The required airflow is based on a target air-change rate. First compute the enclosure volume, then convert air changes per hour into airflow per minute.
- Volume = Length × Width × Height (converted to ft³ for airflow)
- Base CFM = (ACH × Volume) ÷ 60
- Adjusted CFM = (Base CFM × Safety Factor) ÷ (1 − Loss%)
- Machines needed: ceil(Adjusted CFM ÷ Machine CFM)
- Choose the unit system and enter room dimensions.
- Set a target ACH based on containment requirements.
- Add a safety factor to cover leakage uncertainty.
- Enter a realistic loss allowance for filters and ducting.
- Provide the rated airflow of your negative air machine.
- Press Calculate to see airflow and machine count.
- Download CSV or PDF to attach to job documents.
| Scenario | Room (L×W×H) | Target ACH | Safety | Loss% | Machine CFM | Adjusted CFM (Result) | Machines (Result) |
|---|---|---|---|---|---|---|---|
| Small enclosure | 20×15×10 ft | 6 | 1.25 | 20% | 2000 | ~2344 | 2 |
| Medium work zone | 30×20×10 ft | 8 | 1.30 | 25% | 2000 | ~5547 | 3 |
| Large containment | 40×30×12 ft | 10 | 1.35 | 30% | 2500 | ~13,846 | 6 |
Why Negative Pressure Matters
Negative pressure prevents airborne dust, fibers, and odors from migrating to occupied areas. A controlled exhaust stream keeps contaminants inside containment while replacement air enters through planned pathways. In renovation and abatement work, this protects adjacent rooms, reduces cleanup, and supports compliance. The calculator converts an enclosure and air‑change goal into a measurable exhaust airflow.
Selecting a Target Air-Change Rate
Air changes per hour (ACH) expresses how often the enclosure volume is exchanged. Higher ACH improves dilution and capture but increases energy, noise, and make‑up air demand. Tight enclosures often perform well at moderate ACH, while leaky barriers or high‑generation tasks may need higher rates. Use project specs, risk level, and leakage observations, then revisit targets after smoke tests.
Accounting for Losses and Safety
Published machine airflow is commonly measured without restrictive filters, long ducts, or multiple bends. Real installations experience pressure losses that reduce delivered airflow. The loss allowance derates performance so selected fan capacity still meets the ACH target. A safety factor adds margin for door openings, filter loading, and imperfect sealing, reducing under‑sizing risk.
Equipment Planning and Deployment
After the adjusted airflow is known, divide it by the rated airflow of available machines to estimate quantity. Place exhaust units to promote sweep flow from clean to dirty zones, and avoid short‑circuiting by separating supply paths and exhaust locations. Use appropriately sized ducting, minimize sharp bends, and keep filter maintenance schedules visible. If continuous operation is required, plan redundancy and staged filter changes.
Documentation and Ongoing Verification
Calculations support planning, but field verification confirms performance. Record enclosure dimensions, selected ACH, safety factor, and loss allowance, then note equipment model, filter type, and duct routing. Verify airflow using anemometers, flow hoods, or manufacturer test ports, and document readings before work begins and after major changes. Exported CSV and PDF reports communicate assumptions, satisfy inspections, and standardize closeout files. Track filter differential pressure to anticipate airflow drop and replacement timing accurately.