How is a grease interceptor sized? A worked example

The formula has three parts, and each part has a reason.

1. 1Drainage fixture units (DFUs). A DFU is a code number for how hard a fixture can hit the drain. You add up the DFUs of every fixture that sends grease water to the tank.
2. 2Flow rate. Multiply the total by 3 gallons per minute.
3. 3Retention time. Multiply by 12 minutes. If the kitchen has a food waste disposal, multiply by 17 minutes instead.

The result is gallons. Then you round up to the next size the approved manufacturer builds.

Phoenix's manual states it this way: sizes are found by adding the DFUs for each grease bearing fixture, multiplied by 3 gallons per minute and multiplied by a 12 or 17 minute retention time, with 17 minutes if there is a garbage disposal and 12 if there is not. Phoenix's minimum gravity interceptor is 500 gallons.

The National Precast Concrete Association explains why retention time matters: if material passes through too fast, the grease does not have enough time to rise to the top. Phoenix and Scottsdale both give a kitchen with a garbage disposal the longer time.

Phoenix prints a fixture count worksheet as Appendix C of its manual. It lists the DFU value for each fixture type. Here are the values on that sheet.

The sheet ends with the two formulas: gallons equal DFUs times 3 times 12 without a disposal, and DFUs times 3 times 17 with one.

### What is Phoenix's own worked example?

The manual gives this one. The kitchen has one food prep sink and three floor drains, one in the food prep area, one in the grill area, and one receiving the indirect waste from the ice machine and mop sink.

• 3 floor drains at 2 DFUs each is 6 DFUs.
• A mop sink at 3 DFUs is 3.
• A food prep sink at 3 DFUs is 3.
• The total is 12 DFUs.

Then 12 DFUs x 3 gpm x 12 minutes is 432 gallons. The manual sizes the interceptor at 500 gallons, its smallest allowed size.

### What does a bigger kitchen look like? (Example)

This next case is our own arithmetic, built from the Phoenix worksheet values above. It is an example, not a real kitchen and not a city-issued result.

Say a kitchen has a prep sink and a pot sink, one mop sink, three floor drains, one floor sink and a dishwasher with a 2 inch trap arm.

1. 122 DFUs x 3 gpm is 66 gpm.
2. 266 x 12 minutes is 792 gallons.
3. 3That is above 500, so the tank rounds up to the next size the approved maker offers.

Now add a food waste disposal. It adds 3 DFUs, for 25 in all. The retention time jumps to 17 minutes.

1. 125 DFUs x 3 gpm is 75 gpm.
2. 275 x 17 minutes is 1,275 gallons.

One disposal moves the answer from 792 to 1,275. That is why the disposal decision comes before the tank order. Mesa restricts new disposals, which is covered below.

### How does rounding up work?

You round up to a size the approved manufacturer actually makes, and the plan reviewer confirms it. The sizes are not always round numbers. Surprise's example rounds 828 gallons up to 1,050. Phoenix says 750 gallon and larger tanks need three man-ways, so the next size can change the build too.

Same skeleton, different small print. Here is what each city's own document says.

### What does Surprise do?

Surprise publishes a full example. It adds a 3 compartment sink with a 2 1/2 inch outlet (4 units), a mop sink with a 3 inch outlet (5), two floor sinks with 2 inch outlets (6 in all) and four floor drains (8). That is 23 units. Multiplied by 3 it is 69 gpm. Multiplied by 12 it is 828 gallons, rounded up to 1,050.

Surprise's page also puts a limit on the top end. Its formula step says round to the next nearest size, with a minimum of 500 gallons and no more than 2,500. The same page separately lists a minimum of 750 gallons and a maximum of 2,500 in its bullet list of interceptor rules, so ask the city which minimum applies to your job.

