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ACH Calculator

Calculate air changes per hour for a space.

Project & Input Data

Direction
no.

Engineering Assumptions

Direct calculation — Air changes per hour

  • The target ACH used in reverse mode is a user-entered criterion from the applicable code or space type.
  • All percentages and inputs are user-defined; the calculation is purely arithmetic and assumes no particular code or specification.

Formula Summary

  • ACH = airflow(m³/h) ÷ volume(m³)
  • Required airflow = target ACH × volume

Results

Enter your data and press Calculate to see the results.

Engineering Insight

The resulting change rate depends directly on the accuracy of the entered room volume.

Recommended Action

Compare the computed ACH against the minimum required for the space type before finalising the system design.

Next step

This calculator is fully usable as it is. Professional adds saved projects, calculation history and PDF/Excel reports; Premium adds the integrated workflows and the BOQ generator.

Workflow: HVAC Design SuiteAll tools in this disciplineCompare levels and pricing

Ventilation rates are usually stated either as an airflow or as air changes per hour, and reviewers ask for both. This page converts between them for a single space: enter the room volume with the airflow to read the resulting ACH, and a target ACH to read the airflow it demands.

Method and assumptions

Air changes per hour

  • ACH = airflow(m³/h) ÷ volume(m³)
  • Required airflow = target ACH × volume
  • The target ACH used in reverse mode is a user-entered criterion from the applicable code or space type.
  • All percentages and inputs are user-defined; the calculation is purely arithmetic and assumes no particular code or specification.

Worked example

A 150 m³ room supplied with 900 m³/h, checked against a target of 8 air changes per hour.

Worked example results
ResultValue
Air changes per hour6 ACH
Required airflow900 m³/h

These figures are produced by the same calculator engine used on this page.

Frequently asked questions

Where does the target ACH come from?
You enter it. It is a criterion taken from the standard or space classification governing your project; the tool holds no ventilation rate table.
Does it size a fan or duct?
No. It gives the airflow relationship only. Duct dimensions come from the Duct Sizing calculator, and pressure from the external static pressure tool.
Which units are used?
Volume in cubic metres and airflow in cubic metres per hour, so ACH is simply airflow divided by volume.
Does a higher ACH mean better air quality?
Not by itself. ACH describes how often the air volume is replaced; outdoor air proportion, filtration and distribution decide the actual result and are outside this calculation.

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Next step

Free calculators are open to everyone. Project saving and PDF/Excel reports come with Professional, and integrated workflows with Premium.