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Air Change Calculator

Find how many times per hour the air in a room is fully replaced. Enter room dimensions and airflow in CFM to calculate your ACH ventilation rate.

Result

7.5 ACH

The airflow gives 7.5 ACH for a 2,400 cubic foot room.

Quick Answer

An air change calculator converts airflow (CFM) into air changes per hour (ACH) using the formula: ACH = (CFM × 60) / Room Volume. Room Volume = Length × Width × Height (all in feet, result in cubic feet). A 500 sq ft room with 8-foot ceilings has 4,000 cubic feet of volume. At 200 CFM, the ACH is (200 × 60) / 4,000 = 3.0 air changes per hour.

Air Changes Per Hour (ACH) Explained: What It Measures

Air changes per hour (ACH) describes how many times the total air volume of a room is theoretically replaced in one hour by mechanical ventilation. A higher ACH improves indoor air quality, reduces humidity, and dilutes contaminants.

The Air Change Rate Formula And How It Is Calculated

Room Volume (cu ft) = Length (ft) × Width (ft) × Height (ft) ACH = (CFM × 60) / Room Volume

  • Room Volume = Length × Width × Height
  • ACH = (CFM × 60) / Room Volume
  • 60: minutes to hours conversion factor

Using This Air Change Calculator: Step By Step Guide

Inputs

  • Room Length/Width/Height: interior dimensions in feet
  • CFM: airflow rate in cubic feet per minute from fan/HVAC spec

Steps

  1. Measure interior room length, width, and ceiling height in feet.
  2. Find CFM rating of ventilation equipment serving the room.
  3. Enter room dimensions and CFM into the calculator.
  4. Read calculated ACH and compare against ASHRAE 62.1 recommendations.

Air Changes Per Hour Example, Worked In Full Detail

Kitchen prep room 20 ft × 15 ft × 9 ft with a supply diffuser rated at 450 CFM.

  1. Calculate Volume: 20 × 15 × 9 = 2,700 cu ft.
  2. Multiply CFM by 60: 450 × 60 = 27,000.
  3. Divide by Volume: 27,000 / 2,700 = 10.0 ACH.

The room has 10.0 air changes per hour.

When To Use An Air Change Calculator (And When Not To)

Use this calculator to verify ventilation targets, compare fan sizes, or document existing conditions during a building energy audit.

Assumptions

  • CFM represents net airflow to the specific room.
  • Room is simple rectangular shape.
  • Assumes perfect air mixing.

Limitations

  • Does not model MERV filtration efficiency, humidity, or occupant loads.
  • Estimates theoretical ACH from flow rate, not tracer gas test.

In Practice

When reading fan or air handler specifications, make sure the CFM figure you enter is the supply or exhaust flow for that specific room, not the total unit capacity. Many units serve multiple zones. Allocate CFM based on duct design for accurate ACH calculations.

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Frequently Asked Questions: Air Changes Per Hour (ACH)

What does ACH mean in HVAC?

ACH stands for Air Changes Per Hour, the number of times room air volume is replaced per hour.

What is a good ACH for a residential home?

General living spaces target 4 to 6 ACH under ASHRAE guidelines; kitchens target 7.5 to 15 ACH.

How do I find the CFM of my ventilation system?

Check equipment nameplate/spec sheet or have an HVAC technician measure flow hoods.

Can I use this calculator for a portable air purifier?

Yes. Enter CADR or rated CFM directly into the calculator.

Is higher ACH always better?

Not necessarily. Very high ACH increases heating/cooling energy costs.

Sources

Last updated: . Reviewed for accuracy against the formula shown above.