The component nobody sees until the sewer remembers it

The grease trap is the least photographed piece of equipment in a professional kitchen — and the one that decides whether the venue can work in peace. Every service sends hot water loaded with fats and oils down the drain: in the cold collector the grease solidifies, tightens the pipe, and sooner or later presents the bill — the blockage on the busiest night, the water authority's inspection, the fine. The separator breaks the chain: it slows the water in a tank, lets the grease float and the solids settle, and delivers only clarified water to the sewer. But it works on one condition: that the size is right. This calculator estimates it properly — from the design flow of the peak day — and translates it into the nominal NS size of the commercial series, with the metric that says how much margin remains: the size utilisation.

The formula: meals become litres per second

The model is the meals method used in preliminary sizing: the kitchen volume of the peak day — M meals times Vm litres — spread over the service hours and concentrated by the factor F, which turns the average into a peak: the end-of-service spikes, the dishwasher's 60-degree water that keeps grease emulsified, the detergents. The result is the design flow Qs in litres per second — the language catalogues speak. The nominal size is the first step of the commercial series 1, 2, 4, 7, 10, 15, 20 that contains Qs, and the utilisation u = Qs/NS is the dimensionless metric that says whether the size breathes or works at the edge.

The covers scale: the size climbs in steps

A kitchen at 10 litres per meal, 8 service hours, factor 10; only the meals grow:

The size column climbs in steps, not in a straight line: between 300 and 500 meals the tank is the same NS 2 going from 52 to 87% utilisation; at 750 the catalogue jumps to NS 4 and the size breathes again at 65. It is the geometry of the commercial series — and the reason two similar restaurants can have the same separator with opposite margins: the size says nothing without its utilisation.

A complete numeric example

A typical kitchen: 300 meals on the peak day — the established restaurant on a Saturday night — at 10 litres of kitchen water per meal, 8 service hours, factor 10 between spikes and dishwasher. The day's volume is 3,000 litres; the design flow is 300 × 10 × 10 / (3,600 × 8) = 1.04 l/s; the commercial series offers the NS 2, used at 52.1% — the physiology of sizing: half the size for ordinary service, half for the day that is not in the plans. Whoever chooses on the average covers — 180, not 300 — buys a tank that works full from day one.

How to read the size utilisation

The four bands tell four kitchens. Below 45% the size breathes: margin for events, growth and bad days — with the caveat that a large trap must still be emptied on schedule. Between 45 and 70% the trap is well proportioned: the physiology. Between 70 and 90% the size works full: one longer service or one extra dishwasher eats the rest. Beyond 90% you are at the edge of the catalogue: any growth asks for the next step — and the separator is walled in once. No band is «forbidden»: u says how much future is inside the tank.

The factor F: conditions count as much as covers

The variable F is the real kitchen turned into a number. At the bottom — 4-6 — live the mild kitchens: cold plates, diluted service, lukewarm water. In the middle — 8-10 — the ordinary kitchen with its dishwasher and its spikes. At the top — 12-15 — the conditions grease loves: water above 60 degrees keeping it emulsified all the way to the tank, aggressive detergents, the fryer with concentrated discharges. It is the variable you can work on without buying anything: cooling the dishwasher water, spreading the peaks, pre-cleaning the plates — every factor point is worth litres per second of design flow.

The rule the trap does not forgive: no oil straight down

The separator treats water dirty with grease, not pure grease: a fryer emptied into the drain saturates in one day the tank sized for a month. Waste oil has its own chain — the dedicated container, the authorised collection — and the trap is grateful: the computed size holds only if what reaches the tank is what the model expects. Same rule for solids: the strainer basket upstream is the two-penny component that saves the thousand-euro one — and the emptying goes on record, with dates and quantities, because it is the first thing the inspection asks for.

International perspective: NS, gpm and the same tank

The tank is universal, the units are not. Europe sizes by nominal NS in litres per second along the EN 1825 series, with the meals method as preliminary sizing and the fixtures method as verification; North America speaks in gpm and pounds (1 l/s ≈ 15.9 gpm) and distinguishes the small under-sink grease trap from the buried grease interceptor; everywhere the regulatory pressure on FOG — fats, oils and grease — grows, with pump-out logs and inspections. The structure of the count never changes: meals, litres, peak, hours. And the utilisation u compares kitchens across markets on the same scale, because a percentage has no units.

Limits of the model and proper use

This is preliminary sizing with the meals method: the executive verification is done with the fixtures method — summing the flows of sinks, dishwashers and equipment — and with the installation constraints: sewer depth, venting, access for pump-out. The model does not cover mineral oils (another tank, another standard) nor industrial frying with off-scale discharges. And the professional kitchen is a chain of sized machines: the cold room refrigeration calculator keeps the numbers of the cold side, and the septic tank size calculator closes the water chain where the public sewer does not reach — because a kitchen that works well is a chain of right-sized tanks, and each one is sized on the worst day.

Inside the calculation: the conditions knob and the time knob

The six sections that follow put the model on the test bench over its two least intuitive variables. The first is the factor F: the same kitchen, under ever harder conditions — with a surprise produced by the catalogue. The second is the service hours: the same kitchen volume, spread over different days. The two tables use the example kitchen — 300 meals at 10 litres — and move one knob at a time: the cleanest way to see how the commercial series turns a continuous flow into size steps.

