About Ventilation Rate Calculator
The ventilation rate calculator determines the required outdoor airflow for indoor air quality per ASHRAE 62.1, EN 16798-1, and CIBSE Guide B. It is used by mechanical engineers and building services designers to size fresh-air supply, verify air change rates, and document compliance across multiple spaces.
Add each space with its type, floor area, ceiling height, and occupancy; the tool applies the per-person and per-area rates for the chosen standard and returns total airflow in L/s and CFM, weighted average ACH, and a per-space breakdown in real time.
How It Works
- Select the governing standard (ASHRAE 62.1, EN 16798-1, or CIBSE Guide B).
- For each space, look up the per-person rate Rp and per-area rate Ra by space type.
- Compute the required outdoor air as Rp times occupants plus Ra times floor area.
- Convert to CFM and ACH, then weight ACH by volume for a project-level summary.
Worked Example
A 50 m2 office for 10 people under ASHRAE 62.1 (Rp = 2.5, Ra = 0.3) requires 2.5 * 10 + 0.3 * 50 = 40 L/s. With a 2.7 m ceiling (135 m3), ACH = 40 * 3.6 / 135 = 1.07, and 40 L/s converts to about 85 CFM.
Formulas
- Required outdoor air
Q = Rp * N + Ra * A- Air changes per hour
ACH = (Q * 3.6) / V- CFM conversion
CFM = Q * 2.11888
Standards & References
- ASHRAE 62.1
- EN 16798-1
- CIBSE Guide B
Frequently Asked Questions
Which ventilation standards does this calculator support?
It supports ASHRAE 62.1, EN 16798-1 (Category II), and CIBSE Guide B, applying the published per-person and per-area outdoor-air rates for each space type.
How is the required ventilation rate calculated?
It uses the ventilation rate procedure: outdoor air equals the per-person rate times the number of occupants plus the per-area rate times the floor area, summed across all spaces.
What is the difference between L/s, CFM, and ACH?
L/s and CFM are volume flow rates (CFM = L/s * 2.11888); ACH is air changes per hour, the flow rate divided by room volume, which shows how often the entire room air is replaced.
Why do European and UK rates differ from ASHRAE?
EN 16798-1 and CIBSE Guide B generally prescribe higher outdoor-air rates than ASHRAE 62.1 for equivalent spaces, reflecting stricter indoor air quality targets, so the chosen standard changes the result.