Air Changes Per Hour Calculator

Free ACH calculator to determine air changes per hour based on room volume and airflow rate. Essential for HVAC design, ventilation, and indoor air quality.

The Air Changes Per Hour (ACH) Calculator determines how many times the entire volume of air in a room is replaced per hour. ACH is a critical metric for ventilation design, infection control, odor removal, and indoor air quality management.

Higher ACH provides better air quality but increases energy costs. Balance ventilation needs with energy efficiency goals.

What is Air Changes Per Hour (ACH)?

Air Changes Per Hour (ACH) is a measure of how many times the total volume of air in a defined space (room, building, zone) is completely replaced in one hour. For example, if a room has 4 ACH, the entire air volume is replaced 4 times every hour. ACH is critical for HVAC design, infection control in healthcare, manufacturing cleanrooms, odor control in kitchens/bathrooms, and general indoor air quality. Higher ACH rates improve air quality and dilute contaminants but increase energy consumption for heating/cooling fresh outdoor air.

Why ACH Matters

Indoor Air Quality: Higher ACH dilutes pollutants, CO₂, VOCs, and odors
Infection Control: Hospitals use high ACH (12-15+) to reduce airborne pathogens
Moisture Control: Bathrooms/kitchens need high ACH to remove humidity
Comfort: Adequate ACH prevents stuffy, stale air conditions
Code Compliance: Building codes mandate minimum ACH for different spaces
Energy Impact: Higher ACH increases HVAC energy costs significantly

How to Use This Calculator

Select calculation mode: Calculate ACH or calculate required airflow
Choose dimension unit: feet or meters
Enter room dimensions (length, width, height) or direct volume
For ACH calculation: Enter airflow rate in CFM, m³/h, or L/s
For airflow calculation: Enter target ACH value
Optionally select room type to see recommended ACH ranges
Click Calculate to see results and compliance recommendations

ACH Calculation Formulas

1. Air Changes Per Hour (from CFM)

ACH = (CFM × 60) / Room Volume (ft³)

ACH = Airflow (m³/h) / Room Volume (m³)

2. Required Airflow (from ACH)

CFM = (ACH × Room Volume (ft³)) / 60

Airflow (m³/h) = ACH × Room Volume (m³)

3. Room Volume

Volume (ft³) = Length × Width × Height

ACH Standards by Room Type

Residential Living Areas: 0.35-1 ACH (ASHRAE 62.2)
Bedrooms: 2-4 ACH (sufficient for sleeping comfort)
Kitchens: 7-15 ACH (remove cooking odors, heat, moisture)
Bathrooms: 6-10 ACH (moisture and odor removal)
Offices: 4-6 ACH (ASHRAE 62.1 commercial standard)
Classrooms: 4-6 ACH (CDC/ASHRAE recommendations)
Hospital General Rooms: 6-12 ACH (infection control)
Hospital Isolation Rooms: 12-15+ ACH (airborne infection isolation)
Laboratories: 6-20 ACH (depends on chemical use and fume hoods)
Cleanrooms: 60-600+ ACH (ISO cleanroom classifications)

Common HVAC Applications

Ventilation Design: Size exhaust fans and supply air systems
Infection Control: Healthcare facilities, isolation rooms, operating rooms
Indoor Air Quality: Ensure adequate fresh air for occupants
Odor Control: Kitchens, bathrooms, pet areas, smoking rooms
Moisture Control: Prevent mold growth, condensation issues
HVAC Sizing: Determine required fan capacity and ductwork sizing

Factors Affecting ACH Requirements

Occupancy Density: More people require higher ACH for CO₂ dilution
Activities: Cooking, exercising, manufacturing increase ACH needs
Pollutant Sources: Chemicals, smoke, VOCs require higher air changes
Building Tightness: Newer tight buildings need mechanical ventilation
Climate: Hot/humid climates may limit outdoor air to save energy
Building Codes: Minimum ACH mandated by ASHRAE, IMC, local codes

ACH Design Tips

Don't confuse ACH with outdoor air changes - total ACH includes recirculated air
ASHRAE 62.1/62.2 specify minimum outdoor air, not total ACH
Higher ACH improves air quality but significantly increases energy costs
Use heat recovery ventilators (HRV/ERV) to reduce energy penalty of high ACH
Ensure proper air distribution - dead zones won't benefit from high ACH
Consider demand-controlled ventilation (DCV) with CO₂ sensors to vary ACH
Pressurization matters: isolation rooms use negative pressure, cleanrooms positive
Verify ACH with actual airflow measurements, not just design calculations

Energy & Cost Considerations

Each ACH increase requires heating/cooling outdoor air (major energy cost)
In extreme climates, high ACH can double HVAC energy consumption
Heat recovery ventilation (HRV/ERV) can recover 60-90% of energy from exhaust air
Optimize ACH to minimum required by code to save energy
Variable-speed fans with DCV can reduce average ACH during low-occupancy periods

Common ACH Calculation Mistakes

Confusing ACH with outdoor air changes (ACH includes recirculated air)
Using incorrect units: CFM must be converted to ft³/hr (multiply by 60)
Ignoring duct leakage: actual delivered airflow may be 10-30% less than fan rating
Not accounting for air distribution: poorly designed systems have dead zones
Assuming natural infiltration provides adequate ACH in tight modern buildings
Forgetting to include all exhaust fans (kitchen, bath) in total ACH calculation