Ventilation Rate Calculator
A ventilation rate calculator determines the minimum outdoor air ventilation required to maintain acceptable indoor air quality in commercial and institutional buildings per ASHRAE Standard 62.1. Inadequate ventilation results in elevated CO2, VOCs, humidity, and biological pollutants that reduce occupant health and productivity. ASHRAE 62.1's Ventilation Rate Procedure combines a per-person outdoor air component with an area-based component to establish the minimum breathing zone outdoor airflow. This calculator uses ASHRAE 62.1 Table 6-1 default values for common space types and outputs the required CFM and air changes per hour.
ASHRAE 62.1 ventilation rate formula
Vbz = (Rp x Pz) + (Ra x Az)
ACH = (Vbz x 60) / (Az x Ceiling Height)
Rp = per-person outdoor air rate (CFM/person from Table 6-1)
Pz = number of occupants
Ra = per-area outdoor air rate (CFM/sq ft from Table 6-1)
Az = zone floor area (sq ft)
Frequently asked questions
What is the ASHRAE 62.1 ventilation rate procedure?
ASHRAE 62.1 uses the Ventilation Rate Procedure: Vbz = Rp x Pz + Ra x Az, where Vbz is the breathing zone outdoor airflow, Rp is the per-person outdoor air rate (CFM/person), Pz is the number of occupants, Ra is the per-area outdoor air rate (CFM/sq ft), and Az is the zone floor area.
What outdoor air rates does ASHRAE 62.1 specify for offices?
For office spaces, ASHRAE 62.1-2022 specifies Rp = 5 CFM per person and Ra = 0.06 CFM per sq ft. A 1,000 sq ft office with 10 occupants requires: (5 x 10) + (0.06 x 1,000) = 50 + 60 = 110 CFM of outdoor air minimum.
What is air changes per hour (ACH)?
Air changes per hour (ACH) = (CFM x 60) / Volume (cubic feet). It represents how many times per hour the total room volume of air is replaced with fresh air. ASHRAE recommends specific ACH rates for different applications: hospitals 6-12 ACH, classrooms 4-12 ACH, general offices 4-10 ACH.
How do I calculate outdoor air for a residential building?
ASHRAE 62.2 for residential: minimum ventilation = 0.01 x floor area + 7.5 x (bedrooms + 1) CFM. For a 1,500 sq ft house with 3 bedrooms: (0.01 x 1,500) + (7.5 x 4) = 15 + 30 = 45 CFM continuous mechanical ventilation.
What happens if ventilation rates are too low?
Insufficient ventilation leads to indoor air quality (IAQ) problems including elevated CO2, volatile organic compounds (VOCs), humidity, odors, and biological contaminants. ASHRAE 62.1 minimum rates are set to prevent health complaints at acceptable probability. Below-code ventilation increases occupant sick day rates and reduces productivity.
Official sources
- ASHRAE: ASHRAE Standard 62.1-2022 Ventilation and Acceptable Indoor Air Quality in Commercial Buildings.
- EPA: EPA Indoor Air Quality - Ventilation.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.