Home / Trade / Plumbing / Grease Trap Sizing

Grease Trap Sizing

Calculate the required Gallons Per Minute (GPM) flow rate and holding capacity in pounds for a commercial restaurant grease interceptor.

Sink Parameters

Number of Compartments

Bowls

Bowl Length

Bowl Width

Bowl Depth

Drain Time

Required Interceptor Specs

Required Flow Rate

84.2

GPM

Minimum Trap Capacity

168.3

Lbs

Gross Volume (in³)	25,920
Gross Sink Gallons	112.2 gal
75% Operating Capacity	84.2 gal

Email Link Text/SMS WhatsApp

What is Hydro-Mechanical Grease Traps?

Fats, Oils, and Grease (FOG) wreak havoc on city sewage systems by congealing into massive 'fatbergs'. Commercial kitchens must implement internal hydro-mechanical Grease Traps inline before their wastewater hits the municipal sewer. These traps act as speed bumps: they slow the hot, greasy water down long enough for the oils to float to the top and the solid food particles to sink, allowing only the clear wastewater in the middle to escape.

Mathematical Foundation

Total Sink Volume (Gallons)

\text{Gallons} = \frac{L \times W \times D \times C}{231}

L, W, D

= Inside dimensions (Length, Width, Depth) of a single sink bowl in inches.

C

= Total number of identically sized compartments.

231

= Constant: The number of cubic inches in exactly one U.S. Gallon.

Required System Flow Rate

\text{GPM} = \frac{\text{Gallons} \times 0.75}{\text{Drain Time}}

\text{GPM}

= Gallons Per Minute. The structural flow rating the trap must handle.

0.75

= Standard operating fill capacity (sinks are assumed to be filled 75% due to dish displacement).

\text{Drain Time}

= Target time to empty the sink (1 or 2 minutes standard).

Grease Capacity Requirement

Capacity_{lbs} = \text{GPM} \times 2

Capacity_{lbs}

= The physical holding capacity of congealed grease in pounds.

\text{GPM}

= The previously calculated flow rate.

2

= PDI standard ratio. A trap must hold 2 lbs of grease for every 1 GPM of flow rating.

Laws & Principles

Volume to Gallons Rule: 231 cubic inches equals exactly 1 liquid gallon. This is a universally essential plumbing math constant used to convert physical box dimensions into fluid capacity.
The 75% Displacement Rule: When sizing traps based on sink bowl dimensions, you multiply the raw volume by 75% to achieve 'Operating Capacity'. Plumbers assume normal washing scenarios leave headroom, and submerged dishes displace water.
Drain Time Surges: Standard code assumes an entire sink will be drained in 1 to 2 minutes. The faster the drain time configured, the larger the trap you need, because the trap must process the GPM surge without washing the grease cap down the drain.
The 2-to-1 Pound Rule: According to the Plumbing and Drainage Institute (PDI), the grease retaining capacity in physical pounds shall be at least twice the flow rating in GPM. (e.g., A 50 GPM rated trap must securely hold 100 lbs of grease before its efficiency drops below 90%).

Step-by-Step Example Walkthrough

" A plumbing engineer is sizing a trap for a heavy-duty 3-compartment pot sink in a cafeteria. Each bowl measures 24-inch long, 24-inch wide, and 15-inch deep. Draining time is modeled for 2 minutes to save under-counter space. "

1. Total Volume in Cubic Inches: 24 × 24 × 15 × 3 compartments = 25,920 in³.
2. Convert to US Gallons: 25,920 ÷ 231 = 112.2 Gallons total capacity.
3. Apply Operating Displacement: 112.2 × 0.75 = 84.15 Gallons of actual draining fluid.
4. Calculate Peak Flow Rate: 84.15 Gallons ÷ 2 minutes = 42.07 GPM flow rate.
5. Apply PDI Multiplier: 42.07 GPM means 84.14 lbs of required minimum grease capacity.

Final Result: To legally handle the 42 GPM flow rate, the kitchen must install at least a 50 GPM rated interceptor with a minimum 100 lb grease holding capacity, as PDI sizes are manufactured in standard discrete tiers.

Quick Answer: How do you size a Commercial Grease Trap?

Sizing a commercial grease trap involves calculating the total fluid volume of all connected sink compartments, multiplying by 75% to account for displacement, and dividing by a required drain time (usually 1 or 2 minutes) to get the required GPM flow rating. The trap must then be structurally capable of holding 2 pounds of grease for every 1 GPM of its flow rating to comply with PDI-G101 standards.

