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Air Changes per Hour Calculator

Computes the air changes per hour of a room from its volume (or dimensions) and the ventilation airflow in m³/h, L/s or CFM, converts the airflow between units, and gives the airflow needed to reach a target ACH, with typical design values from ASHRAE standards.

When to use

You are checking whether a fan, HRV/ERV, air purifier or HVAC supply provides enough air changes for a room, or sizing the airflow for a target ACH.

Do not use when: You need outdoor-air rates per person or per floor area (ASHRAE 62.1 tables), infiltration from a blower-door test, or the heat lost with ventilation air (use heat-loss for conduction only).

Formula

airflow_m3_per_h = airflow_value × (1 for m³/h, 3.6 for L/s, 1.699 for CFM); ach = airflow_m3_per_h / room_volume_m3; airflow_cfm = airflow_m3_per_h / 1.699; airflow_l_per_s = airflow_m3_per_h / 3.6; required_airflow_for_target_m3_per_h = target_ach × room_volume_m3

ACH = volumetric airflow / room volume, assuming fully mixed air. Reference values: ASHRAE 62.2 (residential) historically used 0.35 ACH of outdoor air as a minimum, offices typically run 4–6 total ACH, and ASHRAE 170 requires 6–12 ACH for hospital patient and treatment rooms; ASHRAE 62.1 itself specifies outdoor-air rates per person and per m², not ACH.

Inputs

Parameter	Type	Unit	Required	Description
`room_volume_m3`	number	m³	no	Room volume in cubic metres; alternatively give room_length_m, room_width_m and room_height_m (which take precedence when all three are given). Range: > 0, ≤ 10000000
`room_length_m`	number	m	no	Optional room length in metres (used with width and height instead of room_volume_m3). Range: > 0, ≤ 1000
`room_width_m`	number	m	no	Optional room width in metres. Range: > 0, ≤ 1000
`room_height_m`	number	m	no	Optional room height in metres. Range: > 0, ≤ 100
`airflow_value`	number		yes	Ventilation or fan airflow in the unit given by airflow_unit. Range: ≥ 0, ≤ 100000000
`airflow_unit`	enum: m3_per_h \| l_per_s \| cfm		yes	Unit of airflow_value (1 CFM = 1.699 m³/h, 1 L/s = 3.6 m³/h).
`target_ach`	number	1/h	no	Optional target air changes per hour, to report the airflow needed to reach it. Range: > 0, ≤ 200

Outputs

Output	Type	Unit	Description
`ach`	number	1/h	airflow_m3_per_h / room_volume_m3.
`room_volume_used_m3`	number	m³	Volume used (given, or length × width × height).
`airflow_m3_per_h`	number	m³/h	Airflow converted to cubic metres per hour.
`airflow_l_per_s`	number	L/s	airflow_m3_per_h / 3.6.
`airflow_cfm`	number	CFM	airflow_m3_per_h / 1.699.
`required_airflow_for_target_m3_per_h`	number	m³/h	target_ach × room_volume_m3; only when target_ach is given.
`required_airflow_for_target_cfm`	number	CFM	required_airflow_for_target_m3_per_h / 1.699; only when target_ach is given.
`guidance`	string		Typical ACH design values from ASHRAE standards and where the result falls.

Example

50 m³ room, 300 m³/h, target 8 ACH: {"room_volume_m3":50,"airflow_value":300,"airflow_unit":"m3_per_h","target_ach":8} → {"ach":6,"airflow_m3_per_h":300,"airflow_cfm":176.6,"airflow_l_per_s":83.3,"required_airflow_for_target_m3_per_h":400,"required_airflow_for_target_cfm":235.4}

6 × 5 × 2.7 m room with a 500 CFM fan: {"room_length_m":6,"room_width_m":5,"room_height_m":2.7,"airflow_value":500,"airflow_unit":"cfm"} → {"room_volume_used_m3":81,"airflow_m3_per_h":849.5,"ach":10.49}

GET https://tttkmbb.com/api/v1/calculate/air-changes-per-hour?room_volume_m3=50&airflow_value=300&airflow_unit=m3_per_h&target_ach=8

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/air-changes-per-hour (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/air-changes-per-hour · Markdown: https://tttkmbb.com/energy/air-changes-per-hour.md · JSON definition: https://tttkmbb.com/energy/air-changes-per-hour.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="air-changes-per-hour"
OpenAPI operationId: calculate_air_changes_per_hour
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Air changes per hour (reference)
ANSI/ASHRAE Standards 62.1 and 62.2 – Ventilation for Acceptable Indoor Air Quality (standard)

FAQ

How many air changes does a room need?

Homes: about 0.35 ACH of outdoor air as a minimum; offices 4–6 ACH of total supply air; hospital rooms 6–12 ACH; for airborne-infection risk reduction 4–6 ACH of clean (outdoor or HEPA-filtered) air is often recommended.

Does an air purifier's CADR count as airflow?

Yes for clean-air changes: CADR (in CFM or m³/h) divided by the room volume gives the equivalent clean air changes per hour, though it does not bring in outdoor air.

What is the difference between L/s, m³/h and CFM?

All are volumetric flow rates: 1 L/s = 3.6 m³/h = 2.119 CFM, and 1 CFM = 1.699 m³/h = 0.472 L/s.

Related calculators

HVAC BTU Sizing Calculator — Heating and cooling capacity of the same room.
Heat Loss Calculator — Conductive losses through the room envelope.
Volume Converter — Convert the room volume between m³ and ft³.