Hydromechanical Grease Trap Sizing per PDI G101: Drainage Flow Rate, Two-Rating Requirement, and 3-Compartment Kitchen Example

Grease trap sizing collapses to a single number in most field conversations: gallons per minute. A 3-compartment commercial kitchen sink drains at roughly 34 gpm, so the crew orders a 35 gpm trap, and the permit gets stamped. The error is that a hydromechanical grease interceptor carries two certified ratings per PDI G101 (Testing and Rating Procedure for Hydromechanical Grease Interceptors): a flow rate in gallons per minute and a grease-retention capacity in pounds. Both must meet or exceed the fixture demand. A unit that passes the flow check while holding too little grease fills to capacity before the cleaning interval, discharging floating FOG into the sanitary sewer: a plumbing code violation and potential municipal FOG-program enforcement issue. This article covers the PDI G101 / ASME A112.14.3 fixture-flow sizing chain from sink dimensions to the certified unit selection, the check mode evaluation, and where the fixture-flow method does not apply.

The IPC (International Plumbing Code) Section 1003.3 requires grease interceptors to comply with PDI G101 or ASME A112.14.3 for sizing and certification. CSA B481 (Canadian Standards Association, Grease Interceptors) is the sister standard governing Canadian installations and is accepted by some US jurisdictions that recognize CSA listings. The IAPMO UPC (Uniform Plumbing Code) Chapter 10 governs Western US states including California, Arizona, Nevada, Utah, Idaho, and Hawaii; UPC Section 1014 uses a fixture unit method in some adoptions that differs from the IPC drainage flow rate basis. ANSI/IAPMO Z1001 and NSF/ANSI 46 (Performance Testing for Wastewater Treatment Devices) establish performance testing standards for hydromechanical grease interceptors. Locally adopted plumbing codes govern the method and the minimum required size. Where an AHJ specifies a flow rate or capacity that differs from the formula result, the AHJ value governs.

Why Two Ratings: Flow Rate vs Grease Capacity

A hydromechanical grease interceptor works by slowing wastewater velocity through a baffled chamber so that FOG, which is less dense than water, floats to the surface while water exits at the bottom. Two independent mechanisms limit its performance: the flow rate and the grease retention.

The certified flow rate is the maximum rate at which the unit can receive wastewater while maintaining effective separation. Above that rate, turbulence disrupts the separation zone and grease passes through. A unit rated at 35 gpm (132 L/min) can handle up to 35 gpm from the connected fixtures; a sink that drains faster than that backs up or passes grease.

The certified grease capacity is the total mass of grease the unit can retain before its available separation volume is consumed. Once grease fills past the design level, it reaches the outlet and discharges downstream. Per PDI G101 and ASME A112.14.3, the minimum grease capacity is set at twice the certified flow rate in pounds: a 35 gpm (132 L/min) unit carries at least 70 lb (31.8 kg) minimum grease retention. The two ratings are correlated at the certification level, but a unit must pass both.

A unit with a 35 gpm (132 L/min) flow rating and a 50 lb (22.7 kg) grease capacity passes the flow check and fails the capacity check. It drains the sink fine but needs cleaning at roughly 71% of the interval a 70 lb unit would allow, which is impractical in most kitchen maintenance schedules and produces recurring FOG discharges if the interval is not shortened accordingly.

Grease Trap Sizing Calculator: Inputs and Outputs

The calculator operates in two modes. Size mode computes the required certified flow rate and grease capacity from the fixture load. Check mode evaluates a proposed interceptor against the computed requirement and names which rating governs any shortfall.

Inputs include the number of compartments (1–10), compartment dimensions or direct volume per compartment, fill fraction (default 0.75), drain time (default 2 min), and in Check mode the proposed unit's certified flow rating and grease capacity. Advanced fields accept AHJ-specified flow and capacity overrides, and soft-check flags for dishwashers and food-waste disposers.

Outputs in Size mode: compartment volume, drainage volume per compartment, total drainage volume, computed drainage flow rate in gpm (L/min), required certified flow rate (next certified size at or above the computed flow), and required grease capacity in lb (kg). In Check mode: flow ratio (required ÷ proposed), capacity ratio (required ÷ proposed), and the governing shortfall (FLOW_RATING, GREASE_CAPACITY, MULTIPLE, or NONE).

