HVAC Duct Calculators

Ventilation and air balance

Room Airflow from ACH Calculator

Enter room volume and an air-change target to calculate the corresponding airflow.

Calculation inputs

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An example is loaded automatically. Replace it with your project values; the result updates as you edit.

ACH is a volume-exchange ratio and does not alone certify contaminant removal or code compliance.
Use inside room volume.
Enter the design/analysis assumption, not a universal default.

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Formula and source basis

Concepts behind this calculation

Convert a selected room air-change rate and volume into an equivalent airflow, or derive ACH from known CFM.

Air changes per hour is volume flow normalized by room air volume. It counts equivalent room volumes, not the number of times every parcel is replaced.

Supply ACH, exhaust ACH and outdoor-air ACH describe different streams. A recirculating supply volume cannot be relabeled as outdoor ventilation without knowing its outdoor fraction.

How to use this calculator

  1. Choose CFM from room volume and ACH for required flow, or ACH from room volume and CFM for a known airflow.
  2. Enter the actual room volume in ft³ and the selected air changes per hour or measured airflow for the identified stream.
  3. Read ACH as a normalized flow quantity and check mixing and outdoor-air requirements separately.

Calculation method and applicability

CFM = room ft³ × ACH ÷ 60; ACH = 60 × CFM ÷ room ft³

CFM = volume flow in ft³/min; room volume is ft³; ACH = equivalent room volumes per hour. The factor 60 converts between minutes and hours.

Input definitions and units

Calculation
ACH is a volume-exchange ratio and does not alone certify contaminant removal or code compliance. Choices: CFM from room volume and ACH; ACH from room volume and CFM.
Room volume (ft³)
Use inside room volume.
Selected air changes per hour (ACH)
Enter the design/analysis assumption, not a universal default. Applies to Calculation: CFM from room volume and ACH.
Airflow (CFM)
For the inverse calculation only. Applies to Calculation: ACH from room volume and CFM.

How to interpret the result

The result is a volume-normalized flow relationship. Retain the stream identity and room-volume boundary when comparing it with a project ACH target.

Worked examples

CFM from room volume and ACH

Inputs: 1,200 ft³ room volume at selected 4 ACH.

Calculation: 1,200×4/60.

Result: 80 CFM nominal airflow.

ACH from room volume and CFM

Inputs: 1,200 ft³ volume with 80 CFM.

Calculation: 60×80/1,200.

Result: 4 ACH.

Assumptions and limits

  • ACH does not predict contaminant removal, ventilation distribution, exposure or code compliance.
  • The result is based on the selected room volume and rate; leakage and changing occupancy are not modeled.

FAQs

Does six ACH mean complete replacement in ten minutes?

It means the equivalent of one room volume flows in ten minutes. Mixing, short circuits and recirculation determine how contaminants actually clear.

Where does the required ACH come from?

Enter an independently selected project or applicable method requirement. This geometry conversion does not choose a room-use requirement or establish air quality.

Sources

  • HVAC Tool SuitePeer directory confirms this is an established HVAC airflow task; directory membership is not engineering validation.

Related calculators

Bathroom Exhaust Fan CFM Calculator Use bathroom floor area, fixture counts, and the selected HVI guideline mode to estimate intermittent exhaust CFM. CO₂ Ventilation Estimate Calculator Use documented CO₂ concentrations or decay readings to estimate ventilation under the selected mixing model.

Related reading

ACH vs. outdoor-air ventilation rate Distinguish total air changes from outdoor-air ventilation, then choose the calculation that matches the room or dwelling task. CO₂ ventilation estimates: steady state vs. decay Choose a steady-generation balance or a source-free decay model, and interpret the estimated flow only within its measurement assumptions.