Air Change Rate (ACH) Calculator

About the Air Change Rate (ACH) Calculator

Air changes per hour (ACH) measures how many times the entire volume of air in a space is replaced in one hour. An ACH of 0.5 means half the air volume is replaced each hour; ACH 2.0 means the entire volume is replaced twice per hour.

ACH is a critical metric for indoor air quality, energy efficiency, and building performance. Too few air changes lead to stale air, moisture problems, and pollutant buildup. Too many air changes waste heating and cooling energy. Building codes specify minimum ACH for different spaces.

This calculator computes ACH from airflow rate (CFM) and room volume. It also shows energy cost implications of the air exchange, helping you balance indoor air quality with energy efficiency.

This measurement provides a critical foundation for energy auditing and sustainability reporting, helping organizations meet regulatory requirements and voluntary environmental commitments. Integrating this calculation into regular energy reviews ensures that conservation strategies are grounded in measured data rather than assumptions about building performance and usage patterns.

When This Page Helps

Knowing your ACH helps you assess indoor air quality and energy loss from air exchange. Whether you're designing a ventilation system, evaluating a blower door test, or troubleshooting moisture issues, ACH is the key metric.

How to Use the Inputs

1. Enter the airflow rate in CFM (cubic feet per minute).
2. Enter the room dimensions or volume.
3. The calculator computes ACH.
4. Compare against recommended ACH for your space type.
5. Optionally estimate the heating/cooling energy cost of the air exchange.

Example Calculation

Tips & Best Practices

• Residential whole-house target: 0.35 ACH (ASHRAE 62.2 minimum).
• Tight new homes often achieve 0.1–0.2 ACH natural with mechanical ventilation needed.
• Kitchens need 2–4 ACH; bathrooms need 8– 10 ACH; commercial 4–12 ACH.
• Each ACH of air exchange above the minimum adds to heating/cooling costs.
• A blower door test at 50 Pa gives ACH50; natural ACH ≈ ACH50 / 20 (rough estimate).
• Energy recovery ventilators (ERV) reduce the energy cost of ventilation by 60–80%.

ACH and Indoor Air Quality

Adequate air exchange removes CO2, moisture, VOCs, and other pollutants. At 0.35 ACH, CO2 levels stay below 1,000 ppm for typical occupancy. Below 0.2 ACH, pollutants accumulate and moisture problems become more likely. Above 1.0 ACH, energy waste becomes significant.

The Energy Cost of Air Exchange

Every cubic foot of infiltration air must be heated (or cooled) from outdoor to indoor temperature. In a 6,000 HDD climate, each CFM of infiltration costs about $3–5/year in heating energy. A drafty house at 0.5 ACH may lose $500–$1,000/year to infiltration compared to a tight house at 0.15 ACH.

Mechanical Ventilation Strategies

Exhaust-only ventilation (bath fans) is simple but provides no heat recovery. Supply ventilation (fresh air duct) is slightly better. Balanced ventilation (ERV or HRV) is optimal — it provides controlled fresh air while recovering 60–80% of the heating/cooling energy from exhaust air.

Frequently Asked Questions

• What is a good ACH for a house?

  ASHRAE recommends a minimum of 0.35 ACH for residential buildings. Modern tight homes achieve 0.1–0.3 ACH naturally and use mechanical ventilation to meet the 0.35 target. Older homes may have 0.5–1.5 ACH from air leakage alone.

• What's the difference between ACH and ACH50?

  ACH50 is measured during a blower door test with 50 pascals of pressure. Natural ACH is the real-world air exchange rate without artificial pressure. As a rough rule, natural ACH ≈ ACH50 / 20. A house with ACH50 of 5 has natural ACH of about 0.25.

• How much energy does air exchange cost?

  Each ACH of uncontrolled air exchange in a 2,000 sq ft home costs roughly $200–$600/year in heating and cooling depending on climate. An ERV recovers 60–80% of this energy, making controlled ventilation much cheaper than air leakage.

• What ACH do commercial spaces need?

  Commercial ventilation varies by use: offices 4–6 ACH, retail 6–8, restaurants 8–12, hospitals 6–25 (depending on area). These are mechanical ventilation rates specified by ASHRAE Standard 62.1.

• Can a house be too tight?

  A house can't be too tight as long as it has adequate mechanical ventilation. "Build tight, ventilate right" is the modern standard. Mechanical ventilation gives you control over air quality that leaky buildings can't match.

• How do I measure ACH in my home?

  A blower door test (about $300–$500) measures ACH50, from which natural ACH is estimated. CO2 monitoring can also indicate ventilation adequacy — CO2 above 1,000 ppm suggests insufficient fresh air exchange.