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North American health care design has developed as a specialty that must meet high standards, including attention to accessibility, safety, technological advances, operational complexity, constant energy consumption and concern for the medical and clinical issues involved in the delivery of care. 

Planners who provide the basic materials associated with a major hospital construction project must produce a project-briefing document that identifies the services to be provided and how they are expected to operate. Once demographic studies have identified the demand for each service, the planner or programmer must prepare a list of the project’s space requirements. 

Information from previous projects has always been used by planners to predict the space requirements for proposed projects. The data collected on past projects comes from area calculation measurements. An important concern is that different consultants, firms and individuals have used slightly different methods to calculate area allocated to departments and buildings.

Health care models

The two most common models for measuring area in North American health care projects are the American Institute of Architects (AIA) methods of calculating areas and volumes of buildings and the Canadian Standards Association (CSA) area measurement for health care facilities. 

ASHE Resource

Area Calculation for Health Care

The AIA document is useful only for calculating the gross area of a building and does not address departmental calculations. The CSA document goes into somewhat greater detail without conflicting with the shorter AIA document. The CSA document introduces calculations for the area allocated to a floor and calculation of net areas.

These standards have not been sufficient for consistent health care planning accuracy. Neither identifies a method for calculating the gross area associated with a department, which is a critical planning category for the program of space requirements in a hospital, clinic or medical school.

Consultants, designers and other users, therefore, have adopted personal and idiosyncratic methods to build sophisticated calculation models that include net square footage (NSF), departmental gross square footage (DGSF), building gross square footage (BGSF) and net-to-gross factors. Without consistent nomenclature for departmental names and hospital services, programmers have been able to use this kind of data with varying predictive accuracy and success to plan health care projects. The questions of accuracy and consistency continue to be a concern, especially because health care space is so expensive to build.

Neither the AIA nor CSA document addresses measurement and reporting for typical building components such as stairs and elevators, or space dedicated to mechanical or electrical services. The information associated with these and other characteristics can provide additional clarity about the historical allocation of area within a health care project. 

Also excluded from the documents is a consistency associated with departmental measurements and naming conventions. In the past, subsets of building gross calculations were rarely called out separately in publicly available documents. Accurate accounting for subsets of building gross space — including mechanical, electrical, communications, nondepartmental corridors, stairs, vertical transport, miscellaneous structure and exterior wall thickness — allows for a more nuanced understanding of the building elements.

Other standards or guidelines for measuring building areas are important for facility and property managers, especially when dealing with a facility extension, sale, lease or renovation. To date, there are two organizations involved in developing area measurement standards for built facilities:

  • The Building Owners and Managers Association (BOMA) publishes "Office Buildings: Standard Methods of Measurement." These standards are appropriate for office buildings only and cannot be used for area measurements in other types of buildings. BOMA’s methodology as a standard for floor measurement is accepted and approved methodology by the American National Standards Institute.
  • The International Facility Management Association (IFMA) publishes area measurement guidelines in association with the American Society for Testing and Materials (ASTM). These standards are published as the ASTM “Standard Classification for Building Floor Area Measurements for Facility Management.” •
  • Recently, BOMA and IFMA published “Building Floor Area Measurements for Facility Management,” which is a document providing common floor area definitions. 

Calculation of the physical area within health care has long been important for planning and design, construction estimating, engineering analysis, capital budgeting, facility management, space allocation and financial reimbursement. For many years, most such calculations in the United States have relied on "Methods of Calculating Areas and Volumes of Buildings," the two-page AIA D101-1995 document. In Canada, "Area Measurement for Health Care Facilities," the 27-page Z317 11-02 CSA document, is similar, compatible and somewhat more extensive.

Despite common usage of these two standard methods, some leeway has been given for individual interpretation, so different individuals and firms have reported statistics that have not been calculated in the same fashion. For some purposes, this has not been a problem because the differences may have been within acceptable limits. For others, such as when developing a program of space requirements and an accompanying budget for a large hospital construction project or for accurately calculating important reimbursements, greater precision and consistency are needed. 

A new methodology

A research team from Texas A&M University, funded in part by the Academy of Architecture for Health Foundation, documented building and departmental areas for recent hospital projects in North America. With advice from a council of practitioners, the team developed precise methods for making calculations to ensure consistency.

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From left: An open nurse work area off a corridor, post-anesthesia care unit stations in a nondepartmental corridor and scrub sinks in alcoves off a nondepartmental corridor.

Image from ASHE monograph "Area Calculation Method for Health Care"

The team used calculations of the areas for 36 recent hospital projects to test the methodology and identify the ranges of areas constructed for each major hospital department. Caution should be used when employing the data from these calculations. The study examples may not represent the full range of possibilities in recent or future hospital construction. 

To test the methodology, the research team used software for computer-aided drafting to conduct consistent area takeoffs from drawings of 36 completed projects provided by practitioners. An attempt was made to be consistent with prevailing methods, as represented by AIA and CSA. As each project was measured, consistency was maintained with identical sequences of measurement operations. Ambiguous situations were identified and policy was adopted and documented. The resolutions of ambiguity not addressed by the AIA or CSA documents were reviewed by a council of practitioners, including programming consultants and representatives of the firms providing projects to be measured.

