Skip to main content
Foodservice Free Pro Features Available

Grease Interceptor / Trap Sizing Calculator

GPM Flow Rate and Grease Retention per IPC/UPC for Hydromechanical Grease Interceptors and Gravity Grease Interceptors

Free grease interceptor sizing calculator for plumbers, mechanical engineers, and foodservice consultants who need to determine the correct size grease interceptor for commercial kitchen waste lines. Enter the number of fixtures, fixture flow rates, and retention time, and the calculator returns the required GPM rating for hydromechanical (under-sink) interceptors or the required gallon capacity for gravity (in-ground) interceptors per IPC Section 1003 and PDI-G101 standards.

Grease interceptors are required by virtually every plumbing code for any commercial kitchen that discharges to a public sewer. Undersized interceptors cause grease to pass through to the municipal sewer, resulting in blockages, fines from the local POTW, and potential surcharge fees. Oversized interceptors waste money on equipment and excavation, and can actually perform worse because the lower flow velocity allows solids to settle and reduces the separation efficiency. The IPC sizing method uses a drainage fixture unit (DFU) approach converted to GPM, while some jurisdictions use a fixture-count method or the PDI-G101 flow rate test standard for hydromechanical interceptors.

The calculator covers both hydromechanical grease interceptors (HGIs, typically 20-100 GPM, installed under sinks or in the floor) and gravity grease interceptors (GGIs, typically 500-2500 gallons, installed in-ground outside the building). It shows the recommended size, the retention time, and whether a point-of-use interceptor or a centralized interceptor is more appropriate for your installation.

Pro Tip: Many municipalities have their own grease interceptor sizing formulas that override the IPC or UPC method. Always check the local FOG (fats, oils, and grease) ordinance before finalizing your size. Some cities require interceptors 50-100% larger than the code minimum, and many now mandate gravity interceptors for any new restaurant regardless of fixture count.

Size the exhaust hood serving the same kitchen

Commercial Hood Exhaust Calculator →

Calculate detention time for water treatment applications

Detention Time Calculator →

Determine fixture unit counts for your plumbing system

Fixture Unit Calculator →

PREVIEW All Pro features are currently free for a limited time. No license key required.

Grease Interceptor / Trap Sizing Calculator
Pop Out

How It Works

  1. Select Interceptor Type

    Choose hydromechanical (HGI, under-sink or floor-mounted, rated in GPM) or gravity (GGI, in-ground, rated in gallons). Hydromechanical interceptors are typically used for individual fixtures or small kitchens; gravity interceptors serve entire facilities.

  2. Enter Connected Fixtures

    List each fixture connected to the interceptor: 3-compartment sinks, pre-rinse stations, dishwashers, floor drains in the cooking area, and prep sinks. Enter the number of each type and its drainage fixture unit (DFU) value or rated GPM flow.

  3. Set Flow and Retention Parameters

    For HGIs, the calculator totals the peak GPM from all connected fixtures. For GGIs, enter the estimated daily water usage and select the retention time multiplier (typically 24-hour or 30-minute, depending on the jurisdiction's FOG ordinance).

  4. Review Sizing Results

    The calculator returns the minimum GPM rating (HGI) or gallon capacity (GGI), the retention time, and the next standard interceptor size available from major manufacturers. Verify this meets the local FOG ordinance requirements before specifying.

Built For

  • Plumbers sizing under-sink grease traps for small cafes and food trucks with limited fixture counts
  • Mechanical engineers specifying in-ground gravity interceptors for new restaurant construction or tenant buildouts
  • Foodservice consultants preparing plumbing specifications for chain restaurant prototype packages
  • Property managers determining whether an existing interceptor is large enough when a tenant changes from a bakery to a full-service restaurant

Features & Capabilities

IPC and PDI-G101 Sizing Methods

Supports the IPC DFU-to-GPM conversion method and the PDI-G101 flow rate standard for hydromechanical interceptors. Shows both methods so you can verify which one the local AHJ requires.

Gravity Interceptor Capacity Calculation

Calculates the required gallon capacity for in-ground gravity interceptors based on peak flow rate and retention time, with standard sizes from 500 through 2500 gallons.

Fixture Library

Built-in fixture database with standard DFU values and flow rates for 3-compartment sinks, pre-rinse sprays, commercial dishwashers, mop sinks, floor drains, and prep sinks per IPC Table 709.1.

Retention Time Check

Verifies that the selected interceptor provides adequate retention time for grease separation, typically 24-30 minutes for HGIs and several hours for GGIs, depending on the jurisdiction.

Frequently Asked Questions

The terms are often used interchangeably, but technically a grease trap is a small (usually under 100 GPM) hydromechanical device installed under or near a fixture, while a grease interceptor is a larger gravity-based unit (500+ gallons) installed in-ground outside the building. The IPC uses the term "hydromechanical grease interceptor" (HGI) for the smaller devices and "gravity grease interceptor" (GGI) for the larger ones. Both separate grease from wastewater by slowing the flow and allowing grease to float to the surface for periodic removal.
It depends on the jurisdiction. Many codes prohibit connecting dishwashers to grease interceptors because the hot water (typically 150-180 F) melts the accumulated grease and pushes it through the interceptor into the sewer, defeating the purpose. Some jurisdictions allow dishwasher connections to gravity interceptors only, not to hydromechanical devices. Always check local requirements — this is one of the most commonly cited plumbing inspection failures in commercial kitchens.
Hydromechanical interceptors (under-sink) should be cleaned weekly to monthly depending on usage volume. Gravity interceptors (in-ground) are typically cleaned on a 30-90 day schedule by a licensed hauler. Most FOG ordinances require cleaning when the interceptor reaches 25% of its grease storage capacity. Many municipalities now require maintenance logs and hauler manifests as a condition of the grease interceptor permit.
Yes, a single gravity interceptor can serve all kitchen fixtures. This is the most common approach for new restaurant construction — all grease-producing fixtures connect to a common waste line that flows to an in-ground gravity interceptor before connecting to the building sewer. For tenant buildouts in existing buildings where excavation is not feasible, individual hydromechanical interceptors under each fixture group are used instead.

Learn More

Foodservice

Grease Trap & Interceptor Sizing Guide

How to size grease interceptors and traps per IPC standards using sink compartment volumes, drain times, and flow rate calculations.

Read Guide →
Foodservice

Sanitizer Dilution & Health Code Compliance

How to calculate exact sanitizer dilutions for chlorine, quat, and iodine using C1V1=C2V2, with FDA Food Code PPM ranges and contact times.

Read Guide →

Related Tools

Foodservice Live

Commercial Hood Exhaust Sizing Calculator

Size commercial kitchen exhaust hoods per IMC 507. Assigns duty classification from appliance types and calculates required CFM by hood style.

Launch Tool →
Foodservice Live

Walk-In Cooler/Freezer Heat Load Calculator

Calculate refrigeration load using the four-load method: transmission, product, internal, and infiltration. Returns BTU/hr and tonnage.

Launch Tool →
Foodservice Live

Sanitizer Dilution & Health Code Checker

Calculate exact sanitizer concentrate amounts using C1V1=C2V2. Supports chlorine, quat, and iodine with FDA Food Code compliance check.

Launch Tool →