Calculating the IBC stair width can be complicated because it is determined by the building’s occupant load, so we have put together this simplified version of the IBC codes.

For stairs serving one floor, multiply the floor’s occupant load by 0.3. For stairs serving multiple floors, multiply the occupant load of the floor that serves the most people by 0.3. These calculations give you the required COMBINED width of all exit stairs serving that floor. However, the minimum clear width between handrails is 44″. If the stairway serves a floor with an occupant load of less than 50, the minimum clear width is reduced to 36″. 

IF there is a compliant sprinkler system installed throughout the building, multiply the occupant loads by 0.2, not 0.3. The minimum widths stated above still apply.

Here’s where it gets tricky.

There is a minimum number of exits that is based on the occupant load per story:

Occupant Load Per StoryMinimum Number of Exits or Access to Exits from Story
1-5002
501-1,0003
More than 1,0004

The combined width calculation performed above must be distributed between the minimum number of required exits while still meeting the minimum width requirement.

AND if one exit stairway is “lost” (i.e. occupants cannot access it due to a fire) the remaining stairway width(s) cannot be reduced to less than half the required combined width. This applies typically when there are only two stairways because if you lose one, there is only one stairway left. If you originally have three stairways and lose one, you still have two stairways left.

Summary: calculate the total width per floor you need by multiplying the occupant load by 0.3″ and in some cases 0.2″. Take the largest width that you calculate, and use it for the entire stairway system. Keep in mind, the minimum clear width is typically 44″, but it is sometimes reduced to 36″ for low-capacity floors. Then, figure out how many exits are required by referencing the chart above, and divide the required width between the required number of stairways. That’s it! Reference the IBC codes below to check your work.

Referenced IBC Code:

1005.3.1 Stairways  The capacity, in inches, of means of egress stairways shall be calculated by multiplying the occupant load served by such stairways by a means of egress capacity factor of 0.3 inch per occupant. Where stairways serve more than one story, only the occupant load of each story considered individually shall be used in calculating the required capacity of the stairways serving that story.

Exceptions: For other than Group H and I-2 occupancies, the capacity, in inches, of means of egress stairways shall be calculated by multiplying the occupant load served by such stairways by a means of egress capacity factor of 0.2 inch per occupant in buildings equipped throughout with an automatic sprinkler system installed in accordance with Section 903.3.1.1 or 903.3.1.2 and an emergency voice/alarm communication system in accordance with Section 907.5.2.2.

1005.4 Continuity The maximum capacity required from any story of a building shall be maintained to the termination of the means of egress.

1005.5 Distribution of Minimum Width and Required Capacity Where more than one exit, or access to more than one exit, is required, the means of egress shall be configured such that the loss of any one exit, or access to one exit, shall not reduce the available capacity or width to less than 50 percent of the required capacity or width.

1011.2 Width & Capacity The required capacity of stairways shall be determined as specified in Section 1005.1, but the minimum width shall be not less than 44 inches. See Section 1009.3 for accessible means of egress stairways.

Exceptions: Stairways serving an occupant load of less than 50 shall have a width of not less than 36 inches.

1022.1 Exits Once a given level of exit protection is achieved, such level of protection shall not be reduced until arrival at the exit discharge.

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