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Ventilation ACH Calculator

Map room dimensions and fresh air CFM to air changes per hour, required ventilation capacity, and ASHRAE space application compliance with live IAQ safety evaluation.

Ventilation ACH Inputs

Spatial Boundary Metrics

Ventilation Fan Performance

System Profile Designation

Manual Target Adjuster Override

Live Results

Calculated Air Turnover Volume

1.0 ACH

Required Airflow Capacity

1,000.0 CFM Required

Air Stagnation Threat — Air changes run drastically below safe target bounds (<50% of target). Stale room atmosphere will trap carbon dioxide, humidity, and micro-particulates. Step up active mechanical fan sizes or introduce fresh outside air intake lines.

Calculated Room Volume

12,000 cu. ft.

Active Design Target: 5.0 ACH Target (Office code default)

How to Use This Ventilation ACH Calculator

  1. Enter spatial boundary metrics. Input room length, width, and finished ceiling height in feet. Room volume is calculated as length × width × height in cubic feet.
  2. Set introduced fresh air CFM. Enter the mechanical ventilation airflow volume delivered to the space in CFM. This represents outside air or dedicated fresh air intake, not total supply air unless fully outdoor air.
  3. Select the space application profile. Choose Residential (0.5 ACH), Office (5.0 ACH), Classroom (7.0 ACH), Gym (10.0 ACH), or Hospital (15.0 ACH). These ASHRAE-aligned targets drive required CFM and compliance evaluation.
  4. Optionally override the target ACH. Leave Custom Target ACH blank to use the selected profile default. Enter a positive value to override with a project-specific design target.
  5. Review ACH turnover and IAQ compliance. The Live Results panel shows calculated ACH, required CFM to meet the active target, room volume, and a RED/YELLOW/GREEN IAQ compliance badge based on turnover relative to the design target.

Formulas & Example

Volumetric air exchange ties linear room geometry to turnover rate and required mechanical fresh air delivery against ASHRAE space application targets.

Room Volume (cu. ft.) = length × width × height

ACH = (fresh air CFM × 60) ÷ room volume
Required CFM = (target ACH × room volume) ÷ 60

Active Target ACH = custom target (if > 0) else profile default

Profile ACH targets:
  Residential: 0.5 ACH
  Office:      5.0 ACH
  Classroom:   7.0 ACH
  Gym:        10.0 ACH
  Hospital:   15.0 ACH

Compliance:
  NONE   → CFM = 0 (awaiting airflow inputs)
  RED    → ACH < 50% of target
  YELLOW → ACH below target, or above 2× target
  GREEN  → ACH within target through 2× target

Worked Example

40 ft × 30 ft × 10 ft office, 200 CFM fresh air, 5.0 ACH target:

Volume = 40 × 30 × 10 = 12,000 cu. ft.
ACH = (200 × 60) ÷ 12,000 = 1.0 ACH
Required CFM = (5.0 × 12,000) ÷ 60 = 1,000.0 CFM

1.0 ACH < 50% of 5.0 ACH target → RED badge

Increasing fresh air to 500 CFM yields 2.5 ACH — still below the 5.0 ACH office target but above the 50% threshold, producing a YELLOW sub-standard badge until delivery reaches 1,000 CFM for GREEN compliance.

Frequently Asked Questions

How is Air Changes per Hour (ACH) calculated?▾
ACH equals the hourly volume of fresh air divided by room volume. With CFM input: ACH = (CFM × 60) ÷ room volume (cu. ft.). Example: 200 CFM in a 12,000 cu. ft. room yields (200 × 60) ÷ 12,000 = 1.0 ACH.
How is required CFM derived from a target ACH?▾
Required CFM = (target ACH × room volume) ÷ 60. For a 12,000 cu. ft. office targeting 5.0 ACH: (5.0 × 12,000) ÷ 60 = 1,000 CFM of fresh air delivery needed to meet the design turnover rate.
What ACH targets apply to each space type?▾
Default code-aligned targets are Residential 0.5 ACH, Office 5.0 ACH, Classroom 7.0 ACH, Gym 10.0 ACH, and Hospital 15.0 ACH. Use the Custom Target ACH field to override with a project-specific value when engineering documents specify a different rate.
What do the RED, YELLOW, and GREEN compliance badges mean?▾
Red indicates calculated ACH is below 50% of the active target — severe stagnation risk. Yellow (low) means turnover is below target but above half. Green means ACH is within target through 2× target. Yellow (high) means over-ventilation beyond 2× target, increasing conditioning load.
Does this calculator size total supply air or only fresh air?▾
Enter introduced fresh air CFM — outside air or dedicated OA lines — not mixed return/recirculated supply unless the entire delivered volume is outdoor air. ACH here measures volumetric exchange of fresh ventilation against enclosed room volume for IAQ diagnostics.

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