How to Size Grease Trap for a Commercial Kitchen

A grease trap that is too small fills quickly during busy service. One that is oversized can add unnecessary space and installation requirements. Knowing how to size grease trap capacity correctly starts with the actual wastewater produced by your kitchen, not simply the number of seats or appliances on the menu.

For restaurants, hotels, cafeterias, bakeries, and central kitchens, grease trap sizing should be part of the kitchen design process. It affects drainage routing, floor layout, cleaning access, plumbing coordination, and the consistency of daily operations.

Start With the Right Type of Grease Control Unit

The term “grease trap” is often used for several different products. A small passive unit installed below a sink is commonly called a grease trap. A larger external or in-ground unit is generally described as a grease interceptor. Both separate fats, oils, and grease from wastewater before it moves into the drainage system, but their capacities and installation methods differ.

A compact under-sink grease trap can suit a light-duty prep sink, coffee station, or small wash-up point with limited grease discharge. A full commercial kitchen with pot washing, combi ovens, wok stations, fryers, dishwashers, and high-volume food preparation will usually require a larger interceptor or a properly engineered grease management system.

The correct choice depends on peak discharge. A kitchen may have modest average use but still release a large amount of warm, grease-bearing water in a short period when pots are washed after service. Size for that peak period, not for the quiet hours between shifts.

How to Size Grease Trap Capacity From Flow Rate

The most common sizing approach starts with the flow rate from the fixtures connected to the unit. This means calculating how much water can reach the grease trap during the busiest drainage period.

For a manual calculation, first determine the volume of each connected sink compartment. Measure the internal length, width, and water depth in inches, then multiply the three figures. Divide the result by 231 to convert cubic inches to gallons.

For example, a three-compartment sink with each bowl measuring 18 inches by 18 inches by 12 inches has a total volume of:

`18 × 18 × 12 × 3 ÷ 231 = approximately 50.5 gallons`

That figure represents the sink’s full water volume. In practice, sinks are not always filled to the rim, so the usable volume and local calculation method should be checked with the plumbing design team or authority having jurisdiction.

Next, divide the estimated wastewater volume by the drainage time in minutes. If the 50.5-gallon sink drains in one minute, the flow rate is approximately 50.5 gallons per minute, often written as GPM. The grease trap must be rated to handle at least that flow rate, with the required grease storage capacity.

Many commercial designs also apply a flow-rate formula based on fixture capacity, drainage period, and a loading factor. The exact multiplier can vary by code, equipment type, and local approval requirements. That is why a rated unit should never be selected from a formula alone without confirming the installation standard that applies to the project.

Collect the Kitchen Data Before Selecting a Unit

Accurate sizing needs more than a sink measurement. The kitchen consultant, plumbing contractor, and operations team should review the full grease-producing workflow before a unit is specified. Gather these details early:

• Number, dimensions, and drainage times of pot sinks, prep sinks, floor sinks, and wash-up fixtures

• Dishwasher type, discharge rate, and whether its wastewater is permitted to connect to the grease control system

• Menu style and production volume, including frying, grilling, roasting, sauce preparation, and high-fat food handling

• Expected peak service periods, shift pattern, and the number of wash-up cycles during each peak

• Available installation space, pipe routing, ventilation, access for servicing, and location of the final drainage connection

This information helps prevent a common planning gap: selecting a unit based only on the three-compartment sink while overlooking other sources of discharge. Floor drains near cooking equipment, tilt pans, combi ovens, and pot wash areas may require careful drainage planning depending on the kitchen layout and applicable requirements.

Consider Menu and Production Load, Not Just Seating

Two kitchens with the same number of seats may need very different grease trap capacities. A café focused on beverages and light bakery items has a different grease load than a restaurant producing fried foods, grilled meats, rich sauces, and large-batch stock.

High-output kitchens also create more frequent wash-down activity. A hotel banquet kitchen may have a moderate service period followed by concentrated cleaning of cookware, trays, and utensils. A delivery-focused operation may produce repeated wash-up peaks throughout the day. These operating patterns influence the practical storage capacity needed between cleaning visits.

The goal is not simply to install the biggest unit that fits. It is to match flow handling, grease retention, and maintenance access to the kitchen’s working rhythm. A correctly selected interceptor supports predictable servicing and gives the drainage system a more stable operating load.

Check Equipment Connections Carefully

Not every fixture should automatically connect to a grease trap. Connections depend on the fixture, water temperature, detergents used, drainage design, and local plumbing rules. Commercial dishwashers, for example, can release hot water and chemical detergents that may affect grease separation performance. Their connection arrangement must be reviewed as part of the approved plumbing design.

Food waste should also be managed before it reaches the grease unit. Screens, strainers, and solids-handling practices reduce the amount of food debris entering the interceptor. Grease traps are designed to separate fats, oils, and grease, not to replace a complete solids management process.

Where food waste equipment is planned, coordinate it with the grease management strategy from the beginning. The position of the unit, drainage fall, inspection access, and cleaning route should work together rather than being treated as separate construction items.

Allow for Installation Access and Maintenance

Capacity is only one part of proper grease trap sizing. The unit must be installed where it can be inspected and cleaned safely. A compact internal trap may be appropriate where kitchen space is limited and service access is straightforward. Larger external interceptors can offer greater capacity, but they require suitable site space, accessible covers, and coordination with civil and plumbing works.

Do not place a unit where access will be blocked by fixed equipment, storage racks, or finished flooring. The cleaning team needs enough room to remove covers, inspect internal baffles, and complete routine service without interrupting the kitchen layout.

Maintenance frequency should be based on actual accumulation and the unit manufacturer’s guidance, then adjusted for menu volume and operating hours. Regular records are useful for facilities teams because they show whether the chosen size and service schedule match real kitchen demand.

Confirm Local Requirements Before Final Approval

Grease trap sizing is often subject to municipal, utility, health, and plumbing requirements. These can specify minimum flow rates, grease storage capacity, installation locations, pipe sizing, sampling points, venting, and maintenance procedures.

For commercial kitchen projects in the UAE, the final selection should be coordinated with the approved plumbing drawings and the relevant local authority requirements. This is especially important for hotels, food courts, central production facilities, and sites with multiple food outlets sharing a drainage network.

A kitchen equipment partner can coordinate the operational side of the decision with the project team: sink selection, pot-wash layout, stainless steel fabrication, equipment drainage points, and service access. WAF Kitchen approaches this as part of complete kitchen execution, helping ensure that equipment planning supports the wider back-of-house system.

A Practical Final Check Before Ordering

Before approving a grease trap or interceptor, verify the rated GPM, grease storage rating, connected fixtures, expected peak discharge, installation dimensions, and access for cleaning. Then compare the selection against the approved plumbing design and local requirements.

The best grease trap size is the one that fits the real kitchen workflow, handles its peak wash-up demand, and remains easy to service long after opening day.