Sizing Formula: Q = V_total / t

The drainage flow rate derives directly from the sink geometry and code assumptions. Per PDI G101 and CSA B481 fixture-flow basis:

compartment_volume (gal)  = L(in) × W(in) × D(in) × 0.004329
drainage_volume           = compartment_volume × fill_fraction
total_drainage (gal)      = drainage_volume × number_of_compartments
drainage_flow (gpm)       = total_drainage / drain_time (min)

The 0.004329 factor converts cubic inches to US gallons per NIST (National Institute of Standards and Technology) standard: 1 US gallon = 231 in³; 1/231 = 0.004329. For metric input: 1 US gallon = 3.78541 L per NIST; 1 in = 25.4 mm exact. The fill fraction reflects that a sink is filled to roughly 75% of its geometric volume in use, since the items being washed displace the remaining volume. The drain time is one or two minutes depending on the adopted code; most current IPC adoptions use one minute per ASPE (American Society of Plumbing Engineers) Plumbing Engineering Design Handbook Volume 4, Chapter 8, while some older local codes retain the two-minute basis. Both values are shown in the result and labelled as profile factors.

Once the drainage flow rate is computed, the required certified flow rating is selected by CONSERVATIVE_BRACKET lookup: the smallest certified trap size at or above the computed flow. The PDI G101 / ASME A112.14.3 / CSA B481 certified size series is 5, 10, 15, 20, 25, 35, 50, 75, 100 gpm. A computed flow of 33.6 gpm snaps to 35 gpm; a computed flow of 35.0 gpm also snaps to 35 gpm. The required grease capacity then follows:

required_grease_capacity (lb) = 2 × required_certified_flow (gpm)

Certified Flow (gpm)	Certified Flow (L/min)	Min Grease Capacity (lb)	Min Grease Capacity (kg)
5	19	10	4.5
10	38	20	9.1
15	57	30	13.6
20	76	40	18.1
25	95	50	22.7
35	132	70	31.8
50	189	100	45.4
75	284	150	68.0
100	379	200	90.7

Conversions per NIST: 1 US gallon = 3.78541 L; 1 lb = 0.45359 kg.

Variable Reference

Variable	Definition	Units
L, W, D	Compartment interior dimensions	in (US) or mm (Metric)
fill_fraction	Code assumption: fraction of volume occupied by wastewater in use	dimensionless (default 0.75)
drain_time	Code drainage period	min (default 2)
drainage_flow	Computed fixture drainage flow rate	gpm or L/min
required_flow	Required certified interceptor flow rating (CONSERVATIVE_BRACKET)	gpm or L/min
required_grease_capacity	Minimum required certified grease retention	lb or kg

Worked Example: 3-Compartment Restaurant Sink

A 3-compartment commercial kitchen sink has compartments each 24 in (610 mm) long, 24 in (610 mm) wide, and 12 in (305 mm) deep. Code basis: 0.75 fill fraction, 2-minute drain, all compartments simultaneous. Determine the required interceptor.

Step 1. Compartment volume

V_comp = 24 × 24 × 12 × 0.004329 = 29.9 gal (113.2 L)

Step 2. Drainage volume per compartment

V_drain = 29.9 × 0.75 = 22.4 gal (84.8 L)

Step 3. Total drainage volume (simultaneous)

V_total = 22.4 × 3 = 67.3 gal (254.7 L)

Step 4. Drainage flow rate

Q = 67.3 / 2 = 33.6 gpm (127.2 L/min)

Step 5. Required certified flow rate (CONSERVATIVE_BRACKET)

33.6 gpm (127.2 L/min) is above the 25 gpm (95 L/min) certified size and at or below the 35 gpm (132 L/min) size. Required certified flow = 35 gpm (132 L/min).

Step 6. Required grease capacity

required_capacity = 2 × 35 = 70 lb (31.8 kg)

Result: The sink requires a hydromechanical interceptor with a certified flow rating of at least 35 gpm (132 L/min) and a certified grease capacity of at least 70 lb (31.8 kg). Select the final unit from a manufacturer's certified rating list to PDI G101 or ASME A112.14.3 / CSA B481 at or above both values.

