Free Grease Trap Sizing Calculator: PDI G-101 and IPC 1003 Compliance for US Commercial Kitchens
Select your kitchen type preset or enter fixture counts manually. The calculator sizes both the hydromechanical (indoor, GPM-rated) and gravity interceptor (outdoor, gallon-rated) options side by side, with pumping frequency estimate and FOG compliance context.
PDI G-101 Flow Rate Method: Fixture-by-Fixture GPM Analysis for Hydromechanical and Gravity Interceptor Selection
Select a kitchen preset or enter fixture counts. The calculator totals all fixture GPM per PDI G-101, then sizes the hydromechanical trap (round up to standard GPM) and gravity interceptor (total GPM x 30 min retention, divide by 0.75 for the 25% grease storage rule).
| Fixture | GPM | Count | Subtotal |
|---|---|---|---|
| 3-Compartment Sink | 20 | 0 | |
| 2-Compartment Sink | 15 | 0 | |
| Prep Sink (single) | 10 | 0 | |
| Commercial Dishwasher (under-counter) | 15 | 0 | |
| Commercial Dishwasher (door-type) | 20 | 0 | |
| Floor Drain | 2 | 0 | |
| Mop Sink | 3 | 0 | |
| Pot / Kettle Fill Sink | 15 | 0 | |
| Wok Station / Range | 5 | 0 | |
| Coffee Station | 3 | 0 | |
| Total Design Flow Rate | 0 GPM | ||
Select a kitchen preset or enter fixture counts, then click Calculate to size your grease interceptor.
Teal: working volume zone (75%) | Orange: grease storage zone (25% rule pump trigger) | Hydromechanical shown as gallon equivalent
Why Commercial Kitchens Need Properly Sized Grease Interceptors: FOG Compliance and Federal Enforcement
Fats, oils, and grease, universally shortened to FOG by wastewater engineers and municipal inspectors, are the single largest contributor to sanitary sewer blockages in the United States. The US Environmental Protection Agency estimates that FOG is responsible for approximately 47 percent of all sanitary sewer overflows in the country. Every commercial kitchen, from a 10-seat coffee shop to a 500-seat convention center, generates FOG as a byproduct of cooking, dishwashing, and food preparation. A grease interceptor is the mechanical device that captures FOG before it enters the municipal sewer system, where it solidifies, accumulates, and eventually creates blockages that back up raw sewage into homes, streets, and waterways.
The federal legal foundation for grease trap requirements comes from the EPA’s National Pretreatment Program, codified at 40 CFR Part 403. Specifically, 40 CFR Section 403.5(b)(3) prohibits any food service establishment (FSE) from discharging “solid or viscous pollutants in amounts which will cause obstruction to the flow” in a publicly owned treatment works (POTW). Congealed FOG is the textbook example of such pollutants, and this single clause gives cities the legal authority to cite and fine restaurants under federal authority even when no municipal FOG ordinance has been enacted locally. In practice, most cities have their own FOG ordinances that go further, specifying minimum interceptor sizes, cleaning frequencies, manifest documentation requirements, and inspection protocols.
The EPA’s FOG cap for food service effluent discharging to a municipal sewer is 100 milligrams per liter (mg/L) under the pretreatment program. Most municipal FOG ordinances set the same or stricter limits and require grease interceptors as the primary control technology for achieving that standard. When an interceptor becomes more than 25 percent full of accumulated grease and settled solids, the effective liquid depth in the clear water zone shrinks, hydraulic residence time drops below the design standard, and FOG breakthrough to the sewer begins. This is the basis of the 25 Percent Rule that drives pumping requirements across virtually every US FOG compliance program.
Two Types of Grease Interceptors: Why the Distinction Matters
All grease interceptors work on the same fundamental principle: effluent flows slowly through a baffled chamber, gravity separates the lighter FOG to the top and heavier solids to the bottom, and the clarified water in the middle exits to the sewer. But the two common configurations differ dramatically in size, location, and application, and sizing them incorrectly is one of the most common and costly mistakes in commercial kitchen plumbing.
