Grease interceptor sizing basics
By Stew Israil, GreaseTrapRules · Updated Oct 18, 2026 · Every rule below links to its official source. Rules change; verify with your city.
Short answer: sizing is set by your plumbing code plus your city's FOG program, and the city usually has the final word. Use a code method to get a number, then check the city minimum, and have a licensed plumber or engineer size the final unit. Our sizing calculator runs the three main code methods.
The three code methods
- UPC drainage fixture units (gravity interceptors). Add up the DFUs of the kitchen drains connected to the interceptor and read the size from UPC Table 1014.3.6. For example, 8 DFUs maps to 500 gallons and 35 DFUs to 1,000 gallons (UPC §1014.3.6 and Table 1014.3.6 (gravity interceptor sizing)).
- IPC peak flow × 30 minutes (gravity interceptors). Required capacity in gallons = peak drain flow into the interceptor (gpm) × 30 minutes of retention (IPC gravity grease interceptor sizing (peak flow × 30 minutes)).
- PDI-G101 sink method (hydromechanical units). Sink volume (gallons) = L × W × D in inches ÷ 231, times a 0.75 fill factor, divided by a 1- or 2-minute drain period to get gpm; pick the next larger rated size (PDI-G101 (rev. 2017) hydromechanical interceptor sizing).
City formulas that override the code
- Austin, TX: flow rating = total fixture units × 3 gpm; minimum liquid holding capacity = flow rating × 12 minutes; nothing under 100 gallons; 500 gallons minimum with a dishwasher (Austin Water, Grease Trap Sizing & Design Criteria).
- Illinois (statewide): at least half the liquid capacity of the connected fixtures on the same floor, 60% if the interceptor is a floor below (77 Ill. Adm. Code 890.510 (Illinois Plumbing Code, grease interceptors)).
- Aurora, IL and Oswego, IL (Fox Metro): total fixture units × 125 gallons sets the minimum liquid capacity.
- Chicago, IL: gravity interceptor capacity = peak drain flow × retention time (Chicago Code 18-29-1003.3).
- Raleigh, NC: the pretreatment coordinator approves sizing calculations; the City publishes gravity and hydromechanical sizing templates.
Before you buy
Austin, TX warns not to buy a trap before you have plan approval and an Industrial Waste approval letter with the required size, because unapproved units can be rejected. Raleigh, NC requires interceptor approval before commercial plan review. Check your city page for its approval step.
Every sizing rule we track (quoted)
| City | Sizing rule, from the official source |
|---|---|
| Arlington Heights, IL | Type and capacity must be approved by the Director of Public Works (Sec. 22-405). Illinois Plumbing Code minimum: the interceptor must hold at least half the liquid capacity of the connected fixtures on the same floor (60% if it sits on a floor below); fixture capacity in gallons = length × width × depth in inches ÷ 231. ✓ hand-checked |
| Austin, TX | Flow rating = total fixture units × 3 gpm; minimum liquid holding capacity = flow rating × 12 minutes. Traps under 100 gallons are not approved, and kitchens with a dishwasher need at least a 500-gallon interceptor. ✓ hand-checked |
| Chicago, IL | Gravity grease interceptor capacity = peak drain flow into the interceptor (gpm) × retention time (Chicago Code 18-29-1003.3). ✓ hand-checked |
| Cicero, IL | Illinois Plumbing Code minimum: the interceptor must hold at least half the liquid capacity of the connected fixtures on the same floor (60% if it sits on a floor below); fixture capacity in gallons = length × width × depth in inches ÷ 231. ✓ hand-checked |
| Columbia, SC | The minimum grease interceptor size is 1,000 gallons; structures must be sized so wastewater entering the sewer has no more than 100 mg/L of FOG. ✓ hand-checked |
| Des Plaines, IL | Illinois Plumbing Code minimum: the interceptor must hold at least half the liquid capacity of the connected fixtures on the same floor (60% if it sits on a floor below); fixture capacity in gallons = length × width × depth in inches ÷ 231. ✓ hand-checked |
| Downers Grove, IL | DGSD publishes standard 1,000-gallon and 1,800-gallon grease trap details on its forms page. Illinois Plumbing Code minimum: the interceptor must hold at least half the liquid capacity of the connected fixtures on the same floor (60% if it sits on a floor below); fixture capacity in gallons = length × width × depth in inches ÷ 231. ✓ hand-checked |
| Elgin, IL | In general FRWRD requires outside, 1,000-gallon minimum traps for all food service establishments; existing businesses changing ownership, renovating or expanding must meet the 1,000-gallon outside standard. ✓ hand-checked |
| Elk Grove Village, IL | Illinois Plumbing Code minimum: the interceptor must hold at least half the liquid capacity of the connected fixtures on the same floor (60% if it sits on a floor below); fixture capacity in gallons = length × width × depth in inches ÷ 231. ✓ hand-checked |
