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How to Calculate Occupant Load | Easily Explained with Examples

Chapter 10 of the International Building Code (IBC) provides minimum requirements for designing the Means of Egress system in all buildings and structures.

The primary purpose of this is to establish a method of protecting people in buildings from the presence of a fire. The means of egress system for a building or structure provides a way of travel for occupants to escape while avoiding a fire.

A Means of Egress system has 3 parts to it: (1) Exit Access, (2) Exit, (3) Exit Discharge.

A fundamental component to properly designing a means of egress system is being able to correctly determine the design Occupant Load.

In order to determine the means of egress requirements, the number of occupants (design occupant load) are calculated per Section 1004 of the International Building Code.

Design Occupant Load

What is the Design Occupant Load?

The design occupant load is basically the number of people intended to occupy a building or portion of a building at any one time. This load is the number for which the means of egress system is designed to. It is calculated by applying the requirements of Section 1004 which we will get into.

The code limits the number of occupants within a building or space to ensure a sufficient amount of movement is provided for the occupant in case of a fire.

In chapter 10 the occupant load is used to design the means of egress system however other chapters of the code can use this number to determine other required features within a building or structure.

For example the design occupant load is also used to determine the required number of plumbing fixtures, as well as automatic sprinkler systems and fire alarm detection systems.

With that said, let us take a look at the process involved by which the design occupant load is determined.

Calculating Occupant Load

In order to calculate the occupant load within an area of a building correctly, the code establishes two methods:

(1) Areas without Fixed Seating – (Section 1004.1.2)

(2) Areas with Fixed Seating – (Section 1004.4)

Let us take a look at how the occupant load is calculated using each method.

(1) Occupant Load Calculation for Areas Without Fixed Seating

To calculate the occupant load for an area without fixed seating, the code says to compute the area of the room or space at a rate of one occupant per unit of area using the occupant load factor found in Table 1004.1.2.

(Please note that Table 1004.1.2 referenced in this post is from the 2015 International Building Code (IBC). The 2018 IBC has changed the table reference to 1004.5. For simplicity, this post will reference the Table within the 2015 IBC.)

Therefore the occupant load shall not be less than the number determined when dividing the floor area by the occupant load factor assigned to the function of the space.

To learn how to do this, we must first understand the Occupant Load Factor Table (Table 1004.1.2)

Occupant Load Factor – Table 1004.1.2

The occupant load factor is the maximum floor area allowed per occupant as displayed in Table 1004.1.2.

Table 1004.1.2 displays the occupant load factor based on the function or use of a space or room. The occupant load factor is based on function. Note that it is NOT based on the occupancy group classification.

When this table is used, it results in an occupant load for which a room, space and building is designed to. Once the occupant load is established, the means of egress is then designed for at least this number.

Below is Table 1004.1.2 partially shown for simplicity. The entire Table can be found Here.

2015 International Building Code (IBC)

What happens if a function is not listed in this Table?

The code gives the Building Official the authority to establish a function for the space that most nearly resembles a function within the table.

Now before we run into an example of how to calculate the occupant load based off this table, I want to point out two important factors. You will notice that some of the factors are calculated using “Net” and others “Gross”. Let us see what the code defines these as.

NET Floor Area: “The actual occupied area not including unoccupied accessory areas such as corridors, stairways, ramps, toilet rooms, mechanical rooms and closets.”

This is fairly simple to understand. The net floor area is intended to include only the area of the room used for a specific purpose and does not include the areas mentioned in the above definition and therefore is not included in the net floor area.

GROSS Floor Area: “The floor area within the inside perimeter of the exterior walls of the building under consideration, exclusive of vent shafts and courts, without deduction for corridors, stairways, ramps, closets, the thickness of interior walls, columns or other features. The floor area of a building, or portion thereof, not provided with surrounding exterior walls shall be the usable area under the horizontal projection of the roof or floor above. The gross floor area shall not include shafts with no openings or interior courts.”