Manufacturer selection and economic analysis (continuing 3-compartment sink scenario):

Suitable certified models meeting 35 gpm (132 L/min) / 70 lb (31.8 kg) requirement per PDI G101:

• Schier GB-35 — under-counter floor-mount, fiberglass-reinforced polymer construction, PDI G101 listed, 35 gpm certified / 70 lb capacity, $1,400–$1,800 typical 2026 distributor pricing
• Watts WD-50 — cast iron construction, ASME A112.14.3 listed, $1,200–$1,600 pricing
• Thermaco Trapzilla TZ-300 — high-capacity model exceeding base requirement (300 lb / 136 kg capacity at 35 gpm certified flow), $2,800–$3,500 pricing, extended cleaning interval
• Zurn ZS-880 — under-counter design, ASME A112.14.3 + CSA B481 dual-listed, $1,500–$2,200 pricing

Capital cost analysis (Schier GB-35 base case):

• Interceptor unit: $1,400 (Schier GB-35 from Ferguson, Grainger, or local plumbing supply)
• Plumber installation labor: $800–$1,500 (6–8 hours at $100–200/hr, per IPC Section 1003.5 venting requirements + flow control fitting + sample port per IPC 1003.7)
• Permit fee: $150–$300 (typical municipal commercial plumbing permit)
• Total installed cost: $2,400–$3,200 first-year capital

Operating cost analysis per WEF (Water Environment Federation) Manual of Practice 7 — FOG Reduction in Wastewater Systems:

• Pump-out service: $50–$100 per service from licensed FOG hauler
• Typical 60-day pump-out interval: 6 services/year × $75 average = $450/year operating
• Aggressive 30-day interval (high-volume kitchens): 12 services/year × $75 = $900/year
• Sample port testing per IPC 1003.7: $100–$200/year third-party laboratory analysis where AHJ requires

Lifecycle: 15–20 years per manufacturer warranty (Schier, Watts, Zurn all standard) with pump-out servicing throughout. Per WEF Manual of Practice 7: properly-maintained hydromechanical interceptors retain certified performance throughout service life; replacement driven by code revision or structural corrosion rather than functional degradation.

Total cost of ownership over 15 years: $2,400–$3,200 (capital) + $6,750–$13,500 (maintenance) = $9,000–$16,700 lifecycle total per single 3-compartment sink installation. Compared to gravity interceptor alternative ($8,000–$15,000 capital + lower maintenance): hydromechanical wins for kitchens under approximately 200 meals/day per IPC Section 1003.4 sizing methodology.

How Drain Time Governs: 2 Minutes vs 1 Minute

The drain time assumption is the single largest source of variation between code jurisdictions for the same fixture. The IPC 2021 edition and many current local adoptions use a 1-minute drain basis. The older PDI G101 basis uses 2 minutes.

For the same 3-compartment sink above with a 1-minute drain:

Q = 67.3 / 1 = 67.3 gpm (254.7 L/min)
Required certified flow: next size above 67.3 = 75 gpm (284 L/min)
Required grease capacity: 2 × 75 = 150 lb (68.0 kg)

The required interceptor doubles from a 35 gpm / 70 lb (132 L/min / 31.8 kg) unit to a 75 gpm / 150 lb (284 L/min / 68.0 kg) unit. This is not a marginal difference; it affects unit selection, installation space, and cost. Confirm the adopted code's drain time with the AHJ before finalizing the specification. The calculator shows the drain time in the result and labels it as a profile factor so the basis is always visible on the calculation sheet.

Check Mode: Flow Governs vs Capacity Governs

When evaluating a proposed unit, the calculator computes ratios for both ratings:

flow_ratio     = required_flow / proposed_flow
capacity_ratio = required_capacity / proposed_capacity

Ratio ≤ 1.00: Adequate. Ratio 1.00–1.15: At limit. Ratio 1.15–1.50: Undersized. Ratio > 1.50: Significantly undersized. The governing shortfall names which rating controls the overall verdict.

