Ventilation Rate Calculator (ASHRAE 62.1)
How much outdoor air does a space need? Using the ASHRAE 62.1 ventilation rate procedure, this combines a per-person rate and a per-area rate to give the required fresh-air supply — pick the occupancy type, area and headcount.
How Much Fresh Air Does a Space Need?
Ventilation brings in outdoor air to dilute the carbon dioxide, odours and pollutants that people and materials give off. ASHRAE Standard 62.1 sets the minimum outdoor airflow for commercial and institutional spaces with its ventilation rate procedure. The idea is simple: people need a share of fresh air, and so does the floor area itself, so the requirement is the sum of a per-person rate and a per-area rate.
The Two-Part Rate
Rp is the airflow per person (cfm/person) and Ra is the airflow per unit floor area (cfm/ft²). Densely occupied rooms like conference rooms are dominated by the people term; large sparsely used spaces lean on the area term. Adding them gives the breathing-zone outdoor airflow, Vbz — the fresh air that actually has to reach the people.
Air Distribution Effectiveness (Ez)
Not all supply air reaches the breathing zone equally. Ez accounts for that: it is 1.0 for cool air from the ceiling or floor, but drops to 0.8 or 0.7 when warm air is supplied overhead and tends to short-circuit back to the return. Dividing Vbz by Ez gives the zone outdoor airflow Voz the system must actually deliver.
Default Outdoor Air Rates
| Space | Rp (cfm/person) | Ra (cfm/ft²) |
|---|---|---|
| Office | 5 | 0.06 |
| Conference room | 5 | 0.06 |
| Classroom (age 9+) | 10 | 0.12 |
| Lecture hall | 7.5 | 0.06 |
| Retail | 7.5 | 0.12 |
| Restaurant dining | 7.5 | 0.18 |
| Gym / fitness | 20 | 0.06 |
Frequently Asked Questions
How do you calculate required ventilation under ASHRAE 62.1?
Multiply the people rate by the number of occupants, add the area rate times the floor area for the breathing-zone airflow, then divide by the air distribution effectiveness Ez to get the zone outdoor airflow.
What is the difference between Rp and Ra?
Rp is outdoor air per person, covering occupant-generated pollutants; Ra is outdoor air per square foot, covering emissions from the building and furnishings. Both are added together.
What is air distribution effectiveness?
Ez measures how well supply air reaches the breathing zone. It is 1.0 for cool ceiling or floor supply and lower for warm overhead air that short-circuits to the return.
Is this the same as air changes per hour?
No. ACH measures total air movement relative to room volume; the ventilation rate procedure sizes the outdoor (fresh) air specifically, based on people and area.
Related calculators
- CFM & Air Changes Calculator — total airflow and air changes per hour.
- Duct Static Pressure Calculator — size the duct for that air.
- Fan Laws Calculator — match the fan to the airflow.
- BTU Calculator — heating and cooling load.