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Commercial Hood Exhaust Sizing Calculator

CFM Requirements per IMC Chapter 5 for Type I and Type II Hoods Based on Equipment, Hood Style, and Duty Rating

Free commercial kitchen hood exhaust calculator for HVAC contractors, kitchen designers, and mechanical engineers who need to determine exhaust airflow rates for restaurant and institutional cooking operations. Select the hood type (Type I grease-laden or Type II heat/moisture), hood style (wall canopy, island, single island, proximity/backshelf, or eyebrow), equipment duty rating (light, medium, heavy, extra-heavy), and enter your hood dimensions. The calculator returns the required exhaust CFM per IMC Section 507 and the corresponding makeup air requirement so your kitchen stays properly balanced.

Getting hood exhaust wrong causes real problems on the jobsite. Undersized exhaust means grease buildup, smoke in the dining room, and failed health inspections. Oversized exhaust wastes energy, creates excessive negative pressure that makes doors hard to open, and pulls conditioned air out of the building. The IMC exhaust rates are based on CFM per linear foot of hood, with multipliers that vary by hood style and cooking duty. A heavy-duty charbroiler under a wall-canopy hood needs 400 CFM/ft, while a light-duty steam table under a backshelf hood only needs 150 CFM/ft. This calculator applies the correct rate automatically based on your selections.

The output includes the total exhaust CFM, recommended makeup air percentage (typically 80-90% of exhaust to maintain slight negative pressure in the kitchen), and a summary you can include in your mechanical submittal package. For UL 710-listed hoods with published exhaust rates, always defer to the manufacturer's listed rate over the IMC default calculations.

Pro Tip: Always verify whether the local jurisdiction has amended IMC Chapter 5 with stricter requirements. Many cities and counties require 10-20% more CFM than the base IMC table, and some require a minimum of 150 CFM per linear foot regardless of duty rating. Check with the AHJ before finalizing your design.

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Commercial Hood Exhaust Sizing Calculator
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How It Works

  1. Select Hood Type and Style

    Choose Type I (grease-laden vapors from cooking) or Type II (heat, steam, odors only). Then select the hood style: wall-canopy, island canopy, single-island, proximity/backshelf, or eyebrow. Each combination has a different CFM-per-linear-foot rate in the IMC tables.

  2. Set Equipment Duty Rating

    Select light duty (steam tables, ovens below 400 F), medium duty (ranges, griddles, fryers), heavy duty (charbroilers, woks, high-heat equipment), or extra-heavy duty (solid-fuel cooking, mesquite grills). The duty rating determines the CFM multiplier.

  3. Enter Hood Dimensions

    Input the hood length and width in feet. The calculator uses the length (the dimension parallel to the front of the cooking line) to compute total CFM. Width is used for the makeup air balance check.

  4. Review Exhaust and Makeup Air Results

    Check the total exhaust CFM, the recommended makeup air CFM (80-90% of exhaust), and the net negative pressure balance. Use these values for ductwork sizing, fan selection, and your mechanical permit drawings.

Built For

  • HVAC contractors sizing exhaust fans and makeup air units for new restaurant buildouts
  • Kitchen design consultants preparing mechanical specifications for equipment submittals
  • Mechanical engineers calculating exhaust and makeup air loads for permit applications
  • Restaurant owners evaluating whether their existing hood system meets code for a menu change that adds charbroiling

Features & Capabilities

IMC Chapter 5 Exhaust Rates

Applies the correct CFM per linear foot from IMC Section 507 based on hood type, hood style, and equipment duty rating. Covers all standard combinations from 150 CFM/ft (light-duty backshelf) through 550 CFM/ft (extra-heavy-duty island canopy).

Makeup Air Balance

Calculates the recommended makeup air volume at 80-90% of exhaust to maintain slight negative pressure in the kitchen, preventing grease-laden air from migrating to the dining room while avoiding excessive pressure differential on entry doors.

Duty Rating Guide

Includes a built-in reference for classifying cooking equipment by duty rating per IMC Table 507.2.1, so you can confirm whether your specific equipment falls under light, medium, heavy, or extra-heavy duty.

Multiple Hood Zones

Supports calculating exhaust for multiple hood sections with different duty ratings along the same cooking line, then totals the combined CFM for fan and duct sizing.

Frequently Asked Questions

Type I hoods are required over equipment that produces grease-laden vapors — fryers, griddles, charbroilers, ranges, woks, and similar cooking equipment. They must include grease filters (baffle or cartridge type) and connect to a dedicated grease duct with fire suppression. Type II hoods are for equipment that produces only heat, steam, or odors without grease — dishwashers, steam tables, ovens below 400 F. Type II hoods do not need grease filters or fire suppression, and their ductwork requirements are less stringent.
The general guideline is 80-90% of the total exhaust volume. This maintains a slight negative pressure (0.02 to 0.05 inches w.c.) that keeps cooking odors and grease-laden air from migrating into the dining area. If makeup air is less than about 75% of exhaust, doors become hard to open, drafts are uncomfortable for staff, and the exhaust fan may not perform to its rated CFM because of the restriction. Many jurisdictions now require dedicated makeup air units rather than relying on transfer air from the HVAC system.
Yes. If the hood is UL 710 listed, the listed exhaust rate takes precedence over the IMC default calculations. UL 710 testing verifies capture and containment performance at a specific CFM for each hood model and cooking equipment combination. The listed rate may be lower or higher than the IMC table value. Most modern hood manufacturers provide UL 710 listing, and many jurisdictions accept only UL 710-listed hoods for new construction.
An undersized exhaust hood cannot capture all the cooking effluents, leading to grease accumulation on ceiling tiles, walls, and HVAC equipment — which is both a fire hazard and a health code violation. Smoke and odors escape into the dining room, and the grease duct and fan accumulate buildup faster, increasing fire risk and cleaning costs. The health department can shut down cooking operations until the exhaust system is corrected, which typically means replacing the fan, ductwork, or the entire hood.

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