Grease Interceptor Sizing Calculator

Three methods, three devices, and the mistake that gets made every time

The question “what size grease trap” has no single answer because the plumbing code contains three separate sizing procedures, and each one sizes a different kind of equipment in a different unit.

The recurring field error is quoting one of them while being inspected against another. A contractor who has computed 50 gpm from the sink dimensions and is then asked for a 1,000 gallon gravity interceptor has not been given a different answer to the same question — they have been given the answer to a different question, and no arithmetic converts between the two.

The gravity method: fixture units in, gallons out

Section 1014.3.6 is short enough to quote in full: “The volume of the interceptor shall be determined by using Table 1014.3.6. Where drainage fixture units (DFUs) are not known, the interceptor shall be sized based on the maximum DFUs allowed for the pipe size connected to the inlet of the interceptor. Refer to Table 703.2, Drainage Piping, Horizontal.”

That is the whole procedure. Count the DFUs on the kitchen drain lines that will connect to the interceptor, and read the volume off the table.

The third column is not in the published table — it is the result of checking the DFU column against Table 703.2, which footnote 2 cites as the table's own basis. Six of the thirteen rows are exactly the Table 703.2 maxima for horizontal drainage piping at ¼ inch per foot, and four more are those same maxima reduced by the 0.8 factor that Table 703.2 footnote 5 applies at ⅛ inch per foot — 0.8 × 216 truncates to 172, 0.8 × 428 to 342, 0.8 × 720 is exactly 576 and 0.8 × 2,640 is exactly 2,112. Ten of the thirteen rows are therefore confirmed against a companion table rather than against a transcription.

That last point matters more than it looks. The table lists single DFU values, not ranges, and a kitchen almost never lands exactly on one. The code's example settles it: a count falls in the lowest row whose DFU value it does not exceed. Twelve DFU does not go in the “8” row, it goes in the “21” row.

The 6 inch trap: why the fallback is not 5,000 gallons

When the fixture list is not available — a tenant improvement where the existing kitchen is being reused, a shell building, a design-build where the equipment schedule is not out — Section 1014.3.6 sends you to the inlet pipe size and the Table 703.2 maximum for it.

Because every one of those maxima is itself a row in Table 1014.3.6, the fallback never lands between two rows — which is a quiet piece of evidence that the two tables really were built together, and a useful check that you have transcribed either of them correctly.

The fallback is deliberately conservative. It assumes the branch is loaded to the absolute code limit for its diameter, which almost no real kitchen is. If you can count the fixtures, count them.

The hydromechanical method: sink dimensions in, gpm out

Hydromechanical interceptors — the stainless or plastic box under the three-compartment sink, still called a grease trap by nearly everyone — are rated in flow, not volume, and Section 1014.2.1 sizes them from the physical capacity of the fixtures that drain into them. The published procedure is:

231 is cubic inches in a US gallon, which is a definition rather than a code constant. The 0.75 is the fill factor — the assumption that a sink in use is three-quarters full, not brim full. The drainage period is the time the interceptor is given to pass a complete dump of every compartment at once.

Those are the figures the code prints in its own example table, and the calculator above reproduces all of them exactly, which is the cleanest possible confirmation that the constants are right.

Note what the choice of drainage period does: the same sink needs either a 50 gpm or a 25 gpm unit depending on a column heading. The code publishes both and does not tell you which applies — that is the authority having jurisdiction's call. A one-minute unit always satisfies a two-minute requirement; the reverse is never true, which makes one minute the defensible default when nobody has told you otherwise.

Section 1014.2.1 also tabulates the interceptor against the grease waste pipe diameter, for when the sink schedule is not available:

The maximum full pipe flow column is computed at ¼ inch per foot slope using Manning's formula with a friction factor of 0.012, per the table's own footnote. Which is worth remembering the next time somebody hangs the grease line flat: the numbers on this page assume a pipe that is actually sloped.

The 2.5x rule is a second test, and it fails independently

Section 1014.2.1's first sentence is a capacity limit, not a sizing formula: “The total capacity in gallons of fixtures discharging into a hydromechanical grease interceptor shall not exceed two and one-half times the certified gallon per minute flow rate of the interceptor.”

So a unit has to clear two separate hurdles. It needs enough flow rating for the drainage period, and it needs to be big enough that the fixtures hanging on it do not exceed 2.5 × its rating in gallons.

The same 2.5 multiplier shows up independently in municipal codes written on the IPC side — Omaha's Section 49-1131, for instance, states that “the total capacity in gallons of fixtures discharging into a grease trap shall not exceed 2½ times the gpm flow rate.” Two code families, same number, which is reassuring when you are relying on it.

Those same municipal sections also fence in what counts as a grease trap at all: a rated flow of not less than 20 gpm and not more than 50 gpm, no more than four fixtures connected to any one trap, each fixture with its own accessible flow control or restricting device, and no food waste disposer discharging into it. Above 50 gpm you are no longer specifying a trap.

What must not drain through the interceptor

Section 1014.3.2.1 is one sentence and it is absolute: “Toilets, urinals, and other similar fixtures shall not drain through the interceptor.”

