Grease Trap Sizing Calculator
Find the correct interceptor size for your commercial kitchen based on fixture count, flow rate, and plumbing code requirements. Covers UPC, IPC, and PDI sizing methods.
Get the Right Size Before You Buy or Permit
Undersized grease traps fail inspections, back up during peak service, and attract FOG compliance violations. Oversized traps are a waste of capital and still require the same cleaning frequency if not used properly. Sizing a grease interceptor correctly the first time is the cheapest decision you'll make in your kitchen's infrastructure.
This calculator uses the Plumbing and Drainage Institute (PDI) G-101 method and cross-references UPC and IPC fixture flow rates to give you a recommended minimum capacity. Enter your fixture types and count to get a sizing recommendation with the calculation shown step by step.
Important: This tool provides a code-based minimum size recommendation. Your authority having jurisdiction (AHJ) — typically the local building or health department — may require a larger size or a specific approved product. Always confirm with your local FOG ordinance before purchasing or permitting.
Calculate Your Required Trap Size
Enter the number of each fixture type connected to your grease trap. Leave unused fixtures at zero.
Sizing Recommendation
Total Flow Rate
0 GPM
Gallons per minute from all fixtures
Minimum Trap Size
0 gal
PDI/UPC/IPC code minimum capacity
Suggested Install Size
0 gal
Next standard size up for operational buffer
Calculation Breakdown
Next Steps & Considerations
How Grease Trap Sizing Works
The PDI G-101 standard — the basis for most US municipal FOG ordinances — sizes grease interceptors based on the maximum flow rate of all connected fixtures, with a retention time factor applied. The core formula: Trap Size (gallons) = Total Flow Rate (GPM) × Retention Time (minutes). Standard retention time is 30 minutes for passive traps and interceptors under 1,000 gallons; some jurisdictions require 45 minutes for larger units.
Each fixture has a rated drainage flow rate. A 3-compartment sink drains at approximately 20 GPM under full flow. A commercial dishwasher contributes 15 GPM. Floor drains are typically rated at 2 GPM for sizing purposes. The calculator above applies these PDI-standard flow rates and multiplies by 30-minute retention to give you a code-minimum capacity.
The UPC and IPC methods are variants that apply slightly different fixture unit values and retention multipliers — the results are usually within 10–15% of each other. If your jurisdiction has adopted the UPC (common in California and western states) or IPC (common in eastern states and Canada), use that method's result.
Always size up to the next standard manufactured size. Standard interceptor capacities run at 500, 750, 1,000, 1,500, 2,000, 3,000, and 5,000 gallons. If your calculation yields 1,200 gallons, you need a 1,500-gallon unit — never install below the calculated minimum.
How to Calculate Grease Trap Size: Step-by-Step
Grease trap sizing is based on the PDI G-101 standard, which most US municipalities adopt in their FOG ordinances. The formula is straightforward:
- List every fixture that drains to the grease trap: 3-compartment sinks, prep sinks, dishwashers, floor drains, and mop sinks.
- Look up the GPM flow rate for each fixture type (PDI standard rates: 3-compartment sink = 20 GPM, commercial dishwasher = 15 GPM, prep sink = 10 GPM, floor drain = 2 GPM).
- Add all GPM values together to get your total flow rate.
- Multiply by 30 minutes (standard retention time) to get the minimum trap size in gallons.
- Round up to the next standard manufactured size (50, 100, 250, 500, 750, 1,000, 1,500 gallons).
Example: A kitchen with a 3-compartment sink (20 GPM), one prep sink (10 GPM), and a commercial dishwasher (15 GPM) has a total flow of 45 GPM × 30 min = 1,350 gallons minimum — round up to a 1,500-gallon interceptor.
| Kitchen Type | Typical Fixtures | Est. Flow (GPM) | Recommended Trap Size |
|---|---|---|---|
| Coffee shop / Café | 2-comp sink, floor drain | 12–18 GPM | 500–750 gallons |
| Fast food / Quick service | 3-comp sink, dishwasher, 2 floor drains | 40–55 GPM | 1,500–2,000 gallons |
| Full-service restaurant | 3-comp sink, 2 dishwashers, prep sink, floor drains | 55–75 GPM | 2,000–2,500 gallons |
| Hotel / Banquet kitchen | Multiple sinks, conveyor dishwasher, multiple drains | 80–120 GPM | 2,500–4,000 gallons |
| Hospital / Institutional | Central dishroom, multiple prep areas | 100–150 GPM | 3,000–5,000 gallons |
| Ghost kitchen / Dark kitchen | 3-comp sink, dishwasher, floor drain | 35–50 GPM | 1,000–1,500 gallons |
Use the calculator above for a precise calculation based on your actual fixture count. The table above is a planning reference only — your local plumbing code may require additional safety margins.
Not sure what size you need or want to verify your calculation? Grease trap cleaning costs also vary by trap size — larger interceptors cost more to service, so getting the size right matters for your long-term budget.
Grease Trap Sizing — Common Questions
Answers based on PDI G-101, UPC, IPC, and common municipal FOG ordinance requirements.
Passive under-sink grease traps (PDI-rated, typically 7–50 gallons) are only approved for single-fixture applications — usually one or two-compartment prep sinks with low grease output. High-volume commercial kitchens with 3-compartment sinks, dishwashers, and floor drains almost universally require an outdoor in-ground grease interceptor. Check your local FOG ordinance — many municipalities specify minimum trap capacity thresholds that effectively require outdoor interceptors for any food service operation above a certain size.
It depends on your jurisdiction. Many municipalities and plumbing codes require commercial dishwashers to be routed through the grease trap because pre-wash cycles carry significant FOG. Some jurisdictions exempt high-temperature sanitizing dishwashers on the grounds that the elevated water temperature emulsifies grease before it reaches the drain. If your dishwasher uses a chemical sanitizing cycle (rather than high-temperature), it almost certainly needs to be connected to the trap. Confirm with your local health department.
A typical sit-down restaurant (50–150 seats) with a 3-compartment sink, commercial dishwasher, pre-rinse sink, and two floor drains typically calculates to a 1,000–1,500 gallon interceptor under PDI G-101. Fast food operations with high-volume fryers often size up to 2,000–3,000 gallons. Institutional kitchens (hospitals, schools, cafeterias) frequently require 3,000–5,000 gallon units. Use the calculator above with your actual fixture count for a precise recommendation.
Yes — but not in the way most people expect. A larger trap doesn't mean less frequent cleaning by default. Cleaning frequency is governed by the 25% rule: clean when FOG and solids reach 25% of liquid capacity. A larger trap holds more waste before hitting that threshold, which can extend intervals — but only if your kitchen's FOG output doesn't increase proportionally. Oversized traps in low-output kitchens can actually develop odor and anaerobic activity if cleaned too infrequently. Size for your actual fixture load, not for the longest possible cleaning interval.
An undersized trap fills faster than the 25% rule can be effectively applied, meaning FOG passes through into the municipal sewer system. This can result in FOG compliance violations, fines, and in serious cases, contribution to sewer blockages that municipalities trace back to the contributing business. Some jurisdictions conduct downstream sewer camera inspections to identify FOG sources — a failing business can face mandatory retrofitting at its own expense plus fines for the period of non-compliance.
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