Capacity-governs failure path. Using the 3-compartment example (required: 35 gpm / 70 lb / 132 L/min / 31.8 kg), evaluate a proposed unit with 35 gpm (132 L/min) certified flow and 50 lb (22.7 kg) certified grease capacity:

flow_ratio     = 35 / 35 = 1.00 → Adequate
capacity_ratio = 70 / 50 = 1.40 → Undersized

Governing shortfall: GREASE_CAPACITY. The unit drains the fixture without backing up, but the certified grease retention is 28% below the required minimum. The unit will reach its functional limit significantly before the designed cleaning interval, producing periodic FOG discharge.

Flow-governs failure path. Same requirement, proposed unit 25 gpm / 75 lb (95 L/min / 34.0 kg):

flow_ratio     = 35 / 25 = 1.40 → Undersized
capacity_ratio = 70 / 75 = 0.93 → Adequate

Governing shortfall: FLOW_RATING. The grease capacity is ample, but the flow rating is 28% below requirement. The unit may cause the sink to back up or drain slowly at peak load.

Both failure paths produce code-noncompliant installations. The check mode distinguishes them so the correct rating is upgraded in the replacement selection.

Manufacturer Survey: Certified Hydromechanical Models 5 gpm to 100 gpm

Certified hydromechanical grease interceptors are manufactured by approximately 8–10 major US/Canadian manufacturers serving the commercial kitchen market. Selection considers PDI G101, ASME A112.14.3, or CSA B481 certification listing, construction material (cast iron, polymer, fiberglass), installation orientation (under-counter, on-floor, in-ground vault), and pump-out access (top-side cleanout, removable lid, or sample port per IPC Section 1003.7).

Survey of certified hydromechanical units serving the most common 5–100 gpm range (per 2026 manufacturer catalogs and distributor pricing):

Manufacturer	Model Series	Flow Range (gpm / L/min)	Capacity Range (lb / kg)	Construction	Capital Cost (2026 USD)
Schier Products	GB Series	15–100 / 57–379	30–200 / 14–91	Fiberglass-reinforced polymer	$800–$3,500
Watts	WD Series	20–100 / 76–379	40–200 / 18–91	Cast iron	$1,000–$3,200
Zurn	ZS-880 Series	15–75 / 57–284	30–150 / 14–68	Cast iron	$1,200–$2,800
Josam	60000 Series	15–100 / 57–379	30–200 / 14–91	Cast iron	$1,100–$3,000
Endura ABT	ABT Series	25–100 / 95–379	50–200 / 23–91	Polymer	$900–$3,400
Thermaco	Trapzilla TZ	15–100 / 57–379	300–1,000 / 136–454	Polymer	$2,500–$8,500
Big Dipper	W-250/500/750 AST	25–75 / 95–284	250–750 / 113–340	Polymer w/ automatic skimmer	$4,500–$12,000
Canplas	Endura	20–100 / 76–379	40–200 / 18–91	Polymer	$1,000–$3,500 (CSA B481 listed)

Selection considerations per ASPE Plumbing Engineering Design Handbook Volume 4, Chapter 8:

(1) Installation space — under-counter applications favor compact polymer units (Schier, Endura ABT); larger installations benefit from cast iron's structural robustness (Watts, Zurn, Josam).

(2) Extended capacity — Thermaco Trapzilla and Big Dipper AST units carry significantly higher grease capacity than the 2× certified-flow minimum, allowing 90–180-day cleaning intervals vs typical 30–60-day frequency for standard units.

(3) Automatic grease removal — Big Dipper AST units include automatic skimmer mechanisms reducing manual pump-out frequency, but increase capital cost 2–3× vs basic hydromechanical units.

(4) Certification basis — verify the manufacturer's listing to the specific standard adopted in the local jurisdiction: PDI G101 for IPC states, CSA B481 for Canadian provinces, ASME A112.14.3 as alternative dual-listed units.

Per ASPE and WEF Manual of Practice 7: select the manufacturer model from the current certified rating list rather than legacy catalogs — certification listings update periodically per NSF/ANSI 46 retesting requirements.

Cleaning Interval and Pump-Out Frequency per IPC Section 1003.7

The 70 lb (31.8 kg) grease capacity rating from the Section 5 worked example translates to an actual cleaning interval based on the kitchen's daily FOG production rate. Per WEF Manual of Practice 7 and LA County Sanitation District FOG Control Program data, typical commercial kitchen FOG generation varies from 0.3 lb/day (light coffee shop) to 5+ lb/day (high-volume grill or ethnic cuisine with heavy frying).

