IBC Stair Width Calculation

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:

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.