Containment Negative Air Pressure Calculator
Calculate the required exhaust airflow (CFM) to maintain negative air pressure within a containment area, accounting for room volume, air changes per hour, leakage, and pressure differential requirements.
Room Dimensions
Ventilation Parameters
Typical: 6–12 ACH for general containment; 12–20 ACH for infection control Extra airflow added to compensate for envelope leakage (typically 10–20%) Typical: 2.5–8 Pa (0.01–0.03 in. w.g.) below adjacent spacesOpenings / Leakage Area
Estimated total area of gaps around doors, penetrations, etc. Typical value: 0.60–0.70 for cracks and gapsFormulas Used
1. Room Volume:
V = L × W × H (ft³)
2. Base Exhaust CFM (ACH Method):
Qbase = (V × ACH) / 60 (CFM)
3. Exhaust with Leakage Compensation:
Qexhaust = Qbase × (1 + Leakage% / 100) (CFM)
4. Crack Leakage Flow (Orifice / Bernoulli Equation):
v = Cd × √(2 × ΔPlb/ft² / ρ) (ft/s)
Qleak = v × Acrack × 60 (CFM)
where ΔPlb/ft² = ΔPPa × 0.020885, ρ = 0.0765 lb/ft³
5. Maximum Allowable Supply CFM:
Qsupply = max(0, Qexhaust − Qleak) (CFM)
6. Negative Pressure Verification:
Net Outflow = Qexhaust − Qsupply − Qleak ≥ 0 → Negative pressure maintained
Assumptions & References
- Standard air density assumed: ρ = 0.0765 lb/ft³ (at 70°F, sea level).
- Discharge coefficient Cd = 0.60–0.70 is typical for building envelope cracks and gaps (ASHRAE Fundamentals).
- Recommended ACH for containment: 6–12 ACH general; 12–20 ACH for infection control isolation rooms (CDC/ASHRAE 170).
- Target negative pressure: minimum 2.5 Pa (0.01 in. w.g.) per CDC Guidelines for Environmental Infection Control; 8 Pa (0.03 in. w.g.) recommended for robust containment.
- Leakage factor of 10–20% accounts for imperfect sealing of walls, floors, ceilings, and penetrations.
- Supply air must always be less than exhaust air to maintain net negative pressure relative to adjacent spaces.
- References: ASHRAE Standard 170-2021, CDC Guidelines for Environmental Infection Control (2003, updated 2019), OSHA 29 CFR 1926.1153 (silica containment), EPA RRP Rule containment guidance.
- This calculator provides design estimates. Final system design should be verified by a licensed mechanical engineer and tested with a pressure gauge or manometer.