Grease Trap Size: Why a Calculator Answer Is Only the Starting Point

The plumber crouches by the pre-rinse sink in your new fit-out and says the trap on the plans looks small for what you plan to cook. You have a fryer, a combi oven, a three-bay sink and a dishwasher. Opening day is close. You want a straight answer on grease trap size, and every page you open gives you a formula and a different number.

Those pages skip one fact. In Australia, you do not get to pick the size of your grease trap. It is an approved outcome. A formula gives you a rough figure to discuss, and your water authority and your plumber settle the real one.

Who actually decides the size of a grease trap

A sizing calculator asks about seats, meals per day, sink volume and flow rate. Those are fair inputs, and they give you a ballpark. They do not give you permission to install anything.

In a commercial kitchen, the water authority controls what goes down the drain. In Sydney that is Sydney Water, which regulates trade waste for businesses connected to its network. Other cities have their own authority and their own rules. The authority approves the device, and a licensed plumber installs it to the plumbing standards. Those standards sit in the National Construction Code and Plumbing Code of Australia. The detail of grease arrestor design comes from Standards Australia documents your plumber works from.

So take any online result to the people who sign off the trap as a question. It is not the answer.

Why your trade waste agreement matters more than a formula

Your trade waste agreement with the water authority sets the pump-out frequency for your kitchen. Many operators do not know theirs. It matters for sizing because size and frequency pull against each other. A smaller trap fills faster and needs cleaning more often. A larger trap holds more between cleans. If your agreement fixes the frequency, the trap has to be big enough to last that long without overflowing.

The four things that really drive the number

Set the formula aside for a moment and look at what it measures.

• What goes down the drain. Fryer oil, roast trays, pasta water and dishwasher discharge all carry fat. A kitchen that scrapes plates dry and wipes pans before washing sends far less to the trap than one that rinses everything.
• Which fixtures connect to it. A trap on a single pre-rinse sink sees less flow than one that also takes the dishwasher and floor wastes. Our guide to tracing what drains into your trap walks through this check for a commercial kitchen.
• How busy you are at the peak. The trap has to handle your worst hour, not your average one.
• How you plan to clean it. A kitchen on a firm service schedule can run a smaller device than one that cleans only when something goes wrong.

A different menu is a different trap

Two venues with the same floor area can need very different traps. A bakery washing trays and mixers puts a different load on the trap than a pub frying all night. If you run a bakery, the fat load per sink is often lower than at a fryer-heavy site, but flour paste and sugar residue bring their own problems. A cafe with a small menu and one dishwasher sits somewhere in between. Whoever you ask about size, tell them plainly what you cook.

A worked decision: bigger trap or tighter schedule?

Many owners face this choice and few pages cover it. You are fitting out a restaurant. The approved options include a trap at the minimum size and a larger one that costs more to buy and install.

The minimum-size trap saves money on day one. It also fills sooner, so you pay for more pump-outs each year. The larger trap costs more up front and runs longer between cleans.

To decide, get the numbers for your own kitchen:

1. Ask your plumber for the installed price of each option. 2. Ask two or three cleaners to quote a pump-out for each trap size at the frequency your agreement requires. 3. Weigh the difference in install cost against the difference in yearly cleaning cost over the time you expect to hold the lease.

On a short lease, the smaller trap often wins on cost. If you plan to stay for years, or your menu leans heavily on frying, the larger trap often pays for itself. You will not know which until you have real quotes. Do not trust a general rule, including this one.

Cleaners quote on trap size, service frequency, access to the trap and disposal fees. That is why two kitchens with the same trap can get quite different prices. Our breakdown of grease trap cleaning cost shows how those drivers add up.

Signs your trap is too small for the kitchen you run now

You do not size a trap once and forget it. Kitchens change. You add a fryer, extend trading hours or take on catering, and the trap you installed for the old menu now carries a heavier load.

Watch for these signs:

• You need cleans much more often than your agreement or your cleaner expected.
• The trap fills up well before the next booked visit.
• Smells or slow drains come back within days of a clean.
• Your cleaner says the trap is overloaded when they lift the lid.

Most operators only think about the trap when it smells, backs up, or a council or Sydney Water inspection is due. A trap that is too small makes all three happen more often. If you see two or more of these signs, do not just book extra cleans and carry on. Ask your plumber whether a larger or second device makes sense, and check with your water authority before you change anything.

What to bring to the conversation

Before you talk to your plumber, your authority or your cleaner, have these facts ready:

1. Your menu and how much of it is fried. 2. Every fixture that drains to the trap. 3. Your busiest hour and how many covers you serve in it. 4. Your current trade waste agreement, if you have one, and the pump-out frequency it sets. 5. The make, location and approximate size of any existing trap, with a photo.

With these in hand, you get an answer sized for your kitchen, not a generic one. You also stop paying for a trap that is bigger or smaller than you need.

Get a price that matches your trap

Once you know your trap size, you can compare cleaners on equal terms. Grease Trap Quotes sends you three quotes from licensed cleaners by SMS, usually within about a minute of your request. The service covers Sydney today and is expanding to other Australian cities. Get three grease trap quotes and see what cleaning costs for your trap size and frequency.

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Related reading: Grease Trap Servicing: Why Two Kitchens With the Same Trap Pay Different Prices.

Related reading: What Is a Commercial Grease Interceptor, and Does Your Kitchen Need One?.

Related reading: Grease Trap Servicing Around a Closure: When to Book Before You Shut and After You Reopen.