Grease Duct Sizing Calculator
On this page
Calculate
Total exhaust airflow from the Type I hood (CFM)
Typical grease duct velocity: 1500–2500 FPM
Leave blank for round duct only. Enter width to calculate rectangular equivalent height.
Overview
A Grease Duct Sizing Calculator estimates the duct size needed for a Type I commercial kitchen exhaust system using one fixed design model: required duct cross-sectional area is determined from airflow divided by target grease-duct transport velocity, then converted into a round duct diameter or equivalent rectangular size. This is a practical first-pass engineering method for sizing grease exhaust ductwork while keeping velocity in a usable transport range for grease-laden air. Multiple industry and code-guidance sources describe grease-duct sizing in terms of airflow and duct velocity, commonly using a practical Type I grease-duct velocity band around 1500–2500 fpm.
Enter the exhaust airflow, select the target grease duct velocity, and choose whether you want a round duct result or an equivalent rectangular duct result. The calculator first computes the required duct area from airflow and velocity, then converts that area into a round diameter or rectangular dimensions.
How to Use This Calculator
Enter exhaust airflow — in m³/h or CFM.
Enter target grease duct velocity — in m/s or FPM.
Enter rectangular duct width (optional) — in mm or in.
Click "Calculate" — get required duct area, round duct diameter, rectangular duct height, and resulting duct velocity (for the velocity check).
Check that the resulting velocity stays within 1,500–2,500 fpm; then verify NFPA 96 construction, slope, and clearance requirements separately.
Inputs & Outputs
Inputs
- •Exhaust Airflow (m³/h / CFM)
- •Target Grease Duct Velocity (m/s / FPM)
- •Rectangular Duct Width (optional) (mm / in)
Outputs
- •Required Duct Area (m² / ft²)
- •Round Duct Diameter (mm / in)
- •Rectangular Duct Height (mm / in)
- •Resulting Duct Velocity (m/s)
Formula
Calculator Formula
This page uses one fixed airflow-and-velocity duct sizing model.
Step 1: Required Duct Area
Imperial:
A (ft²) = Q (CFM) / V (FPM)
Metric:
A (m²) = Q (m³/s) / V (m/s)
If airflow is entered in m³/h:
Q (m³/s) = Q (m³/h) / 3600
This is the standard duct sizing relation between airflow, area, and velocity.
Step 2: Round Duct Diameter
D = √(4A / π)
Where:
- D = duct diameter (ft or m, then convert to in or mm)
- A = required duct area
Step 3: Rectangular Equivalent
If rectangular sizing is needed:
H = A / W
Where:
- W = duct width
- H = duct height
- A = required duct area
Step 4: Velocity Check
V = Q / A
Where:
- V = resulting duct velocity
- Q = exhaust airflow
- A = actual duct area (round or rectangular)
Variable Reference
| Variable | Meaning | Units |
|---|---|---|
| Q | Exhaust airflow | CFM / m³/h |
| V | Target or resulting duct velocity | FPM / m/s |
| A | Duct cross-sectional area | ft² / m² |
| D | Round duct diameter | in / mm |
| W | Rectangular duct width | in / mm |
| H | Rectangular duct height | in / mm |
What is Grease Duct Sizing
Grease duct sizing is the process of selecting a duct size that can carry grease-laden kitchen exhaust air at a practical transport velocity without becoming excessively large or excessively restrictive. In commercial kitchen exhaust, duct size is not chosen by area alone; it must also support a realistic grease-exhaust velocity so the system can transport grease aerosols effectively while keeping pressure drop manageable.
Practical code-guidance and design references for Type I hoods commonly discuss grease-duct sizing in terms of airflow and velocity. The fundamental relationship is the same as any duct sizing calculation — Area = Airflow ÷ Velocity — but the target velocity range is specific to grease-laden exhaust systems.
Why Grease Duct Sizing Matters
If the duct is too small, velocity and pressure drop can become excessive, increasing fan energy, noise, and turbulence. If the duct is too large, grease transport velocity may become too low, allowing grease to settle and accumulate inside the duct — creating a fire hazard and maintenance problem.
Practical guidance for grease ducts commonly references a working Type I velocity band around 1500–2500 fpm (7.6–12.7 m/s). Staying within this range helps ensure effective grease transport while keeping system resistance manageable.
Grease Duct Velocity Guidelines
| Condition | Velocity (FPM) | Velocity (m/s) | Interpretation |
|---|---|---|---|
| Too high | > 2,500 | > 12.7 | Excessive pressure drop, noise, turbulence |
| Practical range | 1,500–2,500 | 7.6–12.7 | Effective grease transport, manageable resistance |
| Low but workable | 1,200–1,500 | 6.1–7.6 | Large duct, still functional |
| Too low | < 1,200 | < 6.1 | Risk of grease accumulation, poor transport |
Note: These are practical guidance values. Final requirements depend on the adopted code and local authority.
Round vs Rectangular Grease Ducts
Round Grease Ducts
- Lower friction loss per unit of airflow
- Easier to seal and maintain
- Better structural integrity under negative pressure
- Preferred when shaft space allows
Rectangular Grease Ducts
- Useful when vertical space is limited
- Must preserve equivalent area and transport velocity
- Higher friction loss than round ducts of the same area
- More complex fabrication and sealing
When substituting rectangular for round, always verify that the equivalent area is maintained and the resulting velocity stays within the practical grease-duct range.
