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Air Changes Per Hour Calculator

Trying to figure out if a system is moving enough air? Our air changes per hour calculator makes it easy to turn room size and airflow into real numbers you can use on the job.

Whether you’re checking a bathroom fan or a commercial RTU, just plug in the airflow and room dimensions. The calculator handles the math, so you don’t have to second-guess your numbers.

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Disclaimer:

The outputs, any associated images or diagrams, any measurements, and any and all other information (collectively, the “Outputs”) are provided for informational purposes only and do not constitute a binding quote or professional advice. The Outputs are based on general assumptions and the information you provided, which may not reflect the actual specifications or condition of the subject of the calculation, particularly in the case of complex structures or inputs. Accuracy may also vary depending on location, as some data may be more complete in certain regions than others. Actual results or outcomes may vary significantly following an onsite inspection and detailed assessment by a qualified professional. Any auto-generated visuals are for illustrative purposes and may not accurately represent the subject of the calculation. The Outputs should not be relied upon as a final result or used as the sole basis for financial, renovation, operational, or strategic decisions. Jobber recommends consultation with a qualified professional before proceeding with any work, quotes, or financial decisions. Jobber and its affiliates make no warranties, express or implied, regarding the accuracy, completeness, or suitability of the Outputs, and to the fullest extent permitted by law, we disclaim all liability for any damages or losses arising from reliance on the Outputs.

How to use our air changes per hour calculator

Need a quick ACH check between jobs? Or while talking through options with a customer? This calculator is built to make your life easier, and especially when your day’s packed.

Here’s how to use the tool:

  1. Pick your measurement system (Imperial or Metric): If you’re working with cubic feet or working in metric for commercial specs, just select Imperial or Metric.
  2. Enter your airflow rate: Type in this rate, either in cubic feet per minute (CFM) or cubic meters per minute (CMM).
  3. Enter your room size: Add the room volume, if you know it, or enter the room’s length, width, and ceiling height.
  4. Click “Calculate”: One tap and you’ve got your air change rate per hour to help you make smarter calls on fan sizing, duct layout tweaks, or if it’s time to talk to the client about upgrades.


How to calculate ACH

You can use the tool above for your air changes per hour calculation, or follow these steps to do it yourself:

  1. Multiply your airflow by 60: Take your airflow rate (in CFM or CMM) and multiply it by 60. That converts your per-minute flow into an hourly volume. So, if you’re working with 400 CFM, you’re moving 24,000 cubic feet of air per hour (CFH).
  2. Calculate the room’s volume: Measure the room (length, width, and height) and multiply the three numbers to get the volume. Let’s say it’s 20x15x10, which is 3,000 cubic feet.
  3. Divide by the room’s volume: Divide your hourly airflow by the room’s volume to get your air changes per hour. 24,000 CFH divided by 3,000 cubic feet gives you 8 ACH.
  4. [Optional] Compare the result to ventilation standards: Use your ACH result to check if you’re meeting ASHRAE or local code guidelines. It’s an easy way to back up your recommendations with real numbers.

ACH formula

Whether you’re eyeballing duct runs in an old office building or explaining to a customer why their attic feels like a sauna, a clean formula guides your recommendations. Here’s how the math works.

Imperial units for air changes per hour:

ACH = (CFM * 60) / Volume (ft³)

Metric units for air changes per hour:

ACH = (CMM * 60) / Volume (m³)

Example: A residential bathroom fan is rated at 110 CFM, and the room is 8’ by 10’ with an 8’ ceiling. That’s 640 cubic feet.

ACH = (110 * 60) / 640 = 10.31

Or, if you’re juggling enough work as it is, skip the manual calculations and use our calculator above.


Benefits of using our ACH calculator

When you’re juggling installs or callbacks, anything that makes your job easier is worth keeping in the toolbox.

That’s where our indoor air change calculator comes in handy. Here’s how our ACH calculator can help:

  • Saves time on every job: Instead of grabbing a notepad, just plug in your numbers. Whether you’re in a mechanical room during a service call or quoting a retrofit from the passenger seat, this tool gives you a clear ACH number instantly.
  • Makes you look more professional: Customers might not understand airflow, but they can see when you’re confident and prepared. Sharing a calculated ACH figure that’s tailored to their space will build trust.
  • Cuts down on mistakes: We’ve all made math mistakes or forgotten to carry the five when things are moving fast. Our calculator helps keep these errors out of your estimates and recommendations. Fewer mistakes mean fewer callbacks and more money staying in your pocket.


Frequently Asked Questions

ACH (air change per hour) measures how many times the air inside a room is replaced within an hour. It’s a key part of evaluating air exchange rate, and helps you decide if a space has enough ventilation. You’ll see it referenced in HVAC sizing and airflow specs.
ACH calculation matters because air exchange is the backbone of indoor air quality. Proper ACH keeps odors and contaminants from hanging around, and tells you if a system is doing its job or not. In commercial spaces, it helps meet ventilation code.
ASHRAE recommends a home has at least 0.35 air changes per hour for proper ventilation. Kitchens, bathrooms, and laundry rooms usually need more. Every space is different, though, so check local codes and project specs before settling on a number.
Good air change clears out allergens, moisture, and pollutants before they become problems. It also helps HVAC systems control humidity and temperature more efficiently.
Multiply length, width, and height to get the room’s volume in cubic feet or meters. For odd-shaped spaces, split them into rectangles and calculate each section. Then add them up. Getting this right is important since ACH is only as accurate as your volume input.
Improve airflow by increasing fan speed, upgrading equipment, or adding mechanical ventilation. Check for blocked ducts or dirty filters—these are easy fixes that could have a big impact. For trickier jobs, an energy recovery ventilator (ERV) can help get fresh air into tight spaces.

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