How Tall is a "Tall" Building??

What is a tall building? How is a building's height measured? What do terms like "supertall" and "mixed-use" actually mean? These and other questions are answered in the CTBUH Height Criteria, the official rules whereby tall buildings are defined and heights are measured.
What is a Tall Building?
There is no absolute definition of what constitutes a “tall building.” It is a building that exhibits some element of “tallness” in one or more of the following categories:
a) Height Relative to Context:                 It is not just about height, but about the context in which it exists. Thus, whereas a 14-story building may not be considered a tall building in a high-rise city such as Chicago or Hong Kong, in a provincial European city or a suburb this may be distinctly taller than the urban norm.
b) Proportion Again, a tall building is not just about height but also about proportion. There are numerous buildings that are not particularly high, but are slender enough to give the appearance of a tall building, especially against low urban backgrounds. Conversely, there are numerous big/large footprint buildings that are quite tall but their size/floor area rules them out as being classed as a tall building.
c) Tall Building Technologies:         If a building contains technologies which may be attributed as being a product of “tall” (e.g., specific vertical transport technologies, structural wind bracing as a product of height, etc.), then this building can be classed as a tall building. Although number of floors is a poor indicator of defining a tall building due to the changing floor to floor height between differing buildings and functions (e.g., office versus residential usage), a building of perhaps 14 or more stories – or more than 50 meters (165 feet) in height – could perhaps be used as a threshold for considering it a “tall building.”
What are Supertall and Megatall Buildings? The CTBUH defines “supertall” as a building over 300 meters (984 feet) in height, and a “megatall” as a building over 600 meters (1,968 feet) in height. As of June 2015 there were 91 supertall and 2 megatall buildings fully completed and occupied globally.
How is the Height of a Tall Building Measured?
The Council on Tall Buildings and Urban Habitat (CTBUH) recognizes tall building height in three categories:
1. Height to Architectural Top: Height is measured from the level1 of the lowest, significant,2 open-air,3 pedestrian4 entrance to the architectural top of the building, including spires, but not including antennae, signage, flag poles or other functional-technical equipment.5 This measurement is the most widely utilized and is employed to define the CTBUH rankings of the "World's Tallest Buildings."
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2. Highest Occupied Floor: Height is measured from the level1 of the lowest, significant,2 open-air,3 pedestrian4 entrance to the finished floor level of the highest occupied6 floor within the building.
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3. Height to Tip: Height is measured from the level1 of the lowest, significant,2 open-air,3 pedestrian4 entrance to the highest point of the building, irrespective of material or function of the highest element (i.e., including antennae, flagpoles, signage, and other functional-technical equipment).
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Footnotes: 1 Level:  finished floor level at threshold of the lowest entrance door. 2 Significant:  the entrance should be predominantly above existing or pre-existing grade and permit access to one or more primary uses in the building via elevators, as opposed to ground-floor retail or other uses that solely relate/connect to the immediately adjacent external environment. Thus, entrances via below-grade sunken plazas or similar are not generally recognized. Also note that access to car park and/or ancillary/support areas are not considered significant entrances. 3 Open-air: the entrance must be located directly off an external space at that level that is open to air. 4 Pedestrian: refers to common building users or occupants and is intended to exclude service, ancillary, or similar areas. 5 Functional-technical equipment: this is intended to recognize that functional-technical equipment is subject to removal/addition/change as per prevalent technologies, as is often seen in tall buildings (e.g., antennae, signage, wind turbines, etc. are periodically added, shortened, lengthened, removed and/or replaced). 6 Highest occupied floor: this is intended to recognize conditioned space which is designed to be safely and legally occupied by residents, workers or other building users on a consistent basis. It does not include service or mechanical areas which experience occasional maintenance access, etc.
Number of Floors The number of floors should include the ground-floor level and be the number of main floors above ground, including any significant mezzanine floors and major mechanical plant floors. Mechanical mezzanines should not be included if they have a significantly smaller floor area than the major floors below. Similarly, mechanical penthouses or plant rooms protruding above the general roof area should not be counted. Note: CTBUH floor counts may differ from published accounts, as it is common in some regions of the world for certain floor levels not to be included (e.g., the level 4, 14, 24, etc. in Hong Kong).
Criteria for Co-joined Buildings A building complex is considered to be a single, co-joined building (as opposed to two separate buildings connected by skybridges or other elements) when 50 percent or more of the total building height is connected. Exceptions to this 50 percent rule can be made in cases where the form of the building creates a coherent arch, creating a singular architectural expression and thus a co-joined building – to be judged by the CTBUH Height Committee.