| Evanston, IL | The Evanston Building Department determines the number and size of grease traps, grease interceptors or catch basins. ✓ hand-checked |
| Glenview, IL | Illinois Plumbing Code minimum: the interceptor must hold at least half the liquid capacity of the connected fixtures on the same floor (60% if it sits on a floor below); fixture capacity in gallons = length × width × depth in inches ÷ 231. ✓ hand-checked |
| Greensboro, NC | An exterior grease interceptor has a minimum capacity of 1,000 gallons; under-counter grease traps are usually limited to four fixtures and 50 gallons per minute combined. ✓ hand-checked |
| Honolulu, HI | Minimum grease interceptor volume V = F × R × S: peak flow (gpm) × 30-minute retention time × 1.25 storage factor. ✓ hand-checked |
| Los Angeles, CA | LASAN defines a grease interceptor as a device with a minimum size of 750 gallons. ✓ hand-checked |
| Naperville, IL | Illinois Plumbing Code minimum: the interceptor must hold at least half the liquid capacity of the connected fixtures on the same floor (60% if it sits on a floor below); fixture capacity in gallons = length × width × depth in inches ÷ 231. ✓ hand-checked |
| Northbrook, IL | Illinois Plumbing Code minimum: the interceptor must hold at least half the liquid capacity of the connected fixtures on the same floor (60% if it sits on a floor below); fixture capacity in gallons = length × width × depth in inches ÷ 231. ✓ hand-checked |
| Oak Brook, IL | Illinois Plumbing Code minimum: the interceptor must hold at least half the liquid capacity of the connected fixtures on the same floor (60% if it sits on a floor below); fixture capacity in gallons = length × width × depth in inches ÷ 231. ✓ hand-checked |
| Orland Park, IL | Illinois Plumbing Code minimum: the interceptor must hold at least half the liquid capacity of the connected fixtures on the same floor (60% if it sits on a floor below); fixture capacity in gallons = length × width × depth in inches ÷ 231. ✓ hand-checked |
| Peoria, IL | Commercial buildings expected to generate significant FOG (as determined by GPSD) must install a grease interceptor of at least 250 gallons; restaurants generating large quantities of FOG must install at least 1,000 gallons. ✓ hand-checked |
| Phoenix, AZ | The FOG BMP manual states: “The minimum size allowed in the City of Phoenix is 500 gallons.” ✓ hand-checked |
| Raleigh, NC | The pretreatment coordinator reviews and approves interceptor sizing calculations; the City publishes gravity and hydromechanical grease interceptor sizing templates. ✓ hand-checked |
| Rockford, IL | New or altered food service establishments that discharge FOG in quantities large enough to block lines must install a minimum 1,000-gallon interceptor; strip-mall interceptors may be no less than 1,500 gallons. ✓ hand-checked |
| Skokie, IL | Illinois Plumbing Code minimum: the interceptor must hold at least half the liquid capacity of the connected fixtures on the same floor (60% if it sits on a floor below); fixture capacity in gallons = length × width × depth in inches ÷ 231. ✓ hand-checked |
| Springfield, IL | Grease traps: minimum 35 gallons; grease interceptors: minimum 500 gallons (variance possible for traps). New building construction requires a minimum 500-gallon interceptor. ✓ hand-checked |
| Virginia Beach, VA | Size grease control devices to the International Plumbing Code (it governs if it conflicts with the regional technical standards). Sizing questions: Permits & Inspections, (757) 385-4211. ✓ hand-checked |
| Winston-Salem, NC | New in-ground grease interceptors must be at least 1,000 gallons total capacity unless the Director approves otherwise, sized for a 24-minute retention time at actual peak flow. ✓ hand-checked |
| Abilene, TX | Under no circumstances will a grease trap/interceptor smaller than 1,000 gallons be accepted. quoted from source |
| Aurora, IL | The total number of fixture units multiplied by one hundred and twentyfive (125) gallons shall determine the minimum total liquid capacity of the interceptor. quoted from source |
| Boston, MA | Large in-ground grease traps normally hold 500 gallons or more and are usually cleaned by a contractor. equipped to deal with the large quantities of grease, oil, and non-floatables. quoted from source |
| Chattanooga, TN | The minimum grease trap size for bakeries of any type shall be 250 gallon capacity. quoted from source |
| Cincinnati, OH | Minimum 1,000-gallon gravity grease interceptor for each 950 lbs of grease capacity required. quoted from source |
| Colorado Springs, CO | Expected Size Range of Grease Interceptor: 325* – 1000 gallons * Only one chamber is provided in 325 and 500 gallon grease interceptors. quoted from source |
| Decatur, IL | Interceptors thus located shall have a minimum capacity of 1,000 gallons. quoted from source |
