Restaurant Grease Trap Size Requirements: What You Need Before You Install or Upgrade

The plumber is on site, the fit-out is running two weeks behind, and someone has just asked which grease trap you want. You pick a number that sounds reasonable. Six months later, the trap is backing up every three weeks, your council compliance officer is on the phone, and a licensed contractor is telling you the unit is undersized for your kitchen output.

This happens constantly. Grease trap sizing is not guesswork, but it is often treated that way during fit-outs when time pressure pushes decisions through fast. Getting it wrong means higher cleaning costs, faster failure cycles, and a real risk of breaching your trade waste agreement.

Here is how sizing actually works, what local authorities look for, and how to protect yourself before you commit to an installation.

Why Size Matters More Than You Think

A grease trap works by slowing wastewater down long enough for fats, oils, and grease (FOG) to float to the surface and solids to settle at the bottom. The clean water in the middle flows out to the sewer. When the trap is too small, water moves through too quickly for that separation to happen properly. Grease passes straight into the sewer line, which is exactly what the trap is supposed to prevent.

The consequences run in both directions. An undersized trap fills with FOG faster, so you need more frequent pump-outs. That drives up your annual cleaning bill. It also increases the risk of a blockage or overflow, which triggers a council notice and potentially a fine.

On the other side, an oversized trap in a low-output kitchen can lead to anaerobic decomposition of the retained grease, producing hydrogen sulphide gas and foul odours. So bigger is not automatically better.

The Core Variables That Drive the Calculation

Sizing is calculated from your kitchen's wastewater flow rate, which is determined by the fixtures connected to the trap. In Australia and New Zealand, the standard methodology counts fixture units, each sink, floor drain, dishwasher, and pre-rinse spray unit contributes a set number of units, and those convert to a flow rate in litres per minute or litres per second.

The relevant standard in Australia is AS 1825, which covers grease arrestors for commercial food premises. It defines minimum retention time (usually 30 minutes at peak flow for a passive trap) and sets out the formula for calculating the minimum trap volume based on fixture unit loading. Your local council's trade waste team or a licensed hydraulic engineer can walk you through the specific calculation for your setup.

In New Zealand, the equivalent guidance comes from your local territorial authority and the Building Code clause G13, which covers foul water drainage. Requirements vary by region, so always check with your council before specifying a trap size.

What Councils and Regulators Actually Check

When you apply for a trade waste agreement, which you need before discharging any kitchen wastewater to the sewer, your local water authority or council will review your trap specification. They are looking at a few things.

First, whether the trap volume matches the calculated flow rate from your fixture list. Second, whether the unit is an approved product or manufactured to the relevant standard. Third, whether the installation location allows safe and practical access for pump-out, because a trap that is hard to access will not get serviced properly and will fail.

In most Australian jurisdictions, trade waste agreements are managed by the local water utility, Sydney Water, Melbourne Water, SA Water, Queensland Urban Utilities, and so on. Each publishes its own trade waste guidelines, and some publish approved product lists. A trap that is compliant in one state is not automatically compliant in another, so if you are operating across multiple sites, do not assume uniformity.

Indoor Traps vs. External Interceptors

Small kitchens, a suburban café or a small bakery, often use an indoor hydromechanical grease trap (sometimes called a grease arrestor) installed under the sink or in a floor recess. These are compact, rated in litres, and cleaned manually or by a contractor every few weeks depending on output.

Larger operations, full-service restaurants, commercial caterers, food courts, typically need an external grease interceptor installed in the ground outside the building. These are rated in hundreds or thousands of litres. They handle much higher flow rates and are pumped out by a vacuum tanker. The sizing methodology is the same in principle, but the scale is completely different.

If your fit-out involves a high-volume kitchen, say, more than five fixtures discharging to the trap, talk to a hydraulic engineer early, not after the slab is poured. Retrofitting a larger interceptor once a building is finished is expensive and disruptive.

For a closer look at what cleaning and maintenance looks like once your trap is installed, the Grease Trap Cleaning for Restaurant: Cost, Frequency, Contractors | GreaseTrapQuotes guide covers the full service cycle for restaurant-scale traps.

Common Sizing Mistakes and How to Avoid Them

The most common mistake is specifying a trap based on the kitchen footprint or seat count rather than the actual fixture list. A small kitchen running three shifts with a high-capacity dishwasher can produce more wastewater than a large kitchen that only operates for lunch service. Seat count is not a reliable proxy.

The second mistake is ignoring future capacity. If you plan to add a second dishwasher or expand the menu to include more frying, size the trap for where you are going, not just where you are now. Upsizing later means excavation, which is costly.

The third mistake is choosing a trap based on price alone. A cheaper unit that does not meet AS 1825 or your council's approved product list will fail compliance and need to be replaced at your cost. The Grease Trap Cleaning Cost 2026: Real Prices by City and Trap Size | GreaseTrapQuotes article explains how trap size directly affects your ongoing cleaning bills, which adds up fast over several years.

Getting Your Fixture List Right

Before you speak to a plumber or a supplier, list every fixture that will discharge to the trap. Include all hand basins in the kitchen (not customer bathrooms), all prep sinks, the dishwasher, the pot wash sink, any floor drains in the cooking area, and pre-rinse spray units. If you have a mop sink that will be used for kitchen cleaning, that may need to be included too depending on your council's rules.

Once you have the list, a licensed plumber or hydraulic engineer can apply the AS 1825 calculation and give you a minimum trap volume. Take that number to your water utility when you apply for your trade waste agreement, and confirm they accept the product you intend to install.

Cafes and bakeries work through the same process, though the fixture lists tend to be simpler. The Grease Trap Cleaning for Cafe: Cost, Frequency, Contractors | GreaseTrapQuotes and Grease Trap Cleaning for Bakery: Cost, Frequency, Contractors | GreaseTrapQuotes pages have more detail on what those kitchen types typically need.

Maintenance Obligations Once You Are Installed

Once your trap is installed and your trade waste agreement is in place, you have an ongoing obligation to keep the trap working within its design parameters. That means regular pump-outs at the frequency your agreement specifies, and records showing each service was completed by a licensed contractor.

Most agreements require you to keep service records for a minimum period, often two years, and produce them on request. Councils and water utilities do conduct inspections, and missing records are treated the same as missing services.

The required pump-out frequency depends on your output. A high-volume restaurant kitchen may need monthly or bi-monthly service. A lower-output café may be able to go quarterly. The frequency should be set based on the 25% rule: the trap should be pumped out before the combined depth of FOG and solids reaches 25% of the trap's liquid capacity. A good contractor will measure this at each service and advise you on whether your current schedule is right.

For contractors looking to service kitchens in your area, Apply to Join GreaseTrapQuotes, For Licensed Contractors | GreaseTrapQuotes and Contractor Territory Pricing | GreaseTrapQuotes | GreaseTrapQuotes have the details on how the network works.

Get the Right Size from the Start

A grease trap sized correctly for your kitchen is one less compliance problem to manage. It costs less to service over its life, it is less likely to fail at a bad moment, and it keeps your trade waste agreement intact.

If you are fitting out a new kitchen, upgrading an existing trap, or just not sure whether your current unit is right for your output, the best first step is getting a quote from a licensed contractor who can assess your setup on site.

Get 3 Grease Trap Quotes | GreaseTrapQuotes and compare prices and advice from licensed contractors in your area.

Related reading: Bar Grease Trap Cleaning Frequency: How Often Does Your Bar Actually Need a Service?.