Room occupant load calculations: round up or down?

[Tim Mailloux]

When calculating the net occupant load of a room the math almost never results in a whole number. In those cases do you round your math up or down?

As an example, I do a lot of school work, and we typically try and keep classrooms under 1,000sf as to not require two exits and exit doors that have to swing in the direction of egress travel. But say you have a 985sf classroom with an occupant load factor of 1 person per 20sf. This works out to a calculated value of 49.25 people in that classroom. There is no such thing as ¼ of a person, and logic would say you can round down to 49 occupants. But it seems far more common for people to blanketly round up all calculations related to occupant loads.

I attended a chapter 10 egress seminar this past spring, the guest instructor was an ICC Staff member. I asked the instructor this question off-line during one of the breaks and he was in favor of rounding down.

 

[arwat23]

I always round up. You have 1/4 of a person, that's still a person. That's how a few plans examiners and CBOs have told me to do this, so I do that.

 

[RLGA]

I always round down, except when the factor gives me less than 1 occupant.

 

[Tim Mailloux]

I always round down, except when the factor gives me less than 1 occupant.

seeing how you literally wrote one of the books on this subject and the ICC staff member I spoke with helps write IBC chapter 10, I will consider this matter closed

 

[Tim Mailloux]

I always round down, except when the factor gives me less than 1 occupant.

have you ever gotten push back from an AHJ on this approach?

 

[steveray]

Almost every BO I know rounds up....As we were taught...In CT....

 

[RLGA]

have you ever gotten push back from an AHJ on this approach?

Never have.

 

[Tim Mailloux]

Almost every BO I know rounds up....As we were taught...In CT....

I could split the difference and not round at all. Show the fractional occupants (49.25 people) on my plans keeping my occupant load under 50 as to not require 2 doors.

 

[JPohling]

we always round up. been taught that way and have had numerous BO's require it.

 

[mtlogcabin]

I have always rounded up in all rooms in every occupancy type to determine the required exit width for the entire means of egress. More as a safety factor ensuring the means of egress is code compliant. I have no problem rounding down for other code sections that are base on occupant load.

If you have a building that has 120 individual rooms and each room calculates out to 1/4 of a person then that is 30 people. It will rarely make a difference but it opens up a discussion with the DP about how I make my decision.

Exception: Where approved by the building official, the actual number of occupants for whom each occupied space, floor or building is designed, although less than those determined by calculation, shall be permitted to be used in the determination of the design occupant load.

Document why you rounded down for all the rooms, built in cabinets, or shelving for example. Maybe defining the teachers floor area and identify it as a "B" occupancy space.

1004.3 Multiple function occupant load.
Where an area under consideration contains multiple functions having different occupant load factors, the design occupant load for such area shall be based on the floor area of each function calculated independently.

Just don't do it because your only reason is "That's way I have always done it" or "That's how I was taught to do it" Document how and why you made the decision you made.

 

[Tim Mailloux]

Exception: Where approved by the building official, the actual number of occupants for whom each occupied space, floor or building is designed, although less than those determined by calculation, shall be permitted to be used in the determination of the design occupant load.

I am pretty certain that this exception has been deleted from the CT code. We have to handle this sort of thing with formal code modification requests to the state that take months to get reviewed.

 

[Tim Mailloux]

If you have a building that has 120 individual rooms and each room calculates out to 1/4 of a person then that is 30 people. It will rarely make a difference but it opens up a discussion with the DP about how I make my decision.

in the same vein, if you have 120 individual rooms with net occupant load calculations, and each one of those 120 room calculations gets rounded up you have added 120 people to the building.

 

[arwat23]

in the same vein, if you have 120 individual rooms with net occupant load calculations, and each one of those 120 room calculations gets rounded up you have added 120 people to the building.

Yep. I recently had a project where there were about 75 rooms (offices and exam rooms mainly). Originally, I only rounded up after adding the OL of each room. I tried to do it that way because if I hit 200 occupants, I'd need to add 5 more toilets. Using the method we typically use - rounding up each room before adding them together - would push the OL over 200 unless we modified some rooms in a way our client wouldn't accept.

Got a plan check comment saying we needed to round up each room before adding their OL to get the total for the building. Net and Gross mean almost nothing when each room needs to be calculated individually regardless of the function. Almost screwed up the entire project.

Edit: That said, I still prefer it that way. Better to have a complaint or overly compliant space than a non-compliant space imo.

 

[SCBO1]

I typically round down.

When your at that 50 OL, in the case of an "A"-occupancy it may allow you to change to a "B"-occupancy, see sec. 303.1.1.

See 1004.5 for increased occupant load.

 

[mtlogcabin]

in the same vein, if you have 120 individual rooms with net occupant load calculations, and each one of those 120 room calculations gets rounded up you have added 120 people to the building.

120 X .75 = 90 which is still a lot people however I would not use the rounded up total for plumbing fixture count. I would use the actual calculated total OL number because the plumbing code requires you to round up the fixture count.


[P] 2902.1.1 Fixture calculations.
To determine the occupant load of each sex, the total occupant load shall be divided in half. To determine the required number of fixtures, the fixture ratio or ratios for each fixture type shall be applied to the occupant load of each sex in accordance with Table 2902.1. Fractional numbers resulting from applying the fixture ratios of Table 2902.1 shall be rounded up to the next whole number. For calculations involving multiple occupancies, such fractional numbers for each occupancy shall first be summed and then rounded up to the next whole number.