FOG generation rates by kitchen type per WEF Manual of Practice 7, Table 4-2:

Kitchen Type	Daily FOG Generation
Coffee shop / café (low-volume)	0.3–0.5 lb/day (0.14–0.23 kg/day)
Sandwich shop / deli	0.5–1.0 lb/day (0.23–0.45 kg/day)
Restaurant (moderate, 100–200 meals/day)	1.0–2.0 lb/day (0.45–0.91 kg/day)
Burger / fried-chicken / fish-and-chips	2.0–4.0 lb/day (0.91–1.81 kg/day)
Buffet / ethnic cuisine with heavy frying	3.0–5.0 lb/day (1.36–2.27 kg/day)
High-volume commercial cafeteria	4.0–7.0 lb/day (1.81–3.18 kg/day)

Cleaning interval per IPC Section 1003.7 (25% capacity threshold): pump out when accumulated grease reaches 25% of certified capacity. This gives the regulatory minimum cleaning interval:

cleaning_interval (days) = (0.25 × capacity_lb) / daily_FOG_lb

Worked example: 70 lb (31.8 kg) certified capacity, moderate restaurant at 1.5 lb/day FOG production:

cleaning_interval = (0.25 × 70) / 1.5 = 17.5 / 1.5 = 11.7 days

11.7 days is the regulatory minimum per IPC Section 1003.7 for this load. Industry practice per ASPE and Schier installation guides targets 30–60 day intervals to balance compliance with pump-out service cost. For the 70 lb unit at 1.5 lb/day: a 30-day interval accumulates 45 lb (64% of capacity) — compliant where the AHJ enforces pump-out at full capacity. A 60-day interval accumulates 90 lb, exceeding certified capacity and producing FOG discharge between services — non-compliant under any interpretation.

Selection implication: if 30-day pump-out intervals are impractical due to service cost ($50–$100 per service × 12/year = $600–$1,200/year operating), select a unit with higher certified capacity. Schier GB-50 (100 lb / 45.4 kg) extends the compliant interval to approximately 17 days at 1.5 lb/day FOG; Thermaco Trapzilla TZ-300 (300 lb / 136 kg) extends it to approximately 50 days at the same load rate, bringing operating cost in line with the standard 30–60-day service cycle.

Per EPA 40 CFR Part 403 general pretreatment regulations and municipal FOG ordinances (NYC DEP Title 15 RCNY 19-03; LA County Sanitation District Industrial Waste Program, Article 4): maintenance log documentation is required showing pump-out frequency, FOG volume removed, and hauler certification. Some jurisdictions require monthly inspection and quarterly sample port testing per IPC 1003.7 regardless of pump-out frequency.

Hydromechanical vs Gravity Grease Interceptor: When Each Applies

The hydromechanical interceptor sized in this article applies to individual fixtures or small fixture combinations. Larger applications require a gravity grease interceptor sized by entirely different methodology per IPC Section 1003.4 and UPC Section 1014.

Type comparison per ASPE Plumbing Engineering Design Handbook Volume 4, Chapter 8:

Parameter	Hydromechanical	Gravity Interceptor
Typical capacity range	5–100 gpm certified / 10–200 lb	500–5,000 gallons total volume
Sizing method	Fixture-flow per PDI G101 / Section 3 above	Volume per IPC 1003.4 from meals + retention time
Installation location	Interior under-counter or floor-mount	Exterior below-grade vault
Footprint	24 × 18 × 18 in (610 × 457 × 457 mm) typical	8 × 6 × 5 ft (2.4 × 1.8 × 1.5 m) typical excavation
Capital cost (2026)	$1,200–$8,500 installed	$4,000–$25,000 installed (excavation + concrete vault + plumbing)
Cleaning interval	30–90 days typical	30–180 days depending on capacity vs load
Application threshold	Individual sinks, small kitchens (<200 meals/day)	Whole-kitchen high-volume (200+ meals/day)
Certification	PDI G101 / ASME A112.14.3 / CSA B481	ASTM C76 + ASTM C913 structural (precast vault)
Service requirements	Manual pump-out by licensed FOG hauler	Vacuum truck pump-out + sludge disposal
Lifecycle	15–20 years per manufacturer warranty	30–50+ years (concrete vault)

