ACH means air changes per hour. It is the hourly outdoor or clean-air volume divided by the room volume.

Is one air change equal to complete replacement?

No. With perfect mixing, about 63% of the original excess concentration is removed after one theoretical air change.

Should I enter fan label airflow or measured airflow?

Measured delivered airflow is preferable. Label airflow can be reduced by duct resistance, filters, grilles and installation conditions.

Can this calculator size a bathroom fan?

It can convert a chosen ACH into airflow, but bathroom fan selection should also follow local requirements and account for actual installed airflow.

Does filtration count as ventilation?

Filtration can remove particles but does not normally remove CO₂ or replace outdoor air. Clean-air delivery and outdoor-air ventilation should be treated separately.

Why is the decay time idealized?

The equation assumes perfect mixing, constant airflow, no continuing source and a constant incoming/background concentration.

Ventilation, Airflow & ACH Calculator

What this calculator does

Enter room dimensions and either airflow or target air changes per hour. The tool calculates the corresponding ventilation rate and estimates ideal contaminant decay. An optional occupancy rate provides a simple comparison against airflow per person.

How to use this ventilation calculator

1Choose what you know

Select “calculate airflow” when you have a target ACH, or “calculate ACH” when you know the fan or measured airflow.

2Measure the room

Enter internal length, width and average ceiling height. The tool calculates the air volume.

3Add occupants if useful

Enter people and an outdoor-air rate only as a comparison. Local rules may use different methods.

4Choose a removal target

Use the decay section to estimate ideal airing time after a one-time release with no continuing source.

Important: fan label airflow is not always delivered airflow. Duct resistance, filters, grilles and poor air paths can reduce the real value.

Inputs

Quick examples

Calculation mode

Room geometry

Length m

Width m

Height m

Ventilation basis

Air changes per hour h⁻¹

Airflow m³/h

Occupants persons

Outdoor air per person L/s·person

Ideal contaminant decay

Initial concentration relative units

Incoming / background concentration same units

Target concentration relative units

Use the same units for all three values. The target must be above the incoming/background concentration.

Results

Enter valid room and ventilation values.

Room volume—

Required / actual airflow—

Air changes per hour—

Minutes per air change—

Airflow per person—

Occupancy-based airflow—

Ideal decay time—

Air changes for decay—

Occupancy comparison: Enter occupants to compare the calculated airflow with a chosen per-person value.

Worked example: 30 m³ bedroom at 1 ACH

Required airflow

1 ACH × 30 m³ = 30 m³/h of outdoor air.

Air-change time

One theoretical air change takes 60 minutes at 1 ACH.

Ideal 90% removal

With zero background and no continuing source, 90% removal requires about 2.30 air changes, or roughly 138 minutes.

Background

Engineering basis, interpretation, guidance and limitations for room ventilation calculations.

Frequently Asked Questions

Practical questions about ACH, airflow, occupancy and ideal contaminant decay.