Ventilation Dilution Calculator

Calculate required dilution ventilation airflow (CFM) to maintain a target airborne concentration from a known contaminant generation rate.

About this calculator

This calculator sizes general (dilution) ventilation using the formula published in ACGIH's Industrial Ventilation: A Manual of Recommended Practice for Design — the standard reference industrial hygienists use for this calculation: it first converts your liquid evaporation rate from mL/min to pints/min, then solves Q = (G × SG × 403 × K × 10⁶) / (MW × C_target) for the airflow, in cubic feet per minute, needed to hold the room's average airborne concentration at or below your target ppm. G is the solvent's evaporation rate, SG its specific gravity, MW its molecular weight, and K a mixing safety factor — the code defaults K to 4 for good general mixing, but poorly mixed spaces (dead corners, low airflow) call for 6-10 instead, since dilution ventilation controls average room concentration, not the localized cloud right at the evaporating surface. From the required CFM it derives the equivalent m³/hr, the air changes per hour for your entered room volume, and a 99% purge time (room volume × 4.6 ÷ Q) estimating how long it takes the calculated airflow to clear a room from an initial high concentration down to 1% of that level.

Because this is a dilution — not local-exhaust — calculation, it assumes the contaminant is well-mixed throughout the space; it will underestimate the actual breathing-zone exposure of a worker standing directly over an open solvent container. Use molecular weight and specific gravity for the actual solvent (not the mixture) whenever it dominates the vapor generation, and always treat the resulting CFM as a starting design target rather than a guarantee of compliance — real rooms need field verification with a flow hood or anemometer.

Inputs

Generation rate (mL/min)

Molecular weight (g/mol)

Specific gravity

Target concentration (ppm)

Safety factor (K)

Room volume (ft³)

Tab to move between fields

Results

Required airflow (CFM)

19,218

Required airflow (m³/hr)32,651

Air changes per hour115.3

99% purge time (min)2.4

Vapour generation (cfm)0.05

Generation (pints/min)0.01

Figures current as of 2026. Source: American Conference of Governmental Industrial Hygienists (ACGIH), Industrial Ventilation: A Manual of Recommended Practice for Design, 31st Edition

How to Use This Calculator

Enter Generation rate (mL/min), Molecular weight (g/mol), and Specific gravity.
Set Target concentration (ppm), Safety factor (K), and Room volume (ft³).
Review the Required airflow (CFM) result.
Use Required airflow (m³/hr) and Air changes per hour to inform your decision.

How the result changes with Molecular weight (g/mol)

Molecular weight (g/mol)	Required airflow (CFM)
39	38,435
59	25,406
117	12,812
195	7,687

What each input means

Generation rate (mL/min): Rate of liquid solvent evaporation in millilitres per minute.
Molecular weight (g/mol): Molecular weight of the solvent or contaminant (e.g. benzene = 78, toluene = 92).
Specific gravity: Specific gravity of the liquid relative to water.
Target concentration (ppm): Desired maximum airborne concentration in parts per million.
Safety factor (K): Mixing/safety factor (K). Use 3-4 for good mixing, 6-10 for poor mixing conditions.
Room volume (ft³): Total room volume in cubic feet for air change rate calculation.

What each result means

Required airflow (CFM): Dilution ventilation air volume needed in cubic feet per minute.
Required airflow (m³/hr): Equivalent airflow in metric units.
Air changes per hour: How many times per hour the entire room volume is replaced.
99% purge time (min): Minutes to reduce concentration by 99% at the calculated flow rate.
Vapour generation (cfm): Vapour generation rate in cubic feet per minute.
Generation (pints/min): Liquid generation rate converted to pints per minute.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Generation rate (mL/min) = 5, Molecular weight (g/mol) = 78, Specific gravity = 0.88, Target concentration (ppm) = 10 = 6 input(s) provided
Calculate Required airflow
Required airflow = (generationPintsMin * specificGravity * 403 * safetyFactor * 1e6) /
19218 = 19218
Calculate Required airflow
Required airflow = requiredCfm * 1.699
32651 = 32651
Calculate Air changes per hour
115.3 = 115.3

Figures and sources

General (dilution) ventilation design formula and mixing safety factor (K) guidance (2026) — American Conference of Governmental Industrial Hygienists (ACGIH), Industrial Ventilation: A Manual of Recommended Practice for Design, 31st Edition

Engine last updated September 3, 2026. Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does raising the safety factor K increase the required airflow so much?

K enters the formula as a direct multiplier on the required CFM, so it scales linearly — doubling K from 4 to 8 doubles the required airflow. The default of 4 assumes good general mixing with no dead zones; poorly ventilated corners, floor-level pooling of heavy vapors, or intermittent (rather than continuous) evaporation all justify raising K toward 6-10, since the formula otherwise has no way to know how evenly the room actually mixes.

Why does the calculator ask for molecular weight and specific gravity separately?

Molecular weight converts the liquid's evaporation rate into a molar vapor generation rate — lighter molecules produce more vapor moles per gram evaporated — while specific gravity converts your input volume rate (mL/min) into an equivalent mass rate. Both feed directly into the numerator and denominator of the dilution equation, so an error in either one shifts the required CFM proportionally.

What does the 99% purge time actually tell me, and what does it assume?

It estimates how long the calculated airflow would take to clear a room from a high starting concentration down to 1% of that level, using purgeTimeMin = room volume × 4.6 ÷ required CFM. That 4.6 factor comes from the exponential decay math of well-mixed dilution (ln(100) ≈ 4.6), so it assumes the same well-mixed conditions as the main calculation and doesn't apply once the source is still actively generating vapor.

Is this calculator sized for local exhaust ventilation instead of dilution ventilation?

No — this tool only models general dilution ventilation, which controls the average concentration across the whole room volume, not the concentration immediately at the source. A worker leaning directly over an open solvent container will breathe air far more concentrated than the room average this calculator reports, which is exactly the situation local exhaust ventilation (a hood or snorkel at the source) is meant to address instead.

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Air Quality