Gravity interceptor sizing per IPC Section 1003.4:

V_required (gal) = 4 × HR × M

where:
  V_required = required interceptor working volume [gal]
  HR = retention time [hours], typically 2–4 per local code
  M  = meals served per day (counted at peak meal period)

Example: 300 meals/day restaurant, 2.5-hour retention time:

V = 4 × 2.5 × 300 = 3,000 gallons (11,356 L)

Select the next standard precast size at or above 3,000 gal. Typical precast sizes per ASTM C76 / ASTM C913: 750, 1,000, 1,250, 1,500, 2,000, 3,000, 5,000 gallons; 3,000 gal exact match selected.

Alternative sizing methods per local FOG programs include the Drainage Fixture Unit approach per UPC Section 1014, seats × occupancy factor (used by some jurisdictions for stadium and theater concessions), and calculated FOG load per WEF Manual of Practice 7 for unusual cuisine types.

Transition threshold (hydromechanical to gravity) varies by jurisdiction. IPC commonly references 75 gpm (284 L/min) fixture flow rate as a practical upper limit for hydromechanical methodology; UPC and some state codes set 100 gpm (379 L/min). California Plumbing Code (CPC) Chapter 10 adopts the UPC base with state amendments. Above these thresholds, gravity interceptor sizing by volume methodology is the standard professional design approach per ASPE.

When the Fixture-Flow Method Does Not Apply

The PDI G101 fixture-flow method applies to individual fixtures or small combinations served by a hydromechanical interceptor. Several conditions push the design outside its scope.

Whole-kitchen loads. A high-volume kitchen (multiple fryers, griddles, broilers, and prep sinks draining to a single system) normally exceeds the hydromechanical range. Large in-ground or gravity grease interceptors are sized by volume from meals served per day and a retention time per the adopted code (commonly IPC Section 1003.4 or local FOG program rules), not by the drainage flow rate method. See the Hydromechanical vs Gravity Grease Interceptor section above for the volume-based sizing chain.

High-temperature discharge. Commercial dishwashers discharge at 140–180°F (60–82°C). At those temperatures, grease remains liquid and passes through the baffled chamber without separating effectively. IPC Section 1003.3.4 typically prohibits dishwasher discharge from connecting to a hydromechanical interceptor. NFPA 96 (Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations) addresses adjacent kitchen exhaust hood requirements that intersect with grease management system design. The calculator flags a dishwasher connection as a soft check for this reason; the flag does not change the computed size but signals a condition the AHJ or manufacturer may require to be addressed separately. Per WEF Manual of Practice 7: dishwasher waste typically routes to a separate solids interceptor or to sewer directly with a pre-cooling line.

Food-waste disposers. Disposers shred solids into fine particles that raise the suspended solids loading and can accelerate grease capacity consumption. Many FOG programs (NYC DEP Title 15 RCNY 19-03; LA County Sanitation District Article 4) restrict or prohibit food-waste disposer discharge to a grease interceptor. The calculator flags this condition for the same reason.

Unit counts above the hydromechanical range. The certified size series tops at 100 gpm (379 L/min) for standard hydromechanical units per PDI G101 / ASME A112.14.3 / CSA B481. A computed drainage flow above that threshold indicates that the fixture combination is beyond the type's design range. Per WEF Manual of Practice 7 and ASPE Design Handbook: fixtures at this scale are typically served by a whole-kitchen gravity interceptor per IPC Section 1003.4.

These limitations are displayed in the calculator result when the relevant conditions are detected. In each case, the final design follows the adopted code and professional judgment, not the screening result. Vent connections per IPC Section 1003.5 and sample port testing per IPC Section 1003.7 are required for all hydromechanical installations regardless of size.

FAQ

Related Calculators

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Grease Duct Sizing Calculator — Size kitchen exhaust ductwork for grease-laden airflow transport velocity and clearance requirements per NFPA 96. (Adjacent kitchen FOG management system.)

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