Hydromechanical grease interceptors, also called under-sink traps or passive grease interceptors, are relatively small units (typically 10 to 100 GPM rated capacity) installed under or near the fixture inside the kitchen. They are rated by flow rate in gallons per minute under PDI Standard G-101, tested and certified by the Plumbing and Drainage Institute. The Uniform Plumbing Code (UPC) specifically prohibits gravity-type interceptors from being installed inside buildings where food is handled, making hydromechanical units the standard for indoor installation in most US jurisdictions. Gravity grease interceptors are large concrete or fiberglass tanks (typically 500 to 3,000 gallons) buried outside the building, receiving the combined discharge of all kitchen drains before the building sewer connects to the municipal line. They are rated by gallon capacity, sized per the International Plumbing Code (IPC) Section 1003, and are the typical choice for new restaurant construction where site access allows outdoor installation.
Two Methods, One Tool: PDI G-101 for Under-Sink Traps vs. IPC 1003 for In-Ground Interceptors
This calculator performs both sizing calculations simultaneously so restaurant operators, plumbing contractors, and building officials can compare the two options side by side. Understanding the method behind each helps you verify the results and communicate the design rationale to your local authority having jurisdiction (AHJ).
Method 1: PDI G-101 for Hydromechanical (Indoor) Interceptors
The PDI G-101 sizing method is straightforward: for each fixture connected to the interceptor, look up the rated flow rate in gallons per minute from the PDI G-101 fixture table. Add all fixture flow rates together to get the total design flow rate. The minimum hydromechanical interceptor rating equals this total, rounded up to the next standard manufactured GPM size (10, 15, 20, 25, 35, 50, 75, or 100 GPM). This is a minimum rating, not a recommendation for the exact size to install. Most experienced plumbing engineers specify the next size up from the minimum as a buffer against flow surges during peak service and to extend the time between cleanings.
The PDI G-101 fixture flow rates in this calculator are the standard values used by plumbing engineers and inspectors across the US: 20 GPM for a 3-compartment sink, 15 GPM for a 2-compartment sink, 10 GPM for a single prep sink, 15 GPM for an under-counter commercial dishwasher, 20 GPM for a door-type commercial dishwasher, 2 GPM per floor drain, 3 GPM for a mop sink, 15 GPM for a pot/kettle fill sink, 5 GPM for a wok station, and 3 GPM for a coffee station. These values account for the simultaneous operation of all connected fixtures at peak flow, which is the worst-case loading condition the interceptor must handle.
Method 2: IPC Section 1003 for Gravity (Outdoor) Interceptors
The IPC sizing method starts with the same total fixture flow rate from the PDI G-101 table, then converts it to a gallon capacity. The retention time standard for gravity interceptors is 30 minutes: effluent must remain in the interceptor for at least 30 minutes to allow gravity separation of FOG from the wastewater. This gives the working volume in gallons (total GPM times 30 minutes). The total interceptor capacity equals the working volume divided by 0.75, because the PDI G-101 standard and most municipal FOG ordinances require that 25 percent of the interceptor’s liquid capacity be reserved for accumulated grease and solids before the unit needs pumping. Dividing working volume by 0.75 builds that 25 percent reserve into the design capacity.
The 25 Percent Rule and Pumping Frequency
The 25 Percent Rule is the operational standard for grease interceptor maintenance across virtually every US FOG compliance program, deriving from the EPA pretreatment program recommendations, the PDI G-101 standard, and the Canadian standard CSA B481.4. The rule states that the interceptor must be pumped when the combined depth of the floating FOG layer at the top plus the settled solids layer at the bottom reaches 25 percent of the total liquid depth of the unit. At that point, the effective treatment zone (the middle clear water section) has shrunk to the point where hydraulic short-circuiting allows FOG to pass through to the sewer without adequate separation.
How quickly you reach the 25 percent threshold depends almost entirely on the type of food being prepared and the volume of production. A coffee shop steaming milk and making sandwiches may reach the threshold in three to four months. A burger joint running four commercial fryers through lunch and dinner service may reach 25 percent in two to three weeks. The pumping interval estimate in this calculator uses general kitchen intensity categories validated against industry experience from commercial grease trap service providers. The intervals are starting points for service contract negotiation, not fixed schedules. Your service provider should measure actual grease and sludge depths at each pump-out visit and adjust the schedule accordingly.
