ACH Target Gap Calculator
Calculate actual air changes per hour from duct airflow readings and room geometry, then compare the result to your design target. Ideal for HVAC commissioning, infection-control audits, and classroom ventilation spot checks.
For screening purposes only—verify ventilation compliance with certified HVAC testing instruments.
Examples
- 450 CFM, room 30 × 20 × 9 ft, target 6 ACH ⇒ ACH: 5.00 ACH (Gap vs. target: -1.00 ACH)
- 320 CFM, room 22 × 18 × 10 ft, no target ⇒ 4.85 ACH
FAQ
Can I use metric dimensions?
Convert measurements to feet first or use a metric-to-imperial converter so the ACH result remains accurate.
Should I use supply or exhaust airflow?
Use the dominant value for your design—supply for positive pressure rooms, exhaust for negative pressure rooms, or an average if they are balanced.
What if the gap is large and negative?
Increase airflow by adjusting fan speed, cleaning filters, or adding supplemental ventilation until the ACH meets the required guideline.
Additional Information
- Volume equals length × width × height; multiplying airflow (CFM) by 60 converts to cubic feet per hour before dividing by room volume.
- Negative gap values indicate the space is under-ventilated relative to the target ACH, while positive gaps signal excess airflow.
- If you leave the target blank, the tool simply reports the calculated ACH so you can benchmark manually against local codes.