How to Calculate Plumbing Fixture Count

According to the plumbing code, the minimum number of plumbing fixtures required in a building depends greatly on the intended use. Organized in easily defined groups, uses categories include factories, residential, or educational.

After establishing the use, the occupancy load, based on square footage, must be assessed. The final variable is that most cities require an equal number of toilet facilities for men and women. 

Mixed-use buildings can be complicated and may require separate facilities. There are two generally accepted formulas for how to calculate the plumbing fixture count in a facility, which are complex determinations: the Standard Method and the Fractional Method. 

Getting Started

Municipalities provide guidelines for calculating total occupant loads. Determining any plumbing fixture count involves plugging in the correct numbers. Then, use the appropriate tables for your city and apply one of the two methods. Here is an example:

A mixed-use building has 3,800 sq ft of office space, 8,405 sq ft of storage, 1,200 sq ft of kitchen space, and 6,420 sq ft of dining room space. 

Step 1- Calculating Occupant Load for the various spaces:

• Office- 3,800 / 200 = 19
• Storage- 8,405 / 5,000 = 1.68
• Kitchen- 1,200 / 30 = 40
• Dining- 6,420 / 30 = 214

Step 2- Determining Occupancy Load for both males and females (assuming a 50/50 split):

• Office- 19 / 2 = 9.5 (round up to 10) 10 female & 10 male
• Storage- 1.68 / 2 = .84 (round up to 1) 1 female & 1 male
• Kitchen- 40 / 2 = 20, 20 female & 20 male
• Dining- 214 / 2 = 107, 107 female & 107 male

Using the CPC 422.1 Fixture Count Table, you can determine the minimum required fixtures per males and females. Take note that installation of each urinal results in additional water closets for females. These are the results using the Standard Method:

• Office- 1 Water Closet (WC) + 1 urinal for males, 2 WC for females, 1 lavatory for each
• Storage- 1 WC for each, 1 lavatory for each
• Kitchen- 1 WC + 1 urinal for males, 2 WC for females, 1 lavatory for each
• Dining- 2 WC + 1 urinal for males, 3 WC for females, 1 lavatory for each

When using the Fractional Method, determinations are made by taking the occupant load calculations and dividing by the allowable number of fixtures/person as shown in Table 422.1.

With the Fractional Method, you are essentially pinning down the real numbers, rather than solely relying on the ranges provided in Table 422.1. Here is an example (using male WC calculations):

• Office- 10 x 1/50 = .2
• Storage- 1 x 1/100 = .01
• Kitchen- 20 x 1/50 = .4
• Dining Room- 107 x 2/50 = 4.28

Add the fractions, resulting in 4.89, which gets rounded up to 5. Repeat this process with female calculations and the remaining areas. Add all of those results together. The extra work that goes into the Fractional Method results in a minimum number of plumbing fixtures needed. Compare that to the Standard Method, which ultimately could end up costing you more money.

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