PDI G-101 Fixture Flow Rate Reference and Standard Grease Interceptor Sizes
The fixture flow rates below are from the PDI Standard G-101 and the IPC Table 1003.3.4.1, the primary references for grease interceptor sizing in the US. These values represent the rated maximum flow for each fixture type under simultaneous operation conditions.
| Fixture Type | Flow Rate (GPM) | Notes | Common Quantity |
|---|---|---|---|
| 3-Compartment Sink | 20 GPM | Primary fixture in most US commercial kitchens; all compartments count as one rating | 1 to 2 per kitchen |
| 2-Compartment Sink | 15 GPM | Common in smaller operations; smaller than 3-comp but still primary fixture | 1 per kitchen |
| Prep Sink (single comp.) | 10 GPM | Vegetable prep, raw food handling; adds significant flow in high-volume operations | 1 to 3 per kitchen |
| Dishwasher (under-counter) | 15 GPM | Compact commercial unit; common in bars, coffee shops, smaller restaurants | 1 to 2 per kitchen |
| Dishwasher (door-type) | 20 GPM | Full-size commercial unit; higher flow due to larger wash chamber and booster heater | 1 per kitchen |
| Floor Drain | 2 GPM each | Each drain counts separately; common placement at cooking line, walk-in cooler exit | 2 to 6 per kitchen |
| Mop Sink | 3 GPM | Janitorial sink; lower flow but connects FOG from floor cleaning operations | 1 per kitchen |
| Pot / Kettle Fill Sink | 15 GPM | High-volume fill station for stock pots, steam kettles; significant periodic flow | 0 to 2 per kitchen |
| Wok Station / Range | 5 GPM | Wok ranges generate significant oil from high-heat cooking; may need drain | 0 to 4 per station |
| Coffee Station Drain | 3 GPM | Espresso machine drain, coffee urns; relatively low flow but connects in cafes | 0 to 4 per station |
Standard Grease Interceptor Sizes by Type
| Type | Standard Sizes | Sizing Basis | Typical Application | Code Reference |
|---|---|---|---|---|
| Hydromechanical (indoor) | 10, 15, 20, 25, 35, 50, 75, 100 GPM | Flow rate rating per PDI G-101 | Coffee shops, bars, small restaurants, tenant buildouts | PDI Standard G-101 |
| Gravity interceptor (outdoor) | 500, 750, 1,000, 1,250, 1,500, 2,000, 2,500, 3,000 gal | Gallon capacity per IPC 1003 | New restaurant construction, high-volume operations | IPC Section 1003 |
| Automatic grease removal unit | 25, 50, 100, 150, 200 GPM | Flow rate with motorized removal | High-volume operations where manual pumping is impractical | ASME A112.14.4 |
Three Real US Commercial Kitchen Grease Trap Sizing Scenarios with Complete PDI G-101 Calculations
These worked examples trace the complete sizing calculation from fixture count through final interceptor recommendation, using real kitchen configurations common in US food service operations.
A fast food franchise opens a new location in an Austin strip mall. The lease terms restrict outdoor excavation, so an indoor hydromechanical interceptor is required. The kitchen has one 3-compartment sink (20 GPM), two prep sinks (10 GPM each), two floor drains (2 GPM each), and one mop sink (3 GPM). The tenant coordinator needs a size for the plumbing permit submittal.
Total GPM: 20 + 10 + 10 + 2 + 2 + 3 = 47 GPM. Minimum hydromechanical rating: round up to 50 GPM (next standard size above 47). Texas TCEQ and Austin’s local FOG ordinance both accept PDI G-101 sizing for hydromechanical units in tenant buildouts. The licensed plumber specified a 50 GPM rated unit. Gravity equivalent: 47 GPM x 30 min = 1,410 gal working volume; total = 1,410/0.75 = 1,880 gal (not required here since indoor hydromechanical was used). Pumping: medium intensity fast food kitchen, every 60 days, 6 pump-outs per year at $350 avg = $2,100 annual FOG maintenance cost.
A new 120-seat full-service restaurant breaks ground in a Chicago suburb. New construction allows outdoor installation of a gravity interceptor in the parking lot. The kitchen has one 3-compartment sink (20 GPM), two prep sinks (10 GPM each), one door-type commercial dishwasher (20 GPM), three floor drains (2 GPM each), one mop sink (3 GPM), and one pot fill sink (15 GPM). The kitchen operates a 4-well fryer bank, placing it in the heavy FOG intensity category.
