Dilution Ventilation Calculator
Estimate how much clean air keeps the average concentration of a released gas or vapour at or below a target you choose.
Method reviewed October 26, 2026. Version 1.0.0. How it works
This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.
Result
- Dilution airflow needed
- 2,000CFM
About 2,000 CFM of clean air keeps the average concentration at 50 ppm.
- Steady-state estimate for a room where the release rate is constant. Concentrations near the source are higher than the room average; for toxic or flammable materials use local exhaust and get an industrial hygienist’s advice.
- Contaminant release (as vapour)
- 0.1CFM
Show the working
- Steady state: Q = K × G × 10⁶ ÷ (C_target − C_supply)1 × 0.1 × 10⁶ ÷ (50 − 0) = 2,000 CFM
The mass balance
In a room where a contaminant is released at a steady rate and the air is well mixed, the concentration settles where the ventilation carries it away as fast as it is released:
- Airflow = K × release rate × 1,000,000 ÷ (target ppm − supply-air ppm)
K is a mixing factor: 1 for a perfectly mixed room and larger for real rooms where mixing is imperfect. It can be measured with a tracer gas.
Evaporating liquids
For a solvent that evaporates, the tool converts pints per hour to vapour volume using the liquid’s specific gravity and molecular weight (from its safety data sheet) and the ideal-gas law. At 70 °F and sea level one pound-mole of vapour occupies about 387 cubic feet.
Use with care
This is a room-average estimate. Concentrations near the source are higher, and dilution is not suitable for highly toxic materials or large releases. The tool does not supply exposure limits — choose a target from the safety data sheet and get an industrial hygienist’s advice for workplace exposures.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Contaminant released as
- Gas or vapour flow (CFM of pure contaminant)
- Contaminant release rate
- 0.1 CFM
- Target concentration
- 50 ppm
- Concentration in the supply air
- 0 ppm
- Mixing (safety) factor K
- 1
- Room volume (0 to skip air changes)
- 0 ft³
Result
- Dilution airflow needed
- 2,000 CFM
- Contaminant release (as vapour)
- 0.1 CFM
About 2,000 CFM of clean air keeps the average concentration at 50 ppm.
Assumptions
- Steady-state, well-mixed room: generation equals removal by ventilation, C = C_supply + G ÷ Q; the mixing factor K scales the airflow for imperfect mixing.
- Liquid vapour volume from the ideal-gas law at the stated air temperature and the standard-atmosphere pressure for the elevation.
Limitations
- Does not supply exposure limits or judge safety; it only sizes airflow for a target you choose.
- Ignores absorption, chemical decay and the time needed to reach steady state.
Frequently asked questions
What value of K should I use?
K = 1 assumes perfect mixing. Real rooms are less well mixed, so a larger value gives a safety margin; a tracer-gas test can measure it.
Why does the air supply concentration matter?
If the incoming air already contains the contaminant, each cubic foot can absorb less, so more airflow is needed. A target at or below the supply concentration cannot be reached.
How long until the room reaches this level?
Steady state takes several air changes. The tool shows air changes per hour if you enter the room volume.
Sources
- Estimating Personal Exposures Based on Mass Balance Material Usage Rates: Validation of a Ventilation Model in a Spray Paint Booth (Taylor, O’Shaughnessy & Reynolds, JOEH 1:4, 2004) — Journal of Occupational and Environmental Hygiene, via CDC Stacks. Room mass balance V dC/dt = Q·C_in − Q·C_out + G − R·C; at steady state with complete mixing C = G/Q; a correction factor k for imperfect mixing gives C = G·k/Q, and k can be found with tracer gas.
- Carbon Dioxide Generation Rates for Building Occupants (Persily & de Jonge, Indoor Air 2017) — NIST. CO₂ generation per person by age, sex and activity (e.g. about 0.0046 L/s for men and 0.0035 L/s for women aged 30–40 at 1.2 met); steady-state relation between generation, ventilation and the indoor–outdoor CO₂ difference.
- CODATA value: molar gas constant — NIST Physical Measurement Laboratory. R = 8.314 462 618… J mol⁻¹ K⁻¹ (exact, 2022 CODATA).
- NIST Chemistry WebBook, SRD 69 — Thermophysical Properties of Fluid Systems (water) — National Institute of Standards and Technology. Saturated-liquid water density and viscosity, e.g. 62.364 lb/ft³ and 1.1211 cP at 60 °F; 61.377 lb/ft³ and 0.46602 cP at 140 °F.
Spotted an error? Report a problem with this calculator — see our corrections policy.