IMC 403.3.1.1 · ASHRAE 62.1 Table 6-1

How to read minimum ventilation rates (IMC & ASHRAE 62.1)

People rate plus area rate, occupant density, and zone air distribution effectiveness — how the outdoor-air tables work, with a worked office example and a link to the free ICC viewer.

Need the actual table? Read IMC Table 403.3.1.1 in the free ICC code viewer →

How to read the ventilation-rate table, step by step

Step	What you do	Watch out for
1	Find the occupancy category row that matches how the space is actually used (office, classroom, retail sales, etc.)	Use the use of the room, not the building’s overall occupancy group
2	Read the people outdoor-air rate (Rp, cfm per person)	This covers people-generated contaminants (bioeffluents)
3	Read the area outdoor-air rate (Ra, cfm per ft²)	This covers building-generated contaminants (finishes, furniture)
4	Get the zone population (Pz) — design occupancy, or the table’s default density if the actual count isn’t known	The default density column is per 1,000 ft²; don’t read it as per ft²
5	Breathing-zone airflow: Vbz = Rp × Pz + Ra × Az	The two parts add — you don’t pick the larger one
6	Divide by zone air distribution effectiveness: Voz = Vbz ÷ Ez	Ez depends on how air is supplied and returned, and can differ in heating vs cooling
7	For multi-zone recirculating systems, carry Voz into the system-level calc (system ventilation efficiency)	Summing zone Voz is only the answer for single-zone or 100% outdoor-air systems

Our own summary, not the code text. Read IMC Table 403.3.1.1 in the free ICC viewer (codes.iccsafe.org) or ASHRAE 62.1 from ASHRAE, and verify against the edition adopted by your AHJ. Not engineering advice — ventilation design belongs to the mechanical engineer of record.

Which document applies?

Situation	Where to look	Notes
Commercial building in an IMC jurisdiction	IMC Section 403 and Table 403.3.1.1	The IMC table is built on the same people-plus-area method as 62.1
Project specs or LEED call for ASHRAE 62.1	ASHRAE 62.1 Section 6, Table 6-1 (Ventilation Rate Procedure)	Newer editions renumber sections — check the one in the spec
Low-rise residential dwelling	IRC / IMC residential provisions or ASHRAE 62.2	62.1 is written for commercial and high-rise residential
Exhaust-driven spaces (toilets, janitor, kitchens)	The exhaust-rate table (IMC 403.3.1.1 exhaust column / 62.1 exhaust table)	Exhaust is a separate requirement from outdoor air
Engineered alternative (sensors, air cleaning)	IAQ Procedure or demand-controlled ventilation provisions	Requires engineering documentation; not a field decision

Worked example — 1,000 ft² open office, 5 people, ceiling supply and return

Input / step	Value	Source
Floor area (Az)	1,000 ft²	Plans
Occupants (Pz)	5 people	Design occupancy
People rate (Rp), office	5 cfm/person	Office row
Area rate (Ra), office	0.06 cfm/ft²	Office row
Vbz = 5 × 5 + 0.06 × 1,000	25 + 60 = 85 cfm	People part + area part
Cooling: ceiling supply of cool air, Ez	1.0 → Voz = 85 cfm	Ez table
Heating: ceiling supply of warm air, ceiling return, Ez	0.8 → Voz ≈ 106 cfm	Ez table — design to the worse mode

Worked example values only. Look up the full table in the ICC viewer or ASHRAE 62.1.

Common field mistakes

Mistake	What to do instead
Using only cfm-per-person (the old pre-2004 way)	Add the people part and the area part — both apply
Forgetting Ez in heating mode	Warm air dumped from the ceiling short-circuits to the return; that mode usually governs
Adding up zone cfm for a VAV system and calling it done	Multi-zone systems need the system ventilation efficiency step; that’s the engineer’s calc
Setting the OA damper once at startup and walking away	Verify outdoor airflow during TAB at min and max fan speed
Confusing outdoor air with exhaust	Toilet and janitor exhaust are separate rates in a separate column/table

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Outdoor air without the guesswork

Read the table in the free ICC code viewer. ICC offers read-only access to the International Mechanical Code. ASHRAE 62.1 is sold by ASHRAE (ASHRAE has offered free read-only access to some standards — check ashrae.org). This page explains how to use the tables; it does not reproduce them. Edition referenced: the 2021/2024 IMC and ASHRAE 62.1-2019/2022 — verify against the edition adopted by your AHJ.

Since the 2004 edition of 62.1, minimum ventilation has been a two-part number: a people component for what occupants give off and an area component for what the building itself gives off. The IMC table uses the same approach. That’s why a lightly occupied warehouse office still needs outdoor air, and why a packed conference room needs a lot more than its square footage suggests.

Ez is where field conditions change the answer. The same room needs more outdoor air when the system delivers air poorly to the breathing zone. The classic case is a rooftop unit throwing warm air from ceiling diffusers with ceiling returns in winter — the warm air stratifies and short-circuits, so the standard penalizes it. If the diffuser layout changes in a value-engineering round, the ventilation calc should be rechecked.

Worked example. A 1,000 ft² office with 5 people: the office row gives 5 cfm/person and 0.06 cfm/ft², so Vbz = 25 + 60 = 85 cfm. In cooling with ceiling supply, Ez is 1.0 and the zone needs 85 cfm. In heating with warm ceiling supply and ceiling return, Ez drops to 0.8, and the zone needs about 106 cfm. Design to the worse mode.

For a multi-zone VAV system, zone numbers feed a system-level calculation that the engineer of record performs — don’t just total them. For related references, see the IBC occupant load factors guide (egress occupant load is a different number from ventilation population) and the duct construction and Manual J/D/S guide. Field PM keeps TAB reports, submittals, and punch items with the job so the ventilation numbers are on file at closeout.

FAQ

How do I calculate minimum outdoor air for a room?+

Find the room’s occupancy category in IMC Table 403.3.1.1 or ASHRAE 62.1 Table 6-1, multiply the people rate by the number of occupants, multiply the area rate by the floor area, add the two, then divide by the zone air distribution effectiveness (Ez). Read the table free in the ICC viewer at codes.iccsafe.org.

How much outdoor air does an office need?+

Using the office row, a 1,000 ft² office with 5 people works out to 5 × 5 cfm plus 0.06 × 1,000 cfm, or 85 cfm in the breathing zone, before the Ez adjustment. Your edition and AHJ govern — check the current table.

What is zone air distribution effectiveness (Ez)?+

Ez accounts for how well supply air actually reaches the breathing zone. A well-mixed ceiling supply in cooling is treated as fully effective; warm air supplied at the ceiling with a ceiling return is penalized, so the zone needs more outdoor air in heating.

Is the IMC table the same as ASHRAE 62.1?+

They use the same people-plus-area method and generally track each other, but they are separate documents with their own editions and occupancy lists. Use the one your jurisdiction or spec adopts, and don’t mix rows between them.

Do I need to do this on a residential job?+

Single-family and low-rise residential ventilation normally falls under the IRC/IMC residential provisions or ASHRAE 62.2, not 62.1. Check what your AHJ enforces.

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