 

[Yankee Chronicler]

I have always rounded up. That's the way it was always taught, going back to the 1970s under the BOCA Basic Building Code.

"There are no fractions of a person."

 

[Tim Mailloux]

Yep. I recently had a project where there were about 75 rooms (offices and exam rooms mainly). Originally, I only rounded up after adding the OL of each room. I tried to do it that way because if I hit 200 occupants, I'd need to add 5 more toilets. Using the method we typically use - rounding up each room before adding them together - would push the OL over 200 unless we modified some rooms in a way our client wouldn't accept.

I would typically group all those offices, exam rooms and supporting corridors, toilets rooms, under one large 1 per 150 gross business area calculation. any conference rooms & waiting area would have individual net calculations using the appropriate assembly occupant load factor. There would only be a hand full of calculations getting rounded up and this would not have much of an impact on Occupant loads.

In my case I am typically dealing with large schools and the majority of the spaces are net calculations, meaning each classroom / educational spaces has its occupant load individually calculated and each calculation gets rounded up. This has a very big impact on the occupant load in these buildings. And don't even get me started on simultaneous occupancy. All the large assembly spaces (gyms, cafeterias, multipurpose rooms & auditoriums) all get calculated with hundreds of occupants in them while at the same time those very children using the spaces are being accounted for in their classrooms. A school designed for a student body of 800 students and 150 faculty members will end up with a occupant load of 3000 to 4000 people on the code plans and egress is designed for these over inflated numbers.

 

[steveray]

But I would give you the 25sqft or so at the door(s) where no one will be....

 

[Tim Mailloux]

Just found this example on page 379 of the 2021 IBC Code Illustrated Handbook which clearly shows the calculations being rounded down


GIVEN: A 1,600-square-foot conference room in a hotel.
DETERMINE: The design occupant load of the room.
SOLUTION: Because a variety of assembly activities can occur within the room, the use creating the largest occupant load would be evaluated.

(1) Conference/seminar use with tables and chairs
1 person per 15 sq ft = 106.67 = 106 occupants

2) Conference/seminar use with chairs only (auditorium-style seating)
1 person per 7 sq ft for seating = 228.57 = 228

Therefore for egress purposes, a design occupant load of 228 shall be used. Note that other potential uses of the room (dining, receptions, dances, etc.) would also utilize these factors

 

[steveray]

Just found this example on page 379 of the 2021 IBC Code Illustrated Handbook which clearly shows the calculations being rounded down


GIVEN: A 1,600-square-foot conference room in a hotel.
DETERMINE: The design occupant load of the room.
SOLUTION: Because a variety of assembly activities can occur within the room, the use creating the largest occupant load would be evaluated.

(1) Conference/seminar use with tables and chairs
1 person per 15 sq ft = 106.67 = 106 occupants

2) Conference/seminar use with chairs only (auditorium-style seating)
1 person per 7 sq ft for seating = 228.57 = 228

Therefore for egress purposes, a design occupant load of 228 shall be used. Note that other potential uses of the room (dining, receptions, dances, etc.) would also utilize these factors

But just like commentary....unenforceable....

 

[Tim Mailloux]

But just like commentary....unenforceable....

but where in the code does it tell you to round up?

 

[SCBO1]

2012IBC cod and commentary has figure 1004.1.2 with the following calc:

4,660 SQ.FT./15 SQ,FT. per occupant = 311 occupants

May math which always needs to be checked has the following:
4,660 SQ.FT./15 SQ.FT. per occupant = 310.666

So it looks like the commentary people round up?

 

[SCBO1]

My spelling needs to be checked too!

 

[Yankee Chronicler]

In my case I am typically dealing with large schools and the majority of the spaces are net calculations, meaning each classroom / educational spaces has its occupant load individually calculated and each calculation gets rounded up. This has a very big impact on the occupant load in these buildings. And don't even get me started on simultaneous occupancy. All the large assembly spaces (gyms, cafeterias, multipurpose rooms & auditoriums) all get calculated with hundreds of occupants in them while at the same time those very children using the spaces are being accounted for in their classrooms. A school designed for a student body of 800 students and 150 faculty members will end up with a occupant load of 3000 to 4000 people on the code plans and egress is designed for these over inflated numbers.

And for decades the Connecticut Office of State Building Inspector has been granting rubber stamp modifications to allow the plumbing fixture count in schools to be based on non-simultaneous occupancy of schools containing a cafeteria, a gymnasium, and an auditorium. As expressed by the esteemed Dan Tierney of your OSBI many years ago, "Nobody ever died because they had to stand in line to pee." But they do NOT mod exit capacity for non-simultaneous occupancy, because it's not impossible that some group may be putting on a play in the auditorium on the same night there's a basketball game in the gym and a civic meeting in the cafeteria. Egress features must always be designed for the worst case, because people may die if we get it wrong.

I don't regard those numbers as over-inflated. In most schools the exits serving each of those large areas are rather remote from one another and in many designs may be separated from one another (and thereby not available for use) by rolling security grates that are deployed at night. I do not regard those occupant loads as "over-inflated."