Grease Trap Size Calculator
Determines the required grease trap (interceptor) capacity in gallons based on the number of connected fixtures, their individual flow rates, and the required grease retention time. Used by plumbers and kitchen designers for code-compliant commercial kitchen installations.
About this calculator
A grease trap captures fats, oils, and grease (FOG) before wastewater enters the municipal sewer, preventing blockages and environmental violations. Sizing is governed by the Uniform Plumbing Code (UPC) method: multiply the total fixture flow rate by the time water must be retained in the trap for grease to separate and float. The formula is: Capacity (gallons) = fixtures × flowPerFixture × retentionTime, where flowPerFixture is in gallons per minute (GPM) and retentionTime is in minutes. The result is the minimum trap volume in gallons. Longer retention times allow more complete FOG separation. Local health and plumbing codes typically specify a minimum retention time of 2 minutes, and restaurants are commonly required to size traps at 20–50 gallons or more depending on kitchen output.
How to use
A commercial kitchen has 3 sinks, each with a flow rate of 20 GPM, and the local code requires a 2-minute retention time. Step 1 — fixtures × flowPerFixture: 3 × 20 = 60 GPM total flow. Step 2 — multiply by retention time: 60 × 2 = 120 gallons. The required grease trap capacity is 120 gallons. If the inspector requires a retention time of 2.5 minutes instead, the result becomes 3 × 20 × 2.5 = 150 gallons, so you would select the next standard trap size at or above that figure.
Frequently asked questions
How do I determine the correct retention time for sizing a grease trap?
Retention time is the number of minutes wastewater must remain inside the trap for grease to separate from the water and float to the surface. Most plumbing codes and the UPC specify a minimum retention time of 2 minutes for commercial applications. Jurisdictions with stricter FOG ordinances may require 2.5 or even 3 minutes. Always consult your local authority having jurisdiction (AHJ) before finalizing the design, as undersizing can result in failed inspections, fines, or mandatory trap upgrades after installation.
What happens if a grease trap is undersized for a commercial kitchen?
An undersized trap fills with accumulated grease faster than it can be pumped, causing FOG to bypass the trap and enter the sewer. This leads to sewer line blockages, foul odors, and potential sanitary sewer overflows (SSOs) that carry significant municipal fines. Health departments can issue closure orders for restaurants found discharging excess FOG. Beyond compliance, grease blockages in private drain lines cause expensive emergency plumber calls and can trigger property damage from sewer backups into the kitchen.
What is the difference between a grease trap and a grease interceptor?
A grease trap is a smaller, passive device installed under the sink or nearby, typically handling flows up to 50 GPM and requiring manual cleaning every one to three months. A grease interceptor is a large in-ground tank — often 500 to 2,000 gallons — installed outside the building and pumped by a licensed waste hauler. Interceptors are required when total fixture flow exceeds what a trap can handle or when codes mandate underground installation. This calculator covers both; the required capacity output tells you which category applies based on your local code's thresholds.