That is why the fixture picker on this page has no water closet row and no lavatory row, and why a DFU total taken off the building drain calculation is the wrong number to type into the box. Sanitary fixture units are not interceptor fixture units. Footnote 1 of Table 1014.3.6 scopes the count to “the maximum allowable DFUs plumbed to the kitchen drain lines that will be connected to the grease interceptor”.

What must go through it, per Section 1014.3.2 and the IPC-family Section 1003.3.1, is the grease-bearing equipment: pot and pan sinks, prerinse sinks, scullery sinks, soup kettles, wok stations, floor drains or sinks that kettles are drained into, automatic hood wash units, dishwashers without prerinse sinks, and floor drains in areas where grease-containing materials exist.

The persons-served method, and where it actually applies

IPC-family state codes carry a different minimum-capacity rule, written around people rather than fixtures. From Section 1003.2.1 of the Louisiana Plumbing Code: the minimum required capacity shall be determined from the maximum number of persons served during the largest meal period; it shall not be less than 125 gallons below the static water level, which is held sufficient for up to 50 people in a single meal period; and above 50, capacity increases by at least an additional 2½ gallons per person beginning with the 51st person served.

Two details decide whether you get this right. The count is the largest meal period, not the day — a restaurant doing 90 lunches and 180 dinners is a 180-cover calculation, not 270. And the 2.5 gallon charge begins with the 51st person, so 51 covers is 125 + 2.5 = 127.5 gallons, not 125 + 5.

The same section sets a hard floor on any device, however it was sized: “In no case shall a grease interceptor or automatic grease removal device be installed which has an approved rate of flow of less than 20 gallons per minute.”

It also makes clear that the small under-sink unit is the exception, not the default. A point-of-use hydromechanical interceptor is permitted at the code official's discretion where a concrete slab would have to be broken, where the only available outside area is further than 75 feet from the fixtures served, where the kitchen is on an upper floor of a multistory building, or where the menu and mode of operation genuinely produce minimal grease — the section names snowball stands and sandwich shops using single-service tableware. If none of those apply, the expectation is a gravity interceptor.

What this calculator is not

It does not pick a product. Certified ratings live in the manufacturer's listing to ASME A112.14.3, ASME A112.14.4, PDI-G101 or IAPMO Z1001. This page tells you the duty; the listing tells you which model meets it. Note also that a gravity interceptor complying with IAPMO Z1001 may not be installed inside a building where food is handled.

It does not set a cleaning interval. Pump-out frequency, the familiar “25 percent rule”, FOG loading in pounds per day and grease generation rates are pretreatment-ordinance material rather than plumbing code, and the captured code text does not contain them. Quoting a cleaning interval as if it were code would be an invention. Your local sewer authority sets it, and the interval it sets is often monthly or tighter.

It does not do the local ordinance. Sewer authorities routinely impose minimum vessel sizes above the code figure, prohibit food waste disposers upstream of an interceptor outright, require sampling boxes, and mandate recording devices. Where a local pretreatment ordinance conflicts with the plumbing code, the ordinance governs.

It does not size by meals per day or seats. That older method — meals per peak hour times a waste flow rate times a retention time times a storage factor — exists in other editions and in some manufacturer literature, but it is not in the sections captured here, so it is not implemented. Reproducing it from memory is exactly the kind of thing that puts a wrong tank in the ground.

It does not design the piping. Flow control device selection and venting, the downstream vent Section 1014.2.2 requires, the prohibition on a system vent between the flow control and the interceptor inlet, inlet and outlet invert elevations, and the travel distance over which grease cools and sets are all separate work.

It does not evaluate automatic grease removal devices. AGRDs are permitted by the same sections and are sized on their own listed basis.

Frequently asked questions

What size grease trap do I need for a restaurant?

It depends which of the three code methods applies, and they answer in different units. Under UPC Section 1014.3.6 a gravity grease interceptor is sized in gallons from the total drainage fixture units on the kitchen drain lines connected to it: the code's own example of three floor drains, a mop sink and a food prep sink totals 12 DFU and sizes a 750 gallon interceptor. Under Section 1014.2.1 a hydromechanical unit is sized in gallons per minute from the sink compartment volumes. Under IPC-family Section 1003.2.1 the minimum capacity is 125 gallons for up to 50 covers in the largest meal period plus 2½ gallons per person beyond that. Establish which one your authority having jurisdiction enforces before ordering anything.

How do you calculate grease interceptor size from fixture units?

Add the drainage fixture units of only the grease-bearing fixtures that will connect to the interceptor, using UPC Table 702.1 public column values — 2 DFU per floor drain, 3 per service or mop basin, 3 per commercial sink with food waste, 2 per public bar sink, 3 per commercial food waste disposer, 2 per dishwasher with an independent drain. Then read the volume from Table 1014.3.6, placing the count in the lowest row whose DFU value it does not exceed. Toilets, urinals and similar fixtures are excluded by Section 1014.3.2.1 and must not be counted.

What is the formula for hydromechanical grease interceptor sizing?

