Sizing a grease trap is not a matter of picking the biggest that fits. The unit has to hold the flow from the appliances that discharge into it long enough for the grease to separate and float, and it has to be reachable for emptying. Get either wrong and the kitchen has a trap that passes grease through in service and a bill for a blockage anyway.
What the size depends on
The number and type of appliances discharging into it, the peak flow during service, the retention time the unit needs to separate, and how often it will be emptied. A busy fish restaurant and a sandwich bar with the same floor area are not the same kitchen.
Under sink against external
Under sink units are cheap, quick to fit and small, which means frequent emptying and a strict discipline about what goes down the sink. External below ground traps hold far more and are emptied by tanker, which suits high volume kitchens and sites where the internal space does not exist.
What a hundred litre unit is for
Small capacity units suit a single sink line in a light use kitchen. They are the commonest unit sold to small premises and the commonest one found overflowing, because the volume was chosen for the space rather than for the service.
Restaurants, takeaways and multi outlet sites
Where several outlets share a drain, whose trap serves what becomes a lease and a liability question as well as an engineering one. Establish that before the first blockage rather than after it, because a shared trap with no named owner is emptied by nobody and billed to whoever the water company reaches first.
Questions people ask about commercial grease trap
How big a grease trap do I need?
It follows the appliances discharging into it and the flow during service. A contractor will size it from the appliance schedule and the covers rather than from the floor area.
Can one trap serve two kitchens?
Physically often yes, contractually it needs to be clear who maintains it and who pays. On multi occupied sites that is a lease question worth settling in writing.
Are grease traps needed in every commercial kitchen?
Water companies expect grease management wherever significant fats, oils and grease are produced. A kitchen that only reheats packaged food is a different case from one that fries, and the water company's own policy is the answer for your site.