Airflow & CFM Calculator: engineer's guide
Room airflow can be driven by sensible cooling, air changes per hour, or occupant ventilation. The design CFM is the largest of the three.
Open the Airflow & CFM calculator →
What this calculator does
The calculator returns the required CFM by three independent methods (sensible load, ACH, ventilation rate) and reports the controlling case.
Formulas used
- Sensible cooling:
CFM = Qs (BTU/h) / (1.08 × ΔT) - ACH-based:
CFM = Vroom (ft³) × ACH / 60 - Ventilation-based:
CFM = (Rp × P) + (Ra × A)
Variables
- Qs = sensible load, BTU/h
- Vroom = room volume, ft³
- Rp, Ra = ASHRAE per-person and per-area rates
- P = occupants, A = floor area, ft²
Worked example
Scenario: 500 ft² conference room, 20 occupants, 10 ft ceiling, sensible load 15,000 BTU/h, supply delta-T 20°F.
- Sensible: 15,000 / (1.08 × 20) ≈ 695 CFM
- ACH at 6 (conference): 500 × 10 × 6 / 60 = 500 CFM
- Ventilation ASHRAE: (5 × 20) + (0.06 × 500) = 100 + 30 = 130 CFM
- Sensible controls at 695 CFM. Round to 700 CFM.
Common mistakes to avoid
- Sizing only on sensible load and missing ventilation minimum
- Missing ACH minimums for labs, patient rooms, and kitchens
- Using ventilation CFM as total supply (it is a minimum, not the total)
Frequently asked questions
Typical ACH?
4 to 6 offices, 6 to 10 conference, 8 to 12 restaurants, 15+ kitchens, 20+ labs.
ASHRAE 62.1 rates?
Office: Rp 5, Ra 0.06. Classroom: Rp 10, Ra 0.12. Retail: Rp 7.5, Ra 0.12.
Standards and references
ASHRAE 62.1, ASHRAE 170 (healthcare)
Related tools on this site
Open the calculator that pairs with this guide: Airflow & CFM Calculator.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.