Ventilation Rate Calculator
Calculate required outdoor air ventilation rates per ASHRAE Standard 62.1 based on occupancy and space type.
About this calculator
This calculator computes required outdoor air ventilation per ASHRAE 62.1's two-component method: a per-person rate (Rp) times Number of Occupants, plus a per-area rate (Ra) times Floor Area, summed into Total Ventilation, then divided by the zone air distribution effectiveness (Ez = 0.8, a typical value per ASHRAE 62.1 Table 6.2.2.2 for a ceiling supply of warm air) to get Required Outdoor Air -- so Required Outdoor Air is always somewhat larger than the raw people-plus-area total, not identical to it. Space Type is a coded lookup (1 = Office, 2 = Retail, 3 = Classroom, 4 = Restaurant), not a continuous input -- each code selects a fixed pair of Rp/Ra rates from ASHRAE 62.1 Table 6.2.2.1 rather than interpolating between them. At the default 20 occupants and 2,000 sq ft, switching Space Type across its four codes changes Required Outdoor Air from 275 CFM (Office) to 488 CFM (Retail) to 550 CFM (Classroom) to 638 CFM (Restaurant) -- a more than double swing driven entirely by which fixed rate pair is selected.
Within the Office default, Floor Area has a slightly larger effect on Required Outdoor Air than Number of Occupants does, since the area component (120 CFM from 0.06 CFM/sq ft) starts out larger than the person component (100 CFM from 5 CFM/person) at these default values -- though both inputs always move the total in the same direction. This is a minimum-ventilation sizing tool per ASHRAE 62.1's Ventilation Rate Procedure; it doesn't account for demand-controlled ventilation or local code amendments that may require higher rates.
Inputs
Office: 5 CFM/person; Classroom: 10 CFM/person; Restaurant: 7.5 CFM/person per ASHRAE 62.1
ASHRAE 62.1 area rate: Office = 0.06 CFM/ft²; Classroom = 0.12 CFM/ft²
ASHRAE 62.1: Office 5 CFM/person; Classroom 10 CFM/person; Gym 20 CFM/person
Results
Required Outdoor Air
275 CFM
Total Ventilation
220 CFM
Figures current as of 2022. Source: ASHRAE Standard 62.1-2022, Ventilation for Acceptable Indoor Air Quality, Table 6.2.2.1 (minimum ventilation rates in breathing zone) and Table 6.2.2.2 (zone air distribution effectiveness)
How to Use This Calculator
- Enter Number of Occupants, Floor Area, and Space Type.
- Review Required Outdoor Air (CFM) and Total Ventilation (CFM).
- Use People Component (CFM) and Area Component (CFM) to inform your decision.
How the result changes with Floor Area
| Floor Area | Required Outdoor Air | Total Ventilation |
|---|---|---|
| 1,000 | 200 CFM | 160 CFM |
| 1,500 | 238 CFM | 190 CFM |
| 3,000 | 350 CFM | 280 CFM |
| 5,000 | 500 CFM | 400 CFM |
What each input means
- Number of Occupants
- Expected number of occupants in the zone at design conditions. ASHRAE 62.1 Table 6.2.2.1 provides per-person outdoor air rates by occupancy category.
- Floor Area
- Net occupiable floor area of the ventilation zone. ASHRAE 62.1 Table 6.2.2.1 provides per-area outdoor air rates (Ra) that supplement the per-person rates.
- Space Type
- Occupancy category per ASHRAE 62.1 Table 6.2.2.1. 1 = Office (5 CFM/person + 0.06 CFM/ft²), 2 = Retail (7.5 CFM/person), 3 = Classroom (10 CFM/person + 0.12 CFM/ft²), 4 = Restaurant dining (7.5 CFM/person).
How this is calculated
Worked example, using the default values
- Identify Input ParametersNumber of Occupants = 20, Floor Area = 2000, Space Type = 1 = 3 input(s) provided
- Calculate Required Outdoor AirRequired Outdoor Air275 = 275
- Calculate Total VentilationTotal Ventilation220 = 220
- Calculate People ComponentPeople Component100 = 100
- Calculate Area ComponentArea Component120 = 120
Figures and sources
- ASHRAE 62.1 Ventilation Rate Procedure — per-occupancy Rp/Ra rates and zone air distribution effectiveness (2022) — ASHRAE Standard 62.1-2022, Ventilation for Acceptable Indoor Air Quality, Table 6.2.2.1 (minimum ventilation rates in breathing zone) and Table 6.2.2.2 (zone air distribution effectiveness)
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
How much does space type change the required ventilation?
Substantially -- at the default 20 occupants and 2,000 sq ft, Required Outdoor Air goes from 275 CFM for Office to 488 CFM for Retail, 550 CFM for Classroom, and 638 CFM for Restaurant, since each space type selects a different fixed pair of per-person and per-area rates from ASHRAE 62.1 Table 6.2.2.1 rather than a single formula that scales smoothly.
Does occupancy or floor area matter more for ventilation requirements?
At the default Office space type, Floor Area has a slightly larger effect on Required Outdoor Air than Number of Occupants, because the area component (120 CFM from a 0.06 CFM per square foot rate) starts out larger than the person component (100 CFM from a 5 CFM per person rate) at the default 2,000 sq ft and 20 occupants -- both always move the total in the same direction, though.
What is the People Component versus the Area Component?
People Component is the per-person outdoor air requirement (Rp times Number of Occupants) and Area Component is the per-square-foot requirement (Ra times Floor Area); ASHRAE 62.1 requires both to be added together, since occupant density and building materials/furnishings both generate ventilation demand independently.
Why does a classroom need more ventilation than an office of the same size?
ASHRAE 62.1 assigns classrooms a higher per-person rate (10 CFM/person versus 5 for offices) and a higher per-area rate (0.12 CFM per square foot versus 0.06), reflecting denser occupancy and higher activity levels, so a classroom at the same occupant count and floor area as an office computes to roughly double the required outdoor air.
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