The “Area Calculation Method for Health Care” is a process with six major steps: 

  • Measure the entire BGSF of the project, including all area contained by the exterior walls.
  • Measure the component of BGSF attributable to each floor for total gross floor area.
  • Measure the subset of building gross area attributable to the exterior wall thickness.
  • Measure DGSF for each separate department.
  • Measure the NSF for each individual room.
  • Check the accuracy of the measurements. 
  • Various combinations of these calculated results allow for reports of departmental net-to-gross ratios (NSF:DGSF) and departmental gross-to-building gross ratios (DGSF:

BGSF) as well as line items within the overall building gross. 

The line-item calculations allow greater understanding of allocation of services within a health care building. The percentage of building gross area allocated to stairs and elevators, for example, will be different in one-story, low-rise or high-rise configurations. While the area calculations are an important record of what was built, the various ratios and percentages are especially important for projecting space needs in future projects.

The “Area Calculation Method for Health Care” offers a standard compatible with the standards of the AIA and the CSA that goes into greater detail, identifies departmental gross area calculation, calls out subsets of building gross allocations, and offers the potential for higher levels of consistency. 

Scenario-specific calculations

The new methodology covers many scenario-specific considerations to calculate individual health care spaces. A few edited examples of considerations covered in the methodology include:

Departments. Considerations for determining department spaces include:

  • The demising wall, which separates two departments from each other, is sometimes composed of varying thicknesses. Depending on configuration, this calls for the centerlines of each wall to be joined or the perpendicular wall to serve as the joining point for the two centerlines.
  • When the “wet wall” of a toilet room falls along a departmental boundary, the overall thickness should not be split because it includes the plumbing chase. All of the area that includes the plumbing should belong to the department to which the toilet room belongs.
  • Satellite conditions, such as labs, pharmacies and material handling spaces, will be called out separately.
  • The information technology department is defined as “where people work and the main computer frame systems are located.” These areas are separate from “communication-distribution” areas.
  • The space allocated for the storage of folding partitions will be included only in the DGSF, not the NSF.
  • For open telephone and vending areas, 8 feet will be calculated for the adjoining corridor. Anything beyond this measurement will be assigned NSF and included in the administrative and public spaces DGSF.
  • Trash chutes will have their own DGSF and be included in the vertical transport BGSF line item. 
  • Public toilets located throughout the building will be assigned NSF and administrative and public spaces DGSF. 
  • Public waiting areas that are not specifically included in a single department will be classified as administrative and public area spaces. 
  • Large concourse areas will be called out as concourse and be assigned NSF and DGSF. 
  • Cafeterias and bistros are included in the food and nutrition department. 
  • Flex beds between two departments will be included in the department in which the beds are used during the day.
  • Salons and spas will be designated as retail. 
  • Central telemetry monitoring for acute and intensive care unit (ICU) beds will be given to acute care unless used exclusively in the ICU. 
  • The obstetrics department may have spaces that are shared or do not belong to any single department. These spaces will be designated as shared spaces under obstetrics. 
  • Play areas and breastfeeding rooms if near a department will be included in the departmental DGSF. If rooms are located in a public or lobby area, they will be designated as public spaces.

Nurse work areas. Considerations for calculating nurse work areas include:

  • Pneumatic tube stations are included in DGSF, but do not get their own NSF. 
  • Open work areas and chart areas will be assigned NSF and be counted in the departmental gross. The 8-foot minimum requirement for corridors should be respected.
  • Recesses for door swing are part of the nondepartmental corridor BGSF, unless the niche is used as an equipment alcove or crash cart storage. In that case, it is included in the departmental gross, and the equipment area receives NSF. 
  • If the medication supply station is not adjacent to the nurse station, the circulation that is needed to get from the nurse station to the medication supply station is not included in NSF, but is factored into DGSF.
  • If the boundary of the nurse station is not clearly defined on the floor plans by the architect, the NSF boundary will extend to the edges of counters and exterior face of the walls that define the space. The 8-foot minimum requirement for corridors should be respected. If the nurse station is set back from the line of the corridor, the extra square footage between the edge of the nurse station and the corridor line will be given to the nurse station NSF.

Nondepartmental corridors. Factors involved in nondepartmental corridors include:

  • Open patient care areas that are not clearly defined with partitions will be measured as follows: NSF for these spaces will not extend beyond the curtain line that defines the space, and the corners will be squared off. All circulation between patient beds and nurse stations will be designated as DGSF. The 8-foot requirement for corridors should be respected.
  • Scrub and hand-washing sinks located in an alcove off a corridor will be counted in the department NSF. The 8-foot requirement for corridors should be respected.
  • Equipment alcoves located off a corridor in the department will be given NSF. The 8-foot requirement for corridors should be respected.
  • Other scenario-specific calculations covered in the methodology include basic definitions, BGSF line items, furr-outs, circulation areas, exterior covered areas and canopies, columns, connections to other buildings, light wells and atria, shell space, parking, central utility plants, entries, shared spaces and windows.

Accuracy and consistency

This methodology offers accuracy and consistency without varying from compatibility with the most prominent U.S. and Canadian methods for area calculations. 

Every sector of the health care design, construction and facility management field is encouraged to use the proposed calculation method.

D. Kirk Hamilton, FAIA, FACHA, EDAC, is the Julie & Craig Beale Endowed Professor of Health Facility Design and fellow of the Center for Health Systems & Design at Texas A&M University in College Station, where he teaches health care design. He is co-editor of the peer-reviewed Health Environments Research & Design journal. Sarel Lavy is an associate professor in the department of construction science at Texas A&M University, and serves as the associate department head. They can be reached at and, respectively.


About this article 

This feature is one of a series of articles published by Health Facilities Management in partnership with the American College of Healthcare Architects.