Maxi Calculator

Air Changes per Hour Calculator

Calculate a room airflow-to-volume ratio for a specified flow basis

Air Changes per Hour Calculator

Select what you want to calculate

Select the unit for room dimensions

Use one measured flow basis; total supply may include recirculated air

Length of the room

Width of the room

Height of the room

Select the unit for airflow rate

Measured flow for the selected basis; use effective clean-air delivery for filtration

Table of Contents

How to Calculate Air Changes per Hour

Multiply room length × width × height to find volume. Divide the selected airflow in m³/h by that volume in m³: ACH = Q ÷ V. For a user-specified target, reverse the calculation: Q = ACH × V.

Worked example

A 5 m × 4 m × 2.5 m room has a volume of 50 m³. A measured 30 m³/h outdoor-air flow gives 30 ÷ 50 = 0.6 outdoor-air changes per hour. For a separately chosen 2 outdoor ACH target, the corresponding airflow is 2 × 50 = 100 m³/h.

The calculator converts feet, centimeters, or inches to meters and CFM or L/s to m³/h before applying the same equation. The entered flow must represent the flow basis selected in the form.

The CDC/NIOSH ventilation FAQ explains this volumetric definition and distinguishes supply and exhaust flow.

Choosing an Air Change Target

There is no single recommended ACH for every home, office, hospital room, or laboratory. Requirements depend on room function, occupancy, contaminants, system design, and the standard adopted in the location. Use a target from the applicable project or standard; this page does not choose one.

Healthcare requirements can specify separate minimum outdoor and total air changes for each room type. For example, the 2019 ASHRAE Handbook sample of Standard 170 design parameters lists different values for patient and operating rooms. Those 2019 example values are not a current local code determination.

What the Airflow Represents

  • Total supply airflow: all air delivered to the room, including recirculated air. It is not necessarily fresh outdoor air.
  • Outdoor airflow: outdoor air delivered to the room. Use it for outdoor-air ACH, based on an actual outdoor-air flow measurement or system data.
  • Effective filtered clean-air delivery: airflow effectively cleaned for a specified contaminant. It yields equivalent ACH (eACH), not outdoor-air ACH. Filter efficiency and airflow matter.

Supply and exhaust rates describe different flow paths; do not add them as if they were independent outdoor-air delivery. The CDC/NIOSH ventilation guidance (2024) distinguishes equivalent clean-air delivery from outdoor ventilation. Its workplace clean-air goal is context-specific and is not used as a default here.

Interpreting the Result

ACH is a useful arithmetic summary of a measured or planned volumetric flow. It does not verify air distribution, pollutant removal, filtration performance, pressure relationships, comfort, or compliance. A high total supply ACH can coexist with a low outdoor-air ACH.

For ventilation design or regulated spaces, check the applicable room-specific standard, system measurements, and local adoption with a qualified professional.

Frequently Asked Questions

What ACH target should I enter?

Enter a target chosen for your room, activity, system, and applicable standard. This calculator does not determine a required rate. A total supply air change rate, outdoor air change rate, and equivalent filtered-air rate answer different questions.

How do I measure actual air changes?

Measure the relevant airflow at the system or room and divide its hourly volume by the room volume. A flow hood can measure supply or exhaust. An outdoor-air fraction or a filter's effective clean-air delivery requires separate measurement or product data. A calculated ratio does not measure mixing throughout the room.

Does filtered recirculated air count as outdoor ventilation?

No. Filtered recirculated air can provide equivalent clean-air delivery for a specified contaminant, but it does not supply outdoor air. Do not report its equivalent rate as outdoor-air ACH.

Is one air change a complete replacement of room air?

No. ACH is a volumetric flow-to-volume ratio. Mixing, short-circuiting, and airflow patterns affect the removal of actual contaminants, so one nominal air change does not guarantee every parcel of air was replaced.