterior walls". That's the area you have at your disposal.