📊 Required Airflow
⚠ Safety Margin Added
Calculation Breakdown
Full Calculation Table
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How to Use the Dilution Ventilation Calculator
Enter the vapour released per minute in mL, or switch to Liquid Evaporated and enter the liquid rate with its specific gravity and molecular weight.
Pick a reference chip or enter the exposure limit for your contaminant and jurisdiction in ppm.
Select how well air mixes in the space. Poorer mixing or a more hazardous contaminant calls for a larger safety factor, K.
Click Calculate Airflow to see the dilution airflow with and without the safety factor, in L/s and CFM.
The Dilution Ventilation Formula
If a vapour enters a room at a steady rate G, clean air moving through at rate Q holds its average concentration at G divided by Q, once both are in the same volume units. Solving for a target concentration C in ppm and adding a mixing factor K gives Q = K × G × 1,000,000 ÷ C. This calculator takes G in mL of vapour per minute, so the airflow in L/min is K × G × 1,000 ÷ C. Releasing 50 mL/min of vapour and holding it to 250 ppm needs 200 L/min of clean air at perfect mixing, which is 3.3 L/s. At K = 5 that becomes 1,000 L/min, or 16.7 L/s (35 CFM).
From Liquid Evaporated to Vapour Volume
Most people know how much liquid has gone from the container, not how much vapour that makes. One mole of vapour occupies about 24.45 L at 25 °C and 101.3 kPa, so vapour in litres is liquid mL × specific gravity ÷ molecular weight × 24.45. Ten mL/min of acetone (specific gravity 0.79, molecular weight 58.08) is 0.136 mol/min, or 3,326 mL/min of vapour. Held to 250 ppm that needs 221.7 L/s at perfect mixing and 1,109 L/s (2,349 CFM) at K = 5, which shows how quickly an open solvent container outgrows general ventilation. For paints, adhesives and finishes, the VOC calculator looks at the same emission problem from the indoor-air side, where it decays over days instead of being held constant.
Why the K Factor Exists
The formula assumes the vapour mixes instantly and evenly, and no real room does that. Air near the source is stronger than air near the exhaust, and a worker leaning over the source breathes more than the room average. K inflates the airflow to cover the gap, and the options here follow the usual 3 to 10 range: 3 for good mixing with a low-toxicity vapour, 5 for average rooms, 7 for poor mixing or a source near the worker, and 10 where mixing is poor and the contaminant is hazardous. Supply and exhaust placement matter as much as the number. Putting the exhaust close to the source and the supply behind the worker pushes the real mixing toward the good end.
Dilution Ventilation Is a Last Resort, Not a First Choice
Hygiene practice reaches for dilution only after substitution, process change and local exhaust have been considered. The Canadian Centre for Occupational Health and Safety lists the conditions that make it workable: modest amounts of contaminant, moderate toxicity, workers away from the source, and a fairly uniform emission rate. It is a poor fit for dusts, metal fumes, large vapour releases and irregular surges. A hood that captures the vapour at the source needs a fraction of the air, so wherever a hood can reasonably be fitted it wins on both exposure and energy cost.
Getting the Right Exposure Limit
The target concentration should be the limit that applies to the contaminant in the province where the work happens, not a generic number. The reference chips here are ACGIH 8-hour time-weighted averages for orientation only. The NIOSH Pocket Guide to Chemical Hazards lists recommended and permissible exposure limits and IDLH values for hundreds of chemicals, which is a useful cross-check on a safety data sheet. If the vapour is flammable, the concentration must also stay far below its lower explosive limit, commonly held under 25% of it. For acetone that is about 6,000 ppm, so the health limit sets the design long before the fire limit does.
Related Tools
If the source can be captured directly, compare this result with the exhaust fan sizing calculator or the more targeted spot ventilation calculator. Fume hood work is covered by the laboratory ventilation calculator. Every litre per second exhausted has to be replaced, and the makeup air calculator checks that balance. For a whole production floor rather than one room, the industrial HVAC calculator is the wider starting point.
Frequently Asked Questions
Dilution ventilation, also called general exhaust ventilation, controls an airborne contaminant by mixing it with enough clean air to bring its concentration below a target level, rather than capturing it at the source. It suits low-toxicity vapours released at a steady rate away from workers, and it is a weaker control than local exhaust ventilation, which captures the contaminant where it is generated.
The K factor covers imperfect mixing, since the basic formula assumes the contaminant spreads instantly and evenly through the room. Values run from about 3 for good mixing and a low-toxicity contaminant up to 10 for poor mixing, a source close to a worker, or a more hazardous contaminant. Airflow scales directly with K, so doubling K doubles the airflow.
Use dilution ventilation when the contaminant has moderate toxicity, is released in modest amounts at a fairly uniform rate, and workers are not at the source. Where a hood can be placed over the source, or the contaminant is a dust, a metal fume, or a large vapour release, local exhaust needs far less air for the same exposure. The spot ventilation calculator covers the local exhaust side.
In Canada each province and territory sets its own occupational exposure limits in its OHS regulation, and many follow ACGIH Threshold Limit Values. The safety data sheet for the product lists exposure limits in Section 8, but the regulation where the work is done is the authority. Use the current limit for that jurisdiction, not a generic reference value, and check whether it also has a short-term or ceiling limit.