Building Usage Building vs. Telecommunications Tower: A tall “building” can be classed as such (as opposed to a telecommunications/observation tower) and is eligible for the "Tallest” lists if at least 50 percent of its height is occupied by usable floor area.		To enlarge, click here
Single-function and mixed-use buildings
A single-function tall building is defined as one where 85 percent or more of its total floor area is dedicated to a single use. A mixed-use tall building contains two or more functions (or uses), where each of the functions occupies a significant proportion7 of the tower’s total space. Support areas, such as car parks and mechanical plant space, do not constitute mixed-use functions. Functions are denoted on CTBUH “Tallest” lists in descending order (e.g., “hotel/office” indicates hotel function above office function).		To enlarge, click here
Footnote: 7This “significant proportion” can be judged as 15 percent or greater of either: (1) the total floor area, or (2) the total building height, in terms of number of floors occupied for the function. However, care should be taken in the case of supertall towers. For example a 20-story hotel function as part of a 150-story tower does not comply with the 15 percent rule, though this would clearly constitute mixed-use.
Building Status
Complete (Completion) A building is considered to be "Complete" (and officially added to the CTBUH Tallest Buildings lists) if it fulfills all of the following three criteria: 1) Topped out structurally and architecturally8 2) Fully-clad9 3) Open for business, or at least partially occupiable  Footnotes: 8The topping out architecturally of a building implies that ALL structural and finished architectural elements are in place. 9The omission of a small number of cladding panels to allow fixing of a construction hoist while interior fit-out of some building areas is continuing does not affect the status of “fully clad.” Under Construction (Start of Construction) A building is considered to be "Under Construction" once site clearing has been completed and foundation/piling work has begun. Structurally Topped Out A building is considered to be "Structurally Topped Out" when it is under construction, and the highest primary structural element is in place. Architecturally Topped Out A building is considered to be "Architecturally Topped Out" when it is under construction, and has reached its full height both structurally and architecturally (e.g., including its spires, parapets, etc.). On Hold A building is considered to be "On Hold" when construction works had begun, but work on-site has been halted  indefinitely, however there is still an intent to complete the construction to the original design at a future date. Never Completed A building is considered to be “Never Completed” when construction works had begun, but work on-site was halted and never resumed. The site may go on to accommodate a new building, different to the original design, that may or may not retain the original construction. Proposed A building is considered to be "Proposed" (i.e., a real proposal) when it fulfills all of the following criteria: 1) Has a specific site with ownership interests within the building development team 2) Has a full professional design team progressing the design beyond the conceptual stage 3) Has obtained, or is in the process of obtaining, formal planning consent/legal permission for construction 4) Has a full intention to progress the building to construction and completion Only buildings that have been announced publicly by the client and fulfill all the above criteria are included in the CTBUH "proposed" building listings. The source of the announcement must also be credible. Due to the changing nature of early stage designs and client information restrictions, some height data for proposals may be unconfirmed. Vision A building is considered to be a "Vision" when it either: 1) Is in the early stages of inception and does not yet fulfill the criteria under the “proposal” category, or 2) Was a proposal that never advanced to the construction stages, or 3) Was a theoretical proposition Demolished A building is considered to be "Demolished" after it has been destroyed by controlled end-of-life demolition, fire, natural catastrophe, war, terrorist attack, or through other means intended or unintended.
Structural Material
A steel tall building is defined as one where the main vertical and lateral structural elements and floor systems are constructed from steel. A concrete tall building is defined as one where the main vertical and lateral structural elements and floor systems are constructed from concrete. A composite tall building utilizes a combination of both steel and concrete acting compositely in the main structural elements, thus including a steel building with a concrete core. A mixed-structure tall building is any building that utilizes distinct steel and concrete systems above or below each other. There are two main types of mixed structural systems: a steel/concrete tall building indicates a steel structural system located above a concrete structural system, with the opposite true of a concrete/steel building. Additional Notes: 1) If a tall building is of steel construction with a floor system of concrete planks on steel beams, it is considered a steel tall building. 2) If a tall building is of steel construction with a floor system of a concrete slab on steel beams, it is considered a steel tall building. 3) If a tall building has steel columns plus a floor system of concrete beams, it is considered a composite tall building. This post originally appeared on CTBUH.