Although it is not the only one, the dominant standard that defines how to measure and calculate area in commercial buildings is published by BOMA (Building Owners and Managers Association).
Other standards are occasionally used. For example, In Canada, the government (public Works and Government Services) uses their own standard. You can read it here. Even the term 'BOMA Standard' can be misleading as there is more than one in use! There is a 1980 standard, a 1996 standard and there are different methods for calculating office, store and industrial areas.
As a reference, here's a list of North American building measurement standards:
BOMA Standards:
Gross Areas of A Building: Methods Of Measurement
A new floor measurement standard from BOMA International details procedures for measuring construction gross area and exterior gross area for buildings, and provides unequivocal, direct measure of the physical size of a building. The link is to BOMA's online store where you can purchase this guide.
Standard Methods for Measuring Floor Area in Office Buildings: BOMA 1980 (ANSI/BOMA Z65.1-1980)
This is the 'old' BOMA standard and has been replace by the BOMA 1996 standard. It is still important because area numbers based on this standard are in use on leases today. The standard is no longer available from BOMA.
Standard Methods for Measuring Floor Area in Office Buildings: BOMA 1996 (ANSI/BOMA Z65.1-1996)
This is the 'new' BOMA standard. The main difference between the 1980 and 1996 standards is that the 1996 version includes a method to proportionally allocate building common areas as well as floor common areas to suite rentable areas. There are also so minor differences in how area lines are to be determined. The link is to BOMA's online store where you can purchase this guide.
BOMA / SIOR: Industrial Floor Measurement Standard 2004
This standard, developed by BOMA and SIOR (SOCIETY OF INDUSTRIAL AND OFFICE REALTORS) gives users the two alternative methods of measuring an industrial building. Users can choose one of the two dominant market practices observed in North America - the Exterior Wall Methodology or the Drip Line Methodology - depending on their specific building configuration.
Unified Approach For Measuring Office Space; For Use In Facility And Property Management
This publication reveals and harmonizes the differences between the IFMA floor measurement standard for spatial planning and the BOMA standard which is used for leasing purposes. It includes cutting-edge concepts and definitions that will be used by each organization as the basis for future floor measurement standard development. This publication is the 'Rosetta Stone' if you want to know the differences between measuring a building for space planning and measuring it for leasing purposes. A must for all in the real estate industry and for building and interior designers.
IFMA / ASTM Standard
ASTM E1836 - 08 Standard Practice for Building Floor Area Measurements for Facility Management
This is a standard used for facility management and should not be used for lease negotiations.
It provides a definitive procedure for measuring and classifying floor area in buildings for use in facility management, specifying occupant requirements, space planning, and for strategic facility planning.
PWGSC (Public Works and Government Services Canada) Standard
Generic Lease Documentation Package: Schedule 'B' - Measurement Instructions
The government in Canada leases a great deal of office space. They have their own definitions for area that are published online here.
REBNY Standard
Real Estate Board of New York
This standard is used in New York City, Connecticut and New Jersey. It is not available online, but can be ordered by phone: (212) 532-3100
GWCAR Standard
Greater Washington Commercial Association of Realtors
This standard is used in Washington D.C. It can be ordered by phone: (301) 590-2000
