ASHRAE 62.1 Ventilation Rates: The 2022 Ventilation Rate Procedure and 2024 IMC

Minimum outdoor airflow is not determined from one “cfm per person” number. Under the ventilation-rate procedure, start with the exact space type and calculate breathing-zone outdoor airflow from both the people and area components:

Vbz = Rp × Pz + Ra × Az

Then account for zone air distribution effectiveness and, where one system serves multiple zones, complete the applicable system outdoor-air calculation. Exhaust, recirculation, contaminant-source control, filtration, operation, and balancing are separate requirements that can also govern the design.

This page compares the 2022 edition of ANSI/ASHRAE Standard 62.1 with the 2024 International Mechanical Code. They use closely related ventilation-rate concepts, but they are separate authorities. First establish which one the project contract and adopted jurisdiction require; do not present an IMC table row as if it were automatically the ASHRAE requirement.

The calculation sequence

An occupancy group alone is not enough. A Group B building can contain offices, conference rooms, break rooms, corridors, training rooms, storage rooms, and other space types with different rates and exhaust conditions.

Which authority and edition govern the ventilation calculation?

ASHRAE describes Standard 62.1-2022 as the ventilation and acceptable-indoor-air-quality standard for commercial and institutional buildings. Its Ventilation Rate Procedure is in Section 6.2, with minimum breathing-zone rates in Table 6-1. The standard is maintained through official interpretations and addenda, so record the edition and any adopted addenda used for the design.

The 2024 IMC has its own mechanical-ventilation procedure in Chapter 4. Section 403.3.1.1 and Table 403.3.1.1 establish the space rates, §403.3.1.2 addresses zone air distribution effectiveness, and the following sections address zone and system outdoor airflow. The code also references ASHRAE 62.1 in specified provisions, but the two documents should not be collapsed into one unnamed “national minimum.”

Specialized occupancies can invoke other authorities. For example, healthcare ventilation can involve ASHRAE 170 and facility-specific requirements. Process exhaust and hazardous exhaust are not resolved by the general breathing-zone calculation.

What do common occupancy types require per IMC Table 403.3.1.1?

Choose the table row that matches the actual use, occupant density, activity, and construction. The 2024 IMC says that an unlisted occupancy is assigned the listed classification most similar in occupant density, activities, and building construction, or is determined by an approved engineering analysis. That is more disciplined than choosing whichever row gives the lowest airflow.

The following is a limited 2024 IMC extract for common space types; consult the complete current table and notes:

The default density is a design input where the project’s estimated maximum population does not establish a larger value. Under the 2024 IMC, an alternative anticipated density requires approved statistical data. This ventilation population is not automatically identical to an IBC egress occupant load.

What is the ASHRAE 62.1 breathing zone outdoor airflow formula?

For the 2024 IMC, §403.3.1.1 uses:

Vbz = Rp × Pz + Ra × Az

where:

• Vbz is breathing-zone outdoor airflow;
• Rp is the table’s people outdoor-air rate;
• Pz is zone population;
• Ra is the table’s area outdoor-air rate; and
• Az is net occupiable zone floor area.

ASHRAE 62.1-2022 uses the same people-plus-area structure in its Ventilation Rate Procedure. Some rows have only one of the two rate components, so the correct rule is not “both terms are always nonzero”; it is “apply every component shown for the selected row.”

Example: a 2,000-ft² conference room with 30 design occupants

Using the 2024 IMC conference-room row:

• people component: 5 cfm/person × 30 people = 150 cfm;
• area component: 0.06 cfm/ft² × 2,000 ft² = 120 cfm; and
• breathing-zone airflow: Vbz = 270 cfm.

Providing only the people component would omit 120 cfm, or 44 percent of this calculated Vbz. The example stops at the breathing zone; it is not yet the final outdoor-air intake for every possible air-handling system.

What happens when zone air distribution effectiveness drops?

The 2024 IMC determines zone outdoor airflow from:

Voz = Vbz ÷ Ez

Select Ez from Table 403.3.1.2 using the actual supply-air temperature relative to the space, diffuser location, return or exhaust location, and any conditions in the table notes. “Ceiling supply” by itself is not enough.

For example, the 2024 IMC table assigns:

• Ez = 1.0 for ceiling or floor supply of cool air;
• Ez = 0.8 for ceiling supply of warm air with ceiling return, subject to a stated exception; and
• Ez = 0.7 for floor supply of warm air with ceiling return.

The table permits Ez = 1.2 for qualifying low-velocity displacement ventilation with cool floor supply and ceiling return. Do not apply 1.2 to every underfloor-air system, and do not assign 0.8 merely because the return grille is at the floor.

For the 270-cfm example, Ez = 0.8 would produce Voz = 337.5 cfm. The designer must verify that 0.8 actually describes the zone’s operating configuration.

How is the system outdoor-air intake calculated?

Voz is a zone result. A single-zone system, a 100-percent-outdoor-air system, and a multiple-zone recirculating system do not necessarily have the same outdoor-air intake calculation.

The 2024 IMC continues from the zone result through:

• single-zone outdoor-air intake;
• 100-percent-outdoor-air systems;
• primary outdoor-air fraction for multiple-zone systems;
• system ventilation efficiency;
• occupant diversity and uncorrected outdoor-air intake; and
• final system outdoor-air intake.