Length times width times depth in inches, divided by 231 to get gallons, multiplied by a 0.75 fill factor and by the number of compartments, then divided by a drainage period of one or two minutes to get the required flow in gallons per minute. The code's worked example: two compartments of 24 by 24 by 12 inches give 29.922 gallons each, 22.44 gallons after the fill factor, 44.9 gallons for the pair; adding a 3 gallon hydrant and a 2 gallon rated appliance gives 49.9 gallons total, which is a 50 gpm interceptor on a one-minute drainage period or 25 gpm on a two-minute period.

Why does a 6 inch inlet size to 7,500 gallons and not 5,000?

Because the rule runs through the fixture unit column, not the volume column. Section 1014.3.6 says that where DFUs are not known the interceptor is sized on the maximum DFUs allowed for the inlet pipe size from Table 703.2, and the Table 703.2 maximum for 6 inch horizontal drainage piping at a quarter inch per foot is 720 DFU. Table 1014.3.6 puts 720 DFU on the 7,500 gallon row. The 5,000 gallon row is 576 DFU, which is the same 6 inch pipe at an eighth of an inch per foot after Table 703.2 footnote 5's 0.8 factor. Reading the volume column by eye and picking 5,000 understates the vessel by 2,500 gallons.

What is the 2.5 times rule for grease traps?

UPC Section 1014.2.1 states that the total capacity in gallons of fixtures discharging into a hydromechanical grease interceptor shall not exceed two and one-half times the certified gallon per minute flow rate of the interceptor. It is a second test, independent of the flow rate calculation, and a unit can satisfy the required flow while failing it. A 20 gpm unit may carry no more than 50 gallons of fixture capacity; a 50 gpm unit may carry no more than 125 gallons. The same multiplier appears in IPC-family municipal codes, which corroborates it from a second code family.

Can a toilet drain through a grease interceptor?

No. UPC Section 1014.3.2.1 states that toilets, urinals and other similar fixtures shall not drain through the interceptor. This also means a drainage fixture unit total taken from the building drain sizing calculation is the wrong figure to use for interceptor sizing, because it includes sanitary fixtures. Footnote 1 of Table 1014.3.6 scopes the count specifically to the kitchen drain lines connected to the grease interceptor.

Does a food waste disposer go through the grease interceptor?

The plumbing code does not prohibit it, and IPC-family Section 1003.3.2 requires interceptors to be sized and rated for the discharge of food waste disposers where they are connected. However, many local sewer authority pretreatment ordinances prohibit a disposer upstream of a grease interceptor outright, and some municipal plumbing codes do as well — Omaha's Section 49-1131 states that no food waste disposal unit shall be connected to or discharged into a grease trap. Check the local ordinance before piping it, because this is one of the most commonly amended provisions.

Is a grease trap the same thing as a grease interceptor?

Not in code language, although the terms are used interchangeably in the field. A grease trap is defined in IPC-family municipal codes as a passive interceptor with a rated flow of 50 gpm or less located inside the building. A hydromechanical grease interceptor is the current code term for that class of device. A gravity grease interceptor is a much larger tank sized in gallons rather than gpm, usually outside the building, and it is what Section 1014.3 governs. The three sizing procedures are not interchangeable.

What is the minimum size grease interceptor allowed?

Two separate floors apply. IPC-family Section 1003.2.1 requires a minimum capacity of not less than 125 gallons below the static water level for a vessel sized by persons served, and states that in no case shall a grease interceptor or automatic grease removal device be installed with an approved rate of flow of less than 20 gallons per minute. On the UPC gravity table the smallest tabulated interceptor is 500 gallons at 8 drainage fixture units. Local sewer authorities frequently set a higher minimum than any of these.

What does the 0.75 fill factor mean in grease trap sizing?

It is the assumption that a sink compartment in use is drained three-quarters full rather than brim full, and it is applied after converting the compartment's physical dimensions to gallons. Use the full inside depth of the compartment in the calculation and let the factor account for the freeboard. Applying a reduced working depth and the 0.75 factor together double-counts the same allowance and undersizes the interceptor.

How often does a grease interceptor need to be pumped out?

That is not set by the plumbing code sections that size the vessel. Cleaning frequency comes from the local sewer authority's pretreatment ordinance, and the commonly cited trigger is that accumulated grease and solids should not exceed 25 percent of the liquid depth. Some municipal codes do set an outside limit directly — Omaha requires inside grease traps to be cleaned at least every 30 days, with cleaning defined as removing the contents until empty and cleaning the sides and bottom. Confirm the requirement locally rather than assuming a quarterly interval.

Can I use a small under-sink grease trap instead of a buried interceptor?

Only where the authority having jurisdiction allows it. Section 1003.2.1 treats the point-of-use hydromechanical interceptor as an exception, permitted at the code official's discretion where a concrete slab would have to be broken in an existing building, where the available outside area is further than 75 feet from the fixtures served, where the kitchen is on an upper floor of a multistory building, or where the menu and mode of operation produce minimal grease — the section names snowball stands and sandwich shops using single-service tableware. Where none of those conditions exist, a gravity interceptor is expected.

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