### What does Mesa do?

Mesa's city code converts each fixture's outlet or trap size to DFUs. A 2 inch outlet is 3 units, a 3 inch outlet is 5, and dishwashers are listed as double size. Then Mesa multiplies by 3 gpm and by 12 and rounds up. It has no 17 minute step. Instead, Mesa's code says users shall not install or replace a garbage grinder that sends liquefied waste to the sewer, with an exception for domestic sources.

Mesa's minimum interceptor is 500 gallons and its maximum is 2,500, and it must have at least two chambers. The code's table also says one fixture unit equals 7.5 gpm, while the calculation steps use 3 gpm. The steps are what produce the size. Our Mesa sizing code page walks the table.

### What does Scottsdale do?

Scottsdale's guidelines use the amended 2021 plumbing code formula: total DFUs x 3 gpm x the retention time, with 12 minutes without a garbage disposal and 17 with one. Its worksheet lists its own DFU values, which differ from Phoenix's. For example, it lists a 3 compartment sink at 9 DFUs, a 2 compartment sink at 6, a prep sink at 3, a mop sink at 3 and a floor drain at 2. It lists an emergency floor drain at 0.

Scottsdale's worksheet also asks for the dishwasher's cut sheet with the plumbing drawings.

### What does Chandler do?

Chandler skips the formula. Its worksheet form converts each fixture's drain size to DFUs, then reads the interceptor size from a table. For example, 0 to 21 total DFUs gives 750 gallons, 22 to 35 gives 1,000, and 36 to 90 gives 1,250. The table tops out at 3,000 gallons for 308 to 342 DFUs, and anything above 342 must be designed by an Arizona registered professional engineer. That form is dated 2015, so check the Chandler sizing chart page for the code now in force.

Because worksheets use different DFU values, you cannot move one city's total to another city's chart. Start from the fixture list, not the number.

### How do you keep the count honest?

Scottsdale's worksheet notes offer the best tips, and they are written from the city's side of the counter.

• Ask one question per fixture. Can this fixture possibly receive any fats, oils or grease? If not, it may not belong on the count.
• Watch hand sinks. Scottsdale says hand sinks in food prep areas are included unless a note on the plumbing sheet says otherwise.
• Do not count twice. A floor sink in the food prep area that receives discharge from a fixture, such as a 3 compartment sink, is not included in the calculation.
• Watch the 2,500 gallon line. Scottsdale says standard pumping trucks have a 2,500 gallon limit, so a bigger tank needs a special truck or more than one truck.
• Consider zones. Scottsdale says multiple interceptors are acceptable, and suggests designing them to serve zones rather than connecting them in series or parallel, so each can be on its own cleaning cycle.

Surprise also caps its interceptors, saying two smaller interceptors are required once the size reaches over 2,500 gallons.

The model plumbing code is different from these city versions. The 2024 International Plumbing Code says capacity is the peak drain flow times a retention time of 30 minutes. Phoenix amended that section to use the 12 and 17 minute formula, and Scottsdale's guidelines carry the same formula. Always work from your city's amended text.

Start with the steps below. Each one takes the guesswork out of the worksheet.

1. 1List every fixture that touches the kitchen drains. Include the dishwasher's make and model.
2. 2Decide on a disposal before you size. It changes the retention time in Phoenix and Scottsdale, and it is restricted in Mesa.
3. 3Get your city's own worksheet, not a generic calculator.
4. 4Fill it in, then compare the total to the city's minimum and maximum.
5. 5Read the installation steps and the location rules before you buy a tank, since size affects both.

Once the tank is in the ground, the 25 percent rule page explains when it must be cleaned. For a space that never had a grease line, see the former retail space page.

### When is a plumber worth calling?

Call one when the worksheet touches more than a few fixtures, when the count is close to a size break, or when the equipment list is changing. A licensed plumber can fill out the worksheet with you and draw it into the plumbing plan. HQ is licensed and insured and gives free estimates. If a grease line backs up in a working kitchen, we offer 24/7 emergency service. Start at our commercial plumbing page.

For the idea behind the numbers, see what retention time is and why it matters.