Total GPM: 20 + 10 + 10 + 20 + 2 + 2 + 2 + 3 + 15 = 84 GPM. Working volume: 84 x 30 = 2,520 gal. Total capacity: 2,520 / 0.75 = 3,360 gal. Round up to 3,500 gal (some suppliers) or two 2,000-gal units in series (4,000 gal total). Cook County Health requires a minimum 1,000-gal gravity interceptor for all new FSEs and accepts PDI G-101 calculations for sizing approval. Heavy FOG intensity from the fryer bank: pump every 30 days. Twelve pump-outs per year at $500 avg = $6,000 annual FOG maintenance cost. The developer budgeted this as an operating expense in the restaurant proforma.
A coffee shop and light lunch cafe opens in a Seattle commercial space. The menu focuses on espresso drinks, pastries, and cold sandwiches with no deep frying. The kitchen has one 2-compartment sink (15 GPM), one prep sink (10 GPM), one under-counter dishwasher (15 GPM), one floor drain (2 GPM), one mop sink (3 GPM), and two coffee station drains (3 GPM each).
Total GPM: 15 + 10 + 15 + 2 + 3 + 3 + 3 = 51 GPM. Round up to hydromechanical: 75 GPM (next standard size above 51). Gravity equivalent: 51 x 30 = 1,530 gal working; total = 1,530/0.75 = 2,040 gal (round to 2,000 gal). King County DNRP accepted the hydromechanical sizing for the indoor installation. Light FOG intensity from the pastry and cold prep menu: pump every 120 days. Three pump-outs per year at $275 avg = $825 annual maintenance cost. Seattle’s FOG ordinance requires the owner to maintain a pumping log with manifest records for each service visit, retained for three years and available on inspection.
Six FOG Compliance Practices That Keep US Restaurant Operators Out of Violation
Keep Three Years of Pumping Manifests On Site
Every grease interceptor pump-out in the US generates a waste manifest: a document showing the date, volume, hauler’s license number, and disposal site for each service visit. Most US FOG ordinances require food service establishments to retain these manifests for three years and make them available immediately during an inspection. Inability to produce manifests is an automatic violation in most jurisdictions, even if the interceptor is clean. Use a logbook or digital folder to store every manifest as it arrives. Your service provider should give you a copy after every pump-out. If they do not, request it in writing before authorizing future service.
Use a Sludge Judge to Measure Actual Fill Level
The pumping schedule this calculator estimates is a planning tool based on kitchen intensity categories. Actual FOG accumulation in your specific interceptor depends on your production volume, your menu’s fat content, your staff’s grease disposal practices, and your interceptor’s hydraulic efficiency. A sludge judge is a clear acrylic tube, typically 2 inches in diameter and 8 to 12 feet long, used to measure the depth of the floating grease layer at the top and the settled solids layer at the bottom of a gravity interceptor. It costs $25 to $60 and takes 60 seconds to use. Check levels monthly when first establishing your operation, then adjust your service contract frequency based on actual accumulation rate rather than relying only on a calendar schedule.
Train Kitchen Staff on Dry Wipe Before Wash
The single most effective operational practice for extending grease interceptor service intervals is training kitchen staff to dry-wipe all pans, pots, and cooking surfaces with a paper towel before washing. Grease scraped into the trash rather than washed down the drain never reaches the interceptor. In high-volume frying operations, proper dry-wipe training can reduce FOG loading to the interceptor by 30 to 50 percent, potentially doubling the time between pump-outs. This reduces annual maintenance costs and significantly lowers the risk of FOG breakthrough events that trigger city inspection visits and potential violations.
Never Use Biological Additives or Hot Water Grease Cutting
Grease emulsifiers, biological enzyme additives, and hot water flushing are three techniques that seem helpful but are prohibited by most US FOG ordinances and actively harmful to your compliance status. Emulsifiers and hot water break grease into fine droplets that pass through the interceptor in suspension and reach the sewer in a dispersed form that is harder to track but just as damaging to pipes. Biological enzyme additives create the appearance of a cleaner interceptor but actually dissolve the visible grease layer into the effluent stream, increasing downstream FOG loading. Using any of these products while a city inspector is monitoring your discharge can result in a violation even if the interceptor appears clean, because the FOG is now in the sewer line rather than captured in the trap.