ASHRAE 62.1-2022 likewise provides both simplified and alternative multiple-zone procedures. A schedule that lists only the sum of zone Vbz values does not demonstrate the completed system procedure.

Which spaces must exhaust all air and prohibit recirculation?

The exhaust column and notes in 2024 IMC Table 403.3.1.1 are not additional Rp or Ra terms. They establish separate exhaust requirements and recirculation conditions for listed spaces.

Important corrections to common summaries include:

• public toilet-room exhaust is 70 cfm per water closet or urinal for intermittent operation or 50 cfm for continuous operation under the current table and Note e—the earlier version of this page reversed them;
• the science-laboratory row has both outdoor-air components and a separate exhaust rate, with a specific table note for periods when hoods are not in use and hazardous materials are absent;
• a 0.75-cfm/ft² entry applies to common garages for multiple dwelling units, while enclosed parking-garage systems also require the dedicated §404 analysis; and
• commercial-kitchen exhaust is governed by the applicable Chapter 5 hood and exhaust provisions, not a universal 0.7-cfm/ft² shortcut.

For spaces marked by the table’s recirculation notes, apply the current note and §403.2.1. Do not convert a recirculation prohibition into an unsupported statement that one particular equipment type is always mandatory.

How does residential ventilation differ from commercial?

The 2024 IMC’s dwelling-unit ventilation formula (§403.3.2.1, Equation 4-9) is QOA = 0.03 × floor area + 7.5 × (bedrooms + 1), cfm — not the older 0.01 × floor area + 7.5 × (bedrooms + 1) coefficient. The area coefficient moved from 0.01 to 0.03 with the ASHRAE 62.2-2013 update the IMC adopted, tripling the area-driven portion of the required continuous outdoor airflow; do not use the 0.01 coefficient as the current answer. Sections 403.3.2.1 through 403.3.2.5 govern qualifying Group R-2, R-3, and R-4 occupancies three stories or fewer above grade plane.

ASHRAE Standard 62.2 is the separate ASHRAE standard for residential buildings within its scope. Confirm which standard and adopted code apply before transferring a residential formula between them.

What should be documented for the ventilation calculation?

Record at least:

1. governing code or standard, edition, addenda, and local amendments;
2. zone names, actual space uses, table rows, floor areas, and populations;
3. Rp, Ra, Pz, Az, and Vbz for each zone;
4. air-distribution configuration, operating mode, Ez, and Voz;
5. system type, served zones, diversity, efficiency, and outdoor-air intake result;
6. exhaust, recirculation, filtration, source-control, and specialized-occupancy checks;
7. design airflow controls, balancing requirements, and commissioning/TAB records; and
8. assumptions that must be rechecked if use, occupancy, layout, or system operation changes.

Frequently asked questions

Is 5 cfm per person the complete office requirement?

No. In the representative 2024 IMC office row, 5 cfm/person is the people component and 0.06 cfm/ft² is the area component. The zone and system calculations follow.

Does every zone require nonzero people and area terms?

No. Apply the components shown for the selected table row. Public corridors, for example, have an area component but no people component in the 2024 IMC table.

Is outdoor airflow the same as total supply airflow?

No. Supply air can include outdoor and recirculated air. The ventilation procedure establishes outdoor-air requirements; thermal loads and air distribution can require a larger total supply airflow.

Does Standard 62.1 guarantee acceptable indoor air quality?

The standard establishes minimum ventilation and other measures within its scope. Source control, outdoor-air quality, filtration, humidity, operation, maintenance, specialized hazards, and occupant conditions still matter.

Scope and limitations

This guide explains the 2024 IMC procedure and the corresponding ASHRAE 62.1-2022 concepts. It does not reproduce the complete copyrighted standard, replace mechanical engineering, select a row for an unlisted process, calculate a multiple-zone system, or resolve healthcare, laboratory-process, hazardous-exhaust, smoke-control, or local-amendment requirements.

Primary sources

• ASHRAE: Read-only standards, including Standard 62.1-2022
• ASHRAE: Technical FAQs for Standards 62.1 and 62.2
• ASHRAE: Interpretations for Standard 62.1-2022
• ASHRAE: Addenda to Standard 62.1-2022
• 2025 ASHRAE Handbook—Fundamentals, Chapter 16: Ventilation and Infiltration
• 2024 IMC §403.3.1.1 and Table 403.3.1.1
• 2024 IMC, Chapter 4: Ventilation
• UpCodes: IMC §403.3.2.1, Outdoor Air for Dwelling Units, Equation 4-9

Correction history

• 2026-08-31 — Revision 1: Separated ASHRAE 62.1-2022 from the 2024 IMC; corrected the calculation sequence, space-type scope, Ez configurations, public-toilet exhaust order, residential method, and recirculation framing; removed unsupported CO₂, equipment, enforcement, and causal claims; and reassigned the guide from IBC to Codes & Standards.
• 2026-09-08 — Revision 2: Corrected the dwelling-unit ventilation equation — the current IMC §403.3.2.1 Equation 4-9 coefficient is 0.03 (from the ASHRAE 62.2-2013 update), not the 0.01 coefficient this page previously called obsolete; the 0.35 ACH / 15 cfm-per-person figure is the older, superseded method, not the current one.