Core Trap Sizing Equations

GPM to Pound Conversion

Total_Cubic_Inches = L × W × D × Count
Max_Drain_Gallons = (Total_Cubic_Inches / 231) × 0.75
Flow_Rate (GPM) = Max_Drain_Gallons / Target_Minutes
Required_Retainage (Lbs) = Flow_Rate × 2

Any decimal flow rate calculation must be rounded UP to the nearest commercially available trap size (e.g., 22 GPM rounds up to a 25 GPM trap).

Real-World Scenarios

✓ The Coffee Shop Workaround

A small mall kiosk coffee shop needed to install a 3-compartment sink, generating an calculated flow rate of 38 GPM on a 1-minute drain time limit. The required 50 GPM/100 lb trap physically could not fit under their counters. The plumber legally solved this by consulting the master inspector and installing a certified flow-restrictor valve set to exactly 20 GPM ahead of the drain inlet. By intentionally limiting the flow, the water took longer to drain, but the shop was legally permitted to use a compact 20 GPM grease trap.

✗ The Dishwasher Bypass Failure

A restaurant renovation piped their high-temp commercial dishwasher directly into a standard 35 GPM under-sink grease trap. The dishwasher was dumping water at 160°F. The extreme heat essentially emulsified the grease layer inside the trap, turning the 70 pounds of stored fat back into liquid oil, which washed straight out into the sewer. Standard internal traps cannot handle high-temp dishwasher effluent without specialty cooling mechanisms or bypass routing to massive external ground interceptors.

Common Grease Trap Capacities

Trap Flow Rating (GPM)	Required Grease Capacity (Lbs)	Typical Application Profile
10 GPM	20 Lbs	Small handwash sinks, single prep basins in coffee shops.
20 GPM	40 Lbs	Medium 2-compartment pot sinks, standard fast-food prep lines.
35 GPM	70 Lbs	Standard 3-compartment restaurant sink (2-min drain).
50 GPM	100 Lbs	Large 3 or 4-compartment commercial cafeteria sinks.
100 GPM	200 Lbs	Extreme volume. Usually requires multi-trap manifolds or external interceptors.

Note: Sizing rules change dramatically when calculating by DFU (Drainage Fixture Units) rather than direct sink bowl volume. Always verify the required methodology with the local Authority Having Jurisdiction (AHJ).

Pro Tips & Common Mistakes

Do This

✓Verify the flow control valve. Most small indoor traps require a flow-control fitting installed directly upstream of the trap inlet. It bottlenecks the water flow so the trap is never overwhelmed by a massive surge from a filled sink dropping all at once.
✓Account for clear head space. When installing an under-counter trap, you must leave enough vertical and frontal clearance for the business owner to open the lid and scoop out the grease layer daily or weekly.

Avoid This

✗Don't pipe floor drains into hydro-mechanical traps. Floor drains generate a massive volume of water during washdowns that have very little grease content. This clean water surge violently flushes out any grease currently sitting in the trap.
✗Never assume 1 minute drain time is required. A 1-minute drain time requires massive trap footprints. If space is tight, verify if code explicitly permits sizing for a 2-minute drain out, which slashes the GPM requirement in half.

Frequently Asked Questions

What is the 25% Rule for grease traps?

The 25% Rule states that a grease trap must be professionally pumped and cleaned when the combined volume of the floating grease layer and the sunken sludge on the bottom equals 25% of the trap's total liquid capacity. Beyond 25%, the trap loses its certified efficiency and actively spills grease into the city sewer.

What is the difference between a grease trap and a grease interceptor?

While practically identical in function, 'Grease Traps' (Hydromechanical Grease Interceptors) are typically smaller units (under 100 GPM) installed indoors directly next to or under the sinks. 'Gravity Grease Interceptors' are massive, thousands-of-gallons concrete tanks buried outdoors to treat the entire building's effluent at once.

Do I need a trap if I only serve coffee and baked goods?

Yes. Coffee beans contain surprisingly high amounts of oil, and baked goods dairy products (milk, cream) are heavily fat-based. Coffee shops are notorious for causing fatberg blockages if they bypass grease trap requirements on their wash sinks.

Are garbage disposals allowed upstream of the trap?

Typically, no. Food Waste Disposers dump massive quantities of pulverized solid sludge into the piping. This sludge will instantly fill up a small indoor grease trap. Some areas permit it only if a dedicated solids-separator is installed between the disposer and the trap.