Grease interceptor sizing is determined by the Plumbing and Drainage Institute (PDI) formula: flow rate x retention time x storage factor. Most full-service restaurants with 100+ seats need a 1,000-1,500 gallon exterior interceptor. Undersizing is the most expensive mistake in FOG compliance.
Why Sizing Matters
An undersized grease interceptor fills to overflow capacity 2-3x faster than a properly sized one, doubling pump-out costs and increasing the risk of FOG discharge violations. GreaseTrapDispatch internal data shows that 38% of emergency pump-out calls come from facilities with undersized interceptors.
Grease interceptor sizing is not a "pick the cheapest option" decision. An interceptor that is too small will overflow between scheduled pump-outs, discharge FOG into the sewer, and put you in violation of your discharge permit. An interceptor that is too large wastes money on installation and pump-out volume.
The correct size is determined by a formula that accounts for your kitchen's actual flow rate, the type of food you prepare, and the retention time needed for grease separation.
The PDI Sizing Formula
The industry standard for sizing grease interceptors is the PDI G101 formula:
Interceptor Size (gallons) = Flow Rate (GPM) x Retention Time (minutes) x Storage Factor
Calculating Flow Rate:
Flow rate is based on the kitchen fixtures that drain into the interceptor:
| Fixture | Flow Rate (GPM) | |---------|----------------| | Three-compartment sink | 15 GPM | | Pre-rinse spray station | 5 GPM | | Commercial dishwasher | 15-25 GPM | | Floor drain (per drain) | 3 GPM | | Mop sink | 5 GPM | | Wok station | 10 GPM |
Example calculation: A restaurant with a 3-compartment sink, pre-rinse station, commercial dishwasher, and 2 floor drains:
- Flow Rate = 15 + 5 + 20 + 6 = 46 GPM
- Size = 46 x 30 x 2.0 = 2,760 gallons
- Recommended interceptor: 3,000-gallon exterior unit
Interior Traps vs. Exterior Interceptors
The sizing formula determines which type of system you need:
The installation decision is permanent (or at least very expensive to change). Consult with a licensed plumber who specializes in commercial kitchen plumbing before committing to a system type. Your grease trap service provider can also advise based on their experience with similar facilities in your area.
Installation and Ongoing Cost Analysis
| Component | Interior Trap | Exterior Interceptor (1,000 gal) | Exterior Interceptor (2,000 gal) | |-----------|--------------|----------------------------------|----------------------------------| | Unit cost | $200 - $800 | $2,000 - $5,000 | $4,000 - $8,000 | | Installation | $500 - $1,500 | $3,000 - $8,000 | $5,000 - $12,000 | | Pump-out cost (per visit) | $75 - $150 | $200 - $400 | $300 - $600 | | Pump-out frequency | Weekly-Monthly | Every 60-90 days | Every 90-120 days | | Annual maintenance cost | $4,000 - $7,800 | $800 - $2,400 | $1,200 - $2,400 |
Interior traps have lower upfront costs but much higher annual maintenance because of the frequent cleaning required. Exterior interceptors cost more to install but are serviced less frequently and handle higher volumes without risk of overflow.
For any restaurant with more than 50 seats, the exterior interceptor is almost always the better long-term investment.
Related reading: FOG Compliance 101 | Pump-Out Procedures: Step-by-Step
