Size negative air quickly for safer containment work. Include losses, safety margins, and machine count. Clear results support onsite planning, compliance, and reports daily.
| Scenario | Dimensions | ACH | Safety | Duct | Leakage | Rated CFM | Derate | Required CFM | Machines |
|---|---|---|---|---|---|---|---|---|---|
| Small containment room | 30 ft × 20 ft × 10 ft | 6 | 15% | 1.10 | 1.05 | 2000 | 20% | ~ 762 CFM | 1 |
| Long duct run | 40 ft × 25 ft × 12 ft | 8 | 20% | 1.30 | 1.10 | 2000 | 25% | ~ 2,744 CFM | 2 |
| Metric enclosure | 10 m × 6 m × 3 m | 10 | 10% | 1.15 | 1.08 | 2400 | 20% | ~ 1,255 CFM | 1 |
Negative air keeps contaminants moving inward, reducing escape at seams and entry points. Use this calculator to size airflow that supports dust control, odor management, and compliance documentation for temporary enclosures. A pressure differential, commonly checked with a manometer, helps confirm containment performance during active work.
Start with the enclosure volume and target air changes per hour (ACH). For Imperial inputs, base airflow equals volume in cubic feet times ACH, divided by 60. For Metric inputs, the calculator converts cubic meters per hour into CFM for consistent fan sizing. Typical renovation enclosures use 4–12 ACH depending on dust generation and access frequency.
Real setups lose airflow through duct friction, bends, long runs, and imperfect containment. The duct loss factor increases required CFM to reflect resistance, while the leakage factor covers minor gaps, door zipper use, and material stretch. Add a safety margin to protect against layout changes and tool-driven load spikes. If you add extra prefilters or longer ducting, revisit these multipliers.
HEPA filters load over time and motors deliver less at higher static pressure. Derating reduces nameplate CFM to a more realistic effective capacity. Machines needed is the required CFM divided by effective capacity, rounded up, then the delivered CFM is machines needed times effective capacity. Consider staging spare filters and planning swaps to maintain target airflow.
Record inputs and results for daily logs and handoff notes using the CSV or PDF export. Example data: 30 ft × 20 ft × 10 ft, ACH 6, safety 15%, duct 1.10, leakage 1.05 produces about 762 CFM required. With a 2000 CFM unit derated 20%, one machine typically covers the demand; verify with manometer readings and smoke testing.
Many projects start in the 4–12 ACH range, then adjust based on dust generation, door traffic, and monitoring results. Choose a value aligned with your site plan and verification readings.
Duct length, bends, and resistance reduce delivered airflow, while small gaps and door use increase demand. These factors help translate theoretical airflow into a realistic required CFM.
Derate accounts for filter loading, equipment wear, and static pressure effects that reduce real-world airflow. Using a derate helps prevent undersizing and supports steadier containment performance.
Compare required CFM to effective machine CFM and confirm with field checks. Use manometer readings for pressure differential and perform smoke testing at seams and entry points.
Yes. Enter meters for length, width, and height, and the calculator converts the airflow basis to CFM. This keeps equipment selection consistent across unit systems.
A modest safety margin and reasonable derate usually cover filter loading. For critical work, add redundancy or plan a spare unit so airflow is maintained during maintenance.
Save enclosure dimensions, ACH, factors, machine details, and results. Export CSV or PDF for daily logs, audits, and handoff documentation to show your sizing basis.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.