Key Differences from Standard Duct Sizing
Grease duct sizing differs from standard HVAC duct sizing in several important ways:
- Higher minimum velocity — grease ducts need adequate velocity to transport grease aerosols
- Welded construction — grease ducts require liquid-tight welded joints
- Fire-rated enclosures — grease ducts must be enclosed in fire-rated shafts or wrapped with fire protection
- Cleanout access — grease ducts require access panels for cleaning and inspection
- Slope requirements — grease ducts must slope toward a grease collection point
- Code-specific requirements — IMC and NFPA 96 impose additional construction and routing rules
Practical Tips
Always verify the hood exhaust airflow before sizing the grease duct. The duct size is only as good as the airflow input.
Keep grease duct velocity in the 1500–2500 fpm range for effective grease transport. Below 1200 fpm, grease may settle and accumulate.
When using rectangular ducts, maintain the same cross-sectional area as the equivalent round duct. Do not assume a rectangular duct of similar nominal dimensions provides the same performance.
Always check local code requirements. IMC and NFPA 96 have specific requirements for grease duct construction, routing, slope, cleanouts, and fire protection that go beyond airflow/velocity sizing.
Consider pressure drop and fan static pressure. A grease duct sized for velocity may still require a larger fan if the duct run is long or includes many fittings.
Key Facts
- Grease duct sizing must support adequate transport velocity for grease-laden exhaust air.
- Practical Type I grease-duct velocity is commonly referenced around 1500–2500 fpm.
- Too-low velocity may allow grease accumulation inside the duct.
- Too-high velocity increases pressure drop, noise, and fan energy consumption.
- Grease ducts require welded construction, fire-rated enclosures, and cleanout access per code.
Applications
- Type I commercial kitchen hood exhaust.
- Grease duct first-pass sizing.
- Round grease duct sizing.
- Rectangular equivalent grease duct sizing.
- Velocity checks for existing grease duct sizes.
- Restaurant kitchen exhaust planning.
- Fan and static-pressure prechecks.
- Shaft and routing space planning.
Example Calculation
Imperial Example
Given:
- Exhaust airflow = 3,500 CFM
- Target grease duct velocity = 2,000 FPM
Step 1 — Required duct area:
A = 3,500 / 2,000 = 1.75 ft²
Step 2 — Round duct diameter:
D = √(4 × 1.75 / π) ≈ 1.493 ft
D = 1.493 × 12 = 17.9 in
So the required round grease duct is approximately 18 in diameter.
Step 3 — Check resulting velocity with 18 in round duct:
18 in = 1.5 ft diameter
Area = π × (1.5²) / 4 ≈ 1.767 ft²
V = 3,500 / 1.767 ≈ 1,981 FPM
That sits inside a practical Type I grease-duct velocity band.
Metric Example
Given:
- Exhaust airflow = 6,000 m³/h
- Target grease duct velocity = 9.0 m/s
Convert airflow:
Q = 6,000 / 3,600 = 1.667 m³/s
Step 1 — Required duct area:
A = 1.667 / 9.0 = 0.1852 m²
Step 2 — Round duct diameter:
D = √(4 × 0.1852 / π) ≈ 0.486 m = 486 mm
So the required round grease duct is approximately 486 mm.
This follows the exact fixed page logic using airflow and target velocity.
Standards & References
- IMC (International Mechanical Code) — grease duct construction and sizing requirements
- NFPA 96 — Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations
- SMACNA HVAC Duct Construction Standards — duct fabrication and sizing guidelines
- ASHRAE Fundamentals Handbook — HVAC duct design principles
- Local mechanical code — jurisdiction-specific grease duct requirements
Limitations
- This calculator is a first-pass grease duct sizing tool, not a full commercial kitchen code-compliance engine.
- It does not calculate: full duct pressure drop, grease filter pressure loss, fan selection, hood capture performance, cleanout spacing, duct slope and grease drainage, clearance and enclosure requirements, welded construction details, fire suppression integration, or exhaust termination rules.
- Those items are essential to final design but are outside the fixed sizing model used on this page.
- Final code compliance depends on the jurisdiction, adopted code edition, hood type, routing, access, slope, cleanouts, and fire-rated construction details.
Common Mistakes to Avoid
- Sizing grease duct by a comfort-ventilation velocity instead of a grease-exhaust transport velocity.
- Using an oversized duct that drops velocity too far, reducing effective grease transport.
- Switching from round to rectangular duct without preserving the needed area and velocity.
- Assuming that passing a velocity check alone means the system is code-compliant.
- Ignoring duct routing, access panels, slope, and fire-rated construction requirements.
- Not accounting for the difference between net free area and nominal duct size.
Frequently Asked Questions
What does this Grease Duct Sizing Calculator calculate?
What formula does this page use?
What velocity should be used for a grease duct?
Is this the same as ordinary supply or return duct sizing?
Can I use rectangular duct instead of round?
Does this calculator prove code compliance?
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Calculate
Total exhaust airflow from the Type I hood (CFM)
Typical grease duct velocity: 1500–2500 FPM
Leave blank for round duct only. Enter width to calculate rectangular equivalent height.