Airflow from a generation rate and a target
Enter the contaminant generation rate, the target room concentration, the supply air concentration and a mixing factor, and the page returns the airflow needed in cfm with m³/h in parentheses. The generation rate is entered in lb/h by default (g/h and mg/h are in the menu) and the room volume in cubic feet (or m³). It applies the steady state mass balance Q = K x G / (C - C0), and converts the result into air changes per hour when a room volume is given. The perfectly mixed case is shown alongside so you can see how much the mixing factor multiplies the airflow.
Published ranges for the mixing factor differ widely between sources and one study calls the choice highly subjective, so it has not been confirmed in an authoritative source. No suggested value is offered and you enter your own. The equation comes from secondary industrial hygiene material in SI form, so inputs are converted internally (1 lb = 453,592.37 mg, 1 ft³ = 0.0283168 m³, 1 cfm = 1.69901 m³/h). Concentrations stay in mg/m³, the same basis OSHA uses for many exposure limits; a ppm target would need the molecular weight to convert. For example 0.1 lb/h, a 100 mg/m³ target, 5 mg/m³ supply air and K = 3 need about 843 cfm. Figures are current as of October 2026.
Dilution ventilation is not used for dust or fume. Local exhaust hood airflow, confined space purge time and exposure limits are not covered. This calculator is for reference and does not replace design work; review by a qualified engineer is required.
Frequently Asked Questions
No authoritative figure is offered. Enter the value printed in the source or company standard you work from. Entering 1 gives the perfectly mixed result.
Nothing is calculated. When the incoming air is already dirtier than the target, dilution ventilation cannot reach it, so a message appears instead.
No. Dilution ventilation is limited to vapors and gases of low toxicity; dust and fume are handled by local exhaust instead.