The factor scale: when the peak triggers the step

300 meals at 10 litres over 8 hours; only the factor F grows:

The table hides the model's most instructive inversion: from F 8 to F 10 the flow rises, but the utilisation collapses from 83 to 52% — because the extra peak triggers the catalogue step, and the new NS 2 breathes where the NS 1 worked full. Two factor points bought a double tank. It is the honest paradox of commercial series: worsening the conditions on paper can gift real margin — when the calculation crosses a size boundary.

Reading the inversion: the size boundary is the real stake

The inversion teaches the right question to ask a quote: how far is the design flow from the next step? A kitchen at 0.83 l/s on NS 1 and one at 1.04 on NS 2 are neighbours on paper and opposites in substance: the first sits at 83% with no margin, the second at 52 with half the tank free. When the calculation falls close to a boundary — above or below — the professional choice is almost always the upper step: the two tenths of a litre per second that look like prudence today are tomorrow the packed night that sends no grease to the sewer.

The hours scale: the long service dilutes the peak

Same 300 meals at 10 litres, factor 10; only the service hours change:

The same kitchen volume — 3,000 litres — asks for the NS 4 if compressed into 4 hours and the NS 1 if spread over 12: four catalogue steps of difference without changing a single dish. And the utilisation swings between 52 and 69% down the scale: every time the flow crosses a boundary, the new size restarts breathing. The operating lesson: the declared hours are worth as much as the covers — and the restaurant that compresses its service must declare it, because it is the compression, not the volume, that sizes the tank.

The catalogue tie: when the calculation fills the exact size

One boundary case deserves its own paragraph: the kitchen with 400 meals at 12 litres, factor 12 over 8 hours, computes exactly 2.00 l/s — and the series assigns the NS 2 used at 100.0%. The perfect tie is legitimate on paper and fragile in life: no worse-than-expected day, no extra cover, no drift of the factor. It is the row that shows the deep meaning of utilisation: a size filled to one hundred percent is not a right size — it is a size with no future. The next step, here, is not prudence: it is the design.

Before ordering: the four-knob checklist

A readable sizing declares the model's four variables. To put in writing: the peak-day meals with the criterion used (registers, not memories); the litres per meal consistent with the real menu; the factor justified — water temperature, detergents, spikes; the true service hours, not the nameplate ones. Then two questions worth more than any discount: at what utilisation the proposed size works, and how far the calculation sits from the next step of the series. Whoever answers with numbers is selling a sized tank; whoever answers with «it is the one we always fit» is selling the next blockage.

Frequently asked questions

What size grease trap does a restaurant need?

With the meals method: Qs = M·Vm·F/(3600·t) litres per second, then step up to the first size of the commercial series 1-2-4-7-10-15-20. A restaurant with 300 peak meals at 10 litres per meal, factor 10 over 8 hours, computes 1.04 l/s and fits an NS 2 used at 52%.

What is the nominal NS size?

It is the flow in litres per second the separator treats while guaranteeing separation: tanks are sold in the series 1, 2, 4, 7, 10, 15, 20. The size is chosen as the first step that contains the design flow — never rounding down.

What is the size utilisation?

It is the ratio u = Qs/NS between the design flow and the chosen size. Below 45% the size breathes; between 45 and 70% it is the physiology; between 70 and 90% it works full; beyond 90% it is at the edge of the catalogue and any growth asks for the next step.

Why size on the peak day and not the average?

Because the grease that escapes on the worst day ends up in the sewer even if the average is quiet: the tank must hold the packed Saturday, not the Tuesday. The average produces separators working full from day one — and the separator is walled in once.

What does the factor F represent?

The kitchen's real conditions: 4-6 for mild, diluted services, 8-10 for the ordinary kitchen with a dishwasher, 12-15 when water above 60 degrees, aggressive detergents and concentrated discharges keep the grease emulsified all the way to the tank. Every factor point is worth litres per second.

How many litres per meal does a kitchen count?

The 5-15 range tells the registers: 5-7 for cold plates and essential kitchens, around 10 for the ordinary restaurant, 12-15 for fryers and fatty kitchens. It is the number to estimate honestly, because it multiplies everything else.

Can I pour fryer oil into the grease trap?

No: the separator treats water dirty with grease, not pure grease. A fryer emptied into the drain saturates in one day the tank sized for a month. Waste oil has its own chain: dedicated container and authorised collection.

How often must the trap be emptied?

On schedule — typically every 2-4 weeks for an active kitchen — and in any case when the grease layer reaches the design capacity. The practical rule: pump-outs on record with dates and quantities, because it is the first thing the inspection asks. A large trap delays the blockage, not the maintenance.

Does the calculator work for the American grease trap in gpm?

Yes: the flow converts (1 l/s ≈ 15.9 gpm) and the logic is identical — peak flow, commercial size, utilisation. The catalogue series and the local FOG rules change, not the structure of the count. The utilisation stays comparable because it is a percentage.

Is the strainer basket also needed?

Yes, always: kitchen solids clog the tank and steal useful volume from the separation. The upstream basket is the two-penny component that saves the thousand-euro one — provided it is emptied at every service.

Where is the separator installed?

As close as possible to the kitchen, on an accessible run for pump-out, before the foul line and never downstream of sewage. Under the sink in small sizes, buried or in a utility room in restaurant sizes: the installation decides the maintenance for the next twenty years.

What does the calculation NOT cover?

The verification with the fixtures method (summing the real flows of sinks and machines), mineral oils which need another tank, off-scale industrial frying and the installation constraints. It is preliminary sizing: confirmation belongs to the designer on the real kitchen and the local rules.

Updated July 2026