Grease Interceptor Sizing Worksheet: The Inputs to Gather Before You Ask Anyone for a Size

You are standing in the back of your kitchen with a quote for a new grease interceptor in one hand and a half-filled online calculator on your phone. The calculator wants fixture counts, meal numbers and a flow rate. You know the sinks. You do not know the rest. So you guess, and the answer is only as good as the guess.

This guide is the worksheet that comes before the calculator. It lists what to gather, who holds each figure and where operators most often go wrong. It does not give you a litre figure. The size that counts is the one your water authority or plumber approves for your site.

Why the worksheet matters more than the calculator

Every sizing method takes your kitchen facts and runs them through a formula. That holds for flow-rate, fixture-count and meals-served methods alike. The formula is the easy part. The facts are where kitchens go wrong.

An undersized interceptor fills fast, passes grease downstream and sends you back to the cleaner far more often than planned. An oversized one costs more to install and holds old, stale waste for longer. Neither mistake shows up on a calculator screen. Our post on why a calculator answer is only the starting point covers the bigger picture.

Who actually approves the size

In Australia the approved size is not your call, and it is not the calculator's. For a commercial kitchen connected to the sewer, the water authority sets the requirement through your trade waste approval. In Sydney that is Sydney Water. Plumbing work then follows the rules in the National Construction Code as applied in your state. Your worksheet is the evidence you bring to that conversation.

The worksheet: six blocks of inputs

Copy these into a note on your phone or a page on the pass. Fill in every line. Where you do not know, write "unknown". An honest blank beats a guess.

Block 1: Every fixture that drains to the trap

List each sink, dishwasher, combi oven drain, floor waste and pot wash station. Note which ones connect to the interceptor and which go straight to the sewer. Many kitchens find that a fixture they assumed was connected is not, or the reverse. Our guide to tracing what actually drains into your trap shows how to check.

For each fixture, record the bowl or tank dimensions if you can read them off the unit or the spec sheet. Flow-rate methods build on these.

Block 2: Equipment that dumps hot water in bursts

A commercial dishwasher does not drain steadily. It empties in a rush. Write down the make, the model and the drain cycle if the manual states it. Add any tilting kettle, steamer or combi oven that drains to the same line.

Block 3: Trading pattern

Record your opening hours, services a day, seats and meals served on a busy day and a quiet one. Use your till reports, not memory. Peak days drive the load, so write down the peak, not the average.

Block 4: What you cook

Frying, roasting, wok work and slow-cooked meats put more fat and oil into the wash water than salad and sandwich prep. Note your main cooking methods in plain words. A pub with a deep-fryer line and a charcoal grill sits in a different place from a juice bar.

Block 5: Site and access

Note where the trap would sit, how far it is from the kitchen, how deep the pipe runs and whether a truck can reach the lid. Access shapes the choice between an under-bench unit and an in-ground tank. Our post on under bench versus in-ground traps walks through that call.

Block 6: Your paperwork

Find your trade waste agreement or approval. It states the required device and often the pump-out frequency. Many operators do not know what theirs says. If you have never read it, read it before you size anything.

A worked example: the decision a worksheet changes

Take a cafe with one pot sink, one prep sink, a hand basin and a small under-bench dishwasher. The owner plans to add a second dishwasher and start evening service in six months.

A calculator run on today's fixtures gives a modest result, and the owner is tempted to buy to that number. The worksheet changes the decision. Block 2 shows the second dishwasher. Block 3 shows evening service doubling the number of services. Sizing to today's kitchen means replacing the interceptor within a year.

The rule is simple. Size for the kitchen you will run over the next few years, then check it against what your authority allows. If the approval does not permit the larger setup, you learn that before you spend money, not after.

Questions the worksheet should answer before you call

Three questions come up once the numbers are done, and most guides skip them.

Does the size match the schedule you can afford? A smaller trap fills sooner, which means more cleans a year. Ask each cleaner to quote at the pump-out frequency your approval sets, so the comparison is fair. Our guide on how to compare three cleaners line by line shows how.

Can the cleaner reach it? Cleaners quote on trap size, service frequency, access and disposal fees. Two kitchens with the same trap can get quite different prices. Poor access adds cost to every visit for the life of the trap.

Do the quote, the paperwork and the device all name the same thing? A mismatch between the approval and the installed unit causes trouble at inspection time.

Get the worksheet in front of the right people

Once the blocks are filled in, send the same sheet to your plumber, your water authority contact and every cleaner you ask for a quote. Give everyone identical inputs and you can compare their answers like for like.

Most operators only think about the trap when it smells, backs up or an inspection is due. A completed worksheet puts you ahead of that. It keeps paying off, too, because the same sheet tells a new cleaner exactly what they are servicing. If you would rather start with the cleaning side, our post on cleaning a grease trap yourself explains what the law and your approval leave to a licensed professional.

The EPA NSW publishes general guidance on how the state manages waste and wastewater. It is useful background if you want to see why the rules exist.

Next step: put real quotes against your worksheet

Grease Trap Quotes covers Sydney today. We send three quotes from licensed cleaners by SMS, usually within about a minute of your request. Have your worksheet open when they arrive and check each quote against your inputs. Get 3 Grease Trap Quotes and see what the same trap costs across three cleaners. You can also learn about GreaseTrapQuotes or head back to the home page to start.

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Related reading: Grease Trap Servicing When Staff Change: Keep the Booking From Falling Through.

Related reading: How to Dispose of Cooking Oil in Australia: Home, Cafe or Commercial Kitchen.

Related reading: Grease Trap Servicing Before Your Busiest Weeks: How to Time the Clean Around Peak Trade.