How Many Toilets and Sinks Do You Need – Plumbing Fixture Requirements

Have you ever wondered how a mall only seems to have one set of restrooms while an office building has nearly as many on every floor? It all comes back to the plumbing fixture requirements dictated by the building code.

When we design a building, we go through a number of calculations to determine just how many restrooms we are going to need.

1. We look at the number of square feet in the building and what the building is used for (its occupancy). There are two areas to occupancy, first what is the overall space being used for and then what is the specific use of each area of the space. In example, the overall occupancy might be mercantile, with the sales floor at 30 sf per person, but the store room is calculated at 300 sf per person.

2. Once we have all the uses we refer to the occupant load table in IBC chapter 10 and using the factors in the chart we divide the square footage by the occupant load to get a number of occupants. For partial occupants we have to round up and all square footage needs to be accounted for. Interestingly the chart varies between gross and net depending in the occupancy.

3. Then we take that number of occupants to IBC chapter 29. That’s where we find the required plumbing fixture table. We then find the appropriate occupancy and read the requirements. Here are some common factors:

Mercantile: 1 WC per 500, 1 Lav per 750, 1 DF per 1,000, 1 service sink

Education: 1 WC per 50, 1 Lav per 50, 1 DF per 100, 1 service sink

Business: 1 WC per 25 for the first 50 and then 1 per 50, 1 Lav per 40 for the first 80 and then 1 per 40, 1 DF per 100, 1 service sink

Restaurants: 1 WC per 75, 1 Lav per 200, 1 DF per 500, 1 service sink

As you can start to see there is quite a bit of math involved and the numbers vary wildly. Our understanding is that they are all based on historical averages and generally speaking they work. In addition, an owner is always free to go above these requirements, but not below them. Please post or contact us if you need help on your specifics.