Air Changes per Hour (ACH, n = Q/V)

Air Changes per Hour (ACH, n = Q/V) | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Math & Numbers
Input types: select, number
Output type: json
Sample coverage: 4
API ready: Yes

Overview

Calculate air changes per hour (ACH) from outdoor airflow and room volume, or solve for the required airflow or room volume. The tool supports m³/s, m³/h, CFM, m³, ft³, and litres, and reports well-mixed purge time for a selected residual fraction.

When to use

Find the ACH of a room from its outdoor supply airflow and volume.
Calculate the airflow needed to achieve a target ACH in a room.
Estimate well-mixed purge time to reach a chosen residual fraction.

How it works

1Choose whether to solve ACH, airflow, or room volume.
2Enter the known airflow, room volume, or ACH values and select their units.
3The calculator applies n = Q/V, converting airflow and volume to compatible units.
4It calculates purge time with t = −ln(ε)/n and returns the results as JSON.

Use cases

Check ventilation rates for rooms, buildings, and HVAC calculations.
Size outdoor airflow for a target air-change rate when room volume is known.
Estimate the time required to reduce a well-mixed contaminant to a selected residual level.

Examples

1. Calculate ACH for a room

HVAC technician

Background

An HVAC technician has measured an outdoor supply airflow of 300 m³/h for a room with a volume of 60 m³.

Problem

The technician needs the room's air changes per hour and the estimated time to reach 1% residual concentration.

How to use

Select Solve ACH, enter 300 for airflow in m³/h and 60 for volume in m³, then leave the purge target at 0.01.

mode: ach; flowRate: 300; flowUnit: m3/h; volume: 60; volumeUnit: m3; purgeFraction: 0.01

Outcome

The result is 5 ACH, with a well-mixed purge time of 0.921 hours, or about 55.26 minutes.

2. Find airflow for a target ACH

Building engineer

Background

A building engineer is assessing a 2,000 ft³ room and wants to achieve 6 air changes per hour.

Problem

The required supply airflow must be expressed in CFM for the ventilation specification.

How to use

Select Solve Air Flow, enter 6 ACH and 2,000 ft³, choose CFM as the flow unit, and use 0.01 as the purge target.

FAQ

What is the ACH formula?

ACH is calculated as n = Q/V, where Q is airflow and V is room volume.

Which calculation modes are available?

You can solve for ACH from airflow and volume, airflow from ACH and volume, or volume from ACH and airflow.

Which airflow units are supported?

The tool supports m³/s, m³/h, and CFM.

Which room volume units are supported?

You can enter volume in m³, ft³, or litres.

How is purge time calculated?

For a well-mixed single zone, purge time is t = −ln(ε)/n hours, where ε is the target residual fraction.

mode: flow; ach: 6; flowUnit: CFM; volume: 2000; volumeUnit: ft3; purgeFraction: 0.01

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Calculate ACH for a room

2. Find airflow for a target ACH

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