| Denver, CO | MINIMUM GRAVITY GREASE INTERCEPTOR SIZE: For all gravity grease interceptor sizing methods, a 750-gallon two compartment gravity grease interceptor is the accepted minimum liquid capacity allowed to be installed. quoted from source |
| Fox Lake, IL | (Sandwich bars and carryout services, which only prepare noncooked foods such as cold sandwiches, shall install a minimum 400 gallon grease interceptor unless the director of the POTW grants a variance.). quoted from source |
| Irvine, CA | Grease interceptors’ sizing is based on the drainage fixture units (DFU) connected to the grease interceptor, pursuant to the current California Plumbing Code (CPC), table 1014.3.6. quoted from source |
| Lombard, IL | For the purpose of sizing a hydro mechanical grease interceptor the resulting units are taken as gallons per minute, and for the purpose of sizing a gravity grease interceptor, as gallons. quoted from source |
| Louisville, KY | For a FSE approved to install a grease trap, the minimum size requirement is the equivalent of a 25-gallon per minute/50 pound capacity trap. quoted from source |
| Memphis, TN | All grease traps used in conjunction with food service establishments shall have the capacity of 15 gallons per seat of dining capacity, except that no single grease trap shall be smaller than 750 gallons or larger than 3,000 gallons. quoted from source |
| New Orleans, LA | Traps are typically in- floor recessed grease retention units, which are normally not less than 125 gallons capacity. quoted from source |
| Oswego, IL | The total number of fixture units multiplied by one hundred and twentyfive (125) gallons shall determine the minimum total liquid capacity of the interceptor. quoted from source |
| Reno, NV | Detail drawing (cut-sheet) of grease or sand/oil interceptor. * The minimum size allowed for a grease interceptor is 750 gallons in the City of Reno. quoted from source |
| Round Lake, IL | The minimum capacity of a grease interceptor as determined above may be halved for establishments utilizing paper service only but may not be less than 750 gallons. quoted from source |
| Savannah, GA | The grease retention capacity rating in pounds shall be at least two (2) times the GPM flow rate of the type fixture which it serves. quoted from source |
| Scottsdale, AZ | Should your calculations determine you need an interceptor larger than 2500 gallons, operation costs increase. quoted from source |
| Seattle, WA | Total gallon capacity of fixtures discharging to a hydromechanical grease interceptor shall not exceed 2-1/2 times the rated gpm flow rate of the interceptor. quoted from source |
| Tacoma, WA | Calculate the total DFUs and choose an interceptor size based on the total DFU count. quoted from source |
FAQ
How do you size a grease interceptor?
Use the method your plumbing code and city require. Common ones: the UPC table based on drainage fixture units (DFUs), the IPC method (peak drain flow in gpm × 30 minutes of retention), and the PDI-G101 sink-volume method for hydromechanical units. Many cities add a formula or a minimum size on top.
What size grease trap does a small restaurant need?
It depends on the fixtures connected and your city minimum. Use the sizing calculator for an estimate, then check your city: minimums we track run from 100 gallons (Austin; 500 with a dishwasher) up to 1,000 gallons (Elgin, Decatur, Cincinnati, Abilene) or more (1,500 gallons for Rockford strip malls).
What is the grease interceptor sizing formula?
IPC: capacity (gallons) = peak drain flow (gpm) × 30 minutes. Austin: flow rating = fixture units × 3 gpm, then capacity = flow rating × 12 minutes. Illinois Plumbing Code: at least half the liquid capacity of the connected fixtures, with fixture capacity = L × W × D (inches) ÷ 231.
More guides
How often to pump a grease trapThe 25% rule and the 30, 60 and 90-day schedules cities actually setGrease trap vs. grease interceptorHydromechanical vs. gravity: definitions, sizes and why the pumping rules differDoes my business need a grease trap?Coffee shops, bakeries, food trucks and drinks-only businesses, by city ruleFOG inspection checklistWhat grease inspectors check, item by item, with the rule behind each oneGrease trap maintenance log (printable)Free printable log plus how long each city makes you keep recordsWhat affects grease trap cleaning costThe cost drivers your city controls, and the official fees and fines we can sourceGrease interceptor sizing calculatorUPC, IPC and PDI G-101 methods
Sources
- UPC §1014.3.6 and Table 1014.3.6 (gravity interceptor sizing)
- IPC gravity grease interceptor sizing (peak flow × 30 minutes)
- PDI-G101 (rev. 2017) hydromechanical interceptor sizing
- Austin Water, Grease Trap Sizing & Design Criteria
- 77 Ill. Adm. Code 890.510 (Illinois Plumbing Code, grease interceptors)
- City rules quoted on this page come from each linked city page, which lists its official sources and the date checked.