Confined Space Ventilation Calculator

Work out confined space ventilation instantly with clear inputs, formula shown and shareable results.

About Confined Space Ventilation Calculator

Confined space ventilation has two stages. Pre-entry purge replaces the atmosphere several times over, conventionally at least seven air changes, before anyone enters. During work continuous forced ventilation maintains a high air change rate, commonly twenty per hour, to dilute any contaminant generated by the work itself.

Formulas

Working airflow

CFM = volume ft³ x target ACH / 60

Purge time

Minutes = volume ft³ x purge air changes / blower CFM

Worked example

With the default values below, Confined Space Ventilation Calculator returns Airflow required during work of 235. Change any input above and every figure here is recalculated in your browser.

Inputs used

Space volumeminimum 0.1 m³: 20 m³
Target air changes per hour during workminimum 1 ACH: 20 ACH
Air changes for pre-entry purgeminimum 1: 7
Blower capacityminimum 1 CFM: 800 CFM

Results

Airflow required during work: 235 CFM
Pre-entry purge time: 6.18 min
Air changes achieved by the blower: 68.0 ACH
Total purge volume: 140.0 m³

Step by step

1Values used
2Working airflow
3Purge time
4Airflow required during work= 235 CFM
5Pre-entry purge time= 6.18 min
6Air changes achieved by the blower= 68.0 ACH
7Total purge volume= 140.0 m³

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Confined space entry requires a permit, continuous atmospheric monitoring, an attendant and a rescue plan under the applicable regulations.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Confined space entry requires a permit, continuous atmospheric monitoring, an attendant and a rescue plan under the applicable regulations.

Frequently Asked Questions

How is the Confined Space Ventilation figure worked out?

CFM = volume ft³ x target ACH / 60. Confined space ventilation has two stages.

Should air be blown in or drawn out?

Blowing in generally gives better mixing and is the usual choice, but where a heavier-than-air contaminant collects at the bottom, extraction from the low point with fresh air supplied at the top works better.

What values do I need to enter?

This calculator takes 4 inputs: Space volume, Target air changes per hour during work, Air changes for pre-entry purge, Blower capacity. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.