• Article By | CROFT






Aa = {At + [At X If] + [At x Is]}, where

Aa = Allowable area per story
At = Area from Table 503 (above)
If =  Area increase factor due to frontage
Is = Area increase factor due to sprinklers


Let’s break this formula into two sections:

1. [At X If] Frontage

2. [At x Is] Sprinklers

In Section 1, if there is a minimum of 30 feet around the entire perimeter of the building, then At X If =  .75At

If not, then we can find the frontage factor by following this equation:

If = [length of the perimeter w/ min 30 feet around bldg / total perimeter length] – 0.25



In this example, If = [length in red / whole length] -.25 = .63

In Section 2, if the building is one-story and is sprinklered, then At x Is = 3At . If the building is multi-story and is sprinklered, then At x Is = 2At . If the building is not sprinklered, then At x Is = 1At .

Now we combine the 2 sections:

Example: Our building is two stories, sprinklered, and has a minimum of 30ft around the entire building. The construction type is IIB and the occupancy type is A-1. What is our allowable area?

Answer:  Aa = 2.75At, where At = 8500, so Aa = 23,375 S.F.