Register Used Cooking Oil for Recycling, Not Sewer Disposal
Deep fryer oil represents the highest-concentration grease source in most commercial kitchens. When fryer oil is exhausted and changed, it must be placed in a properly labeled waste grease container and picked up by a licensed grease recycler. It must never be poured down any drain. Used cooking oil is valuable as biodiesel feedstock and many licensed recyclers will pick it up free of charge or even pay for large volumes. Pouring used fryer oil down a drain is both an environmental violation and a rapid path to grease interceptor failure. Some city FOG programs conduct random sampling of interceptor waste when grease recycling contracts are not on file and the grease load in the interceptor is inconsistently high, looking for evidence of fryer oil disposal in the drain.
Size Up One Standard Size When Budget Allows
The minimum interceptor size from this calculator is the code floor, not the optimal operating size. Selecting the next standard size above the minimum has two practical benefits. First, it extends the time between pump-outs by increasing the available grease storage volume, reducing the annual maintenance cost enough in most cases to pay for the larger unit within two to three years of operation. Second, it provides a buffer against business growth: adding a fryer, hiring a second dishwasher, or expanding the menu can easily push the actual FOG load above the design minimum of an interceptor sized exactly to code. Upgrading an undersized interceptor after the fact, including excavation and replacement of an outdoor gravity unit, costs $8,000 to $25,000 or more. Specifying one size up from the minimum during initial installation is the most cost-effective insurance against this outcome.
Quick Reference: Grease Interceptor Size by Kitchen Type and Typical Fixture Load
This reference table provides typical interceptor sizes for common US food service establishment types based on standard fixture configurations and the PDI G-101 method. All values represent the PDI-G101 calculated minimum. Local ordinances may require larger sizes.
| Kitchen Type | Typical Total GPM | Hydromechanical Size | Gravity Size | Est. Pump Interval | FOG Intensity |
|---|---|---|---|---|---|
| Coffee shop / cafe | 35 to 55 GPM | 35 to 75 GPM | 1,500 to 2,250 gal | 90 to 120 days | Light |
| Fast food / QSR | 40 to 60 GPM | 50 to 75 GPM | 1,600 to 2,500 gal | 45 to 60 days | Medium |
| Casual dining restaurant | 60 to 90 GPM | 75 to 100 GPM | 2,400 to 3,600 gal | 30 to 60 days | Medium to Heavy |
| Pizzeria | 40 to 65 GPM | 50 to 75 GPM | 1,600 to 2,600 gal | 45 to 90 days | Medium |
| School cafeteria | 80 to 120 GPM | 100 GPM | 3,200 to 4,800 gal | 30 to 45 days | Medium to Heavy |
| Full-service with fryer bank | 75 to 110 GPM | 75 to 100 GPM | 3,000 to 4,400 gal | 14 to 30 days | Heavy to Very Heavy |
| Hospital / institutional kitchen | 100 to 160 GPM | 100 GPM+ | 4,000 to 6,400 gal | 14 to 30 days | Heavy to Very Heavy |
Common Questions About Grease Trap Sizing, Pumping Frequency, and FOG Ordinance Compliance
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Grease trap sizing completes the Wastewater Hub workflow. Use these tools for the rest of your septic system or plumbing project.
This calculator and all content on this page are provided for educational planning purposes only. Hydromechanical sizing uses PDI Standard G-101 fixture flow rates (Plumbing and Drainage Institute). Gravity interceptor sizing uses the IPC Section 1003 method: working volume = total fixture GPM x 30 minutes; total capacity = working volume / 0.75 to account for the 25% grease storage reserve per PDI G-101 and EPA FOG program guidance. Pumping frequency estimates are based on general kitchen intensity categories and are planning guides only, not compliance schedules.
FOG compliance is regulated at the municipal level through local wastewater ordinances derived from the EPA National Pretreatment Program (40 CFR Part 403). The federal enforcement hook is 40 CFR Section 403.5(b)(3). Local ordinances frequently require interceptor sizes, approved products, and pumping schedules that exceed the national minimum. Always verify sizing requirements with your local authority having jurisdiction (AHJ) and licensed plumbing contractor before purchasing or installing any grease interceptor. Fine ranges cited reflect publicly available municipal ordinance data as of 2026 and are for reference only.
USCalculators.com has no affiliation with the Plumbing and Drainage Institute, the EPA, any plumbing code body, or any interceptor manufacturer or service provider referenced herein. See the full Wastewater Design Hub for all five tools in the onsite system design workflow.