This can be somewhat harder to understand but for simplicity the gross floor area would include the area of all occupiable and nonoccupiable spaces. Therefore based on the definition, only exterior walls, vent shafts and courts can be deducted from the building area, but other accessory areas such as corridors, stairways, etc… (as mentioned in the definition above) are not deducted.

Now lets look at an example:

Occupant Load Calc without Fixed Seats
Illustration: © Building Code Trainer, 2020

It is important to mention that the code does have an exception where the Building Official has the authority to permit a design occupant load that is less than the actual number calculated by the table for an occupied space.

Although this exception is there it is best to understand its intent. It is understandable that some occupancies may not typically reflect an occupant load that is consistent with the occupant load factors within the Table. However it is not the intent of this exception to reduce code requirements, instead it is an alternative to address limited unique circumstances where the actual occupant load might be less than the calculated load.

Although the Building Official can make this determination, he/she may want to create specific conditions for the space or building prior to approving. If such a change is made, it should be documented and justified, as well as understood that such consideration can impact the use of the building related to egress and other features addressed in the code. As always, when making such a determination, it is best to discuss it with the Authority Having Jurisdiction.

And note in some cases, the Building Official may permit the design occupant load to be greater than what is being calculated. More on this a little later…

(2) Occupant Load Calculation for Areas With Fixed Seating

To calculate the occupant load for an area with fixed seating, the code says that the occupant load shall be determined by the number of fixed seats installed within the area or space. However portions of the space that do not contain fixed seating shall be determined per Table 1004.1.2 as previously explained above and added to the number of fixed seats.

This can be fairly easy to compute in places like auditoriums or stadiums however what about restaurants that contain fixed booths or benches?

The code recognizes this and has 2 factors to use. For fixed seating without dividing arms (such as church pews), the occupant load is calculated at one person for each 18 inches of seat length.

As for fixed booth seating without dividing arms (such as a fixed bench at a fixed table), the occupant load is calculated at one person for each 24 inches of booth seat length measured from the backrest of the booth seat. More space is required per occupant in this scenario to accommodate for people eating.

Now lets look at an example:

Occupant Load Calc with Fixed Seats
Illustration: © Building Code Trainer, 2020

Occupant Load for Outdoor Areas

Outdoor areas such as yards, patios, courts and similar areas for example shall have the occupant load assigned by the Building Official based on how it is expected to be used.

Multiple Occupancies

What happens when a building contains more than one occupany?

Remember what was said earlier in the post? The design occupant load determines the means of egress requirements therefore establishing the correct occupant load is important.

Each portion of a building must be based on the occupancy of that space. Again remember what was said previously. The occupant load factor within Table 1004.1.2 used to determine the occupant load is based on the function or use of a space, NOT on the occupancy group classification.

Therefore if you have several occupancies within a building, be mindful of how these separate occupancies integrate with the means of egress system. In other words, if a means of egress component serves multiple occupancies, such as a corridor for example, it must be designed to meet the most stringent requirements of all the occupancies it serves.

Increased Occupant Load

We previously discussed that the Building Official has the authority to permit an occupant load that is less than the actual number calculated in limited unique circumstances, however the Building Official also has the authority to permit a greater occupant load than calculated.

There are some conditions that the code establishes for such a case. The Building Official is permitted to approve an increased occupant load provided that all other requirements of the code are met based on the modified number. Also whatever the increase is, it shall not exceed a factor of one occupant per 7 square feet of occupiable floor area to allow for sufficient movement for the occupants in an actual fire situation.

In addition to these conditions, the Building Official may require an aisle, seating, or fixed equipment diagram to show the established increase in occupant load. Also the Building Official may require this diagram to be posted.

Posting the Occupant Load | Occupant Load Signage

Rooms or spaces used for assembly purposes require the occupant load to be posted in a visible location near the main exit or exit doorway from the room or space. This is a way to ensure the maximum approved occupant load will not be exceeded by the building users.

The sign must also be permanent and legible show that the code official, such as the Building Official or Fire Marshall, can reference it during a periodic inspection.

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* Reference Source – 2015 International Building Code – [Buy on Amazon]

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