{"success":true,"version":"v1","request":{"tool":"get_calculator_schema","calculator_id":"air-changes-per-hour"},"result":{"entity_type":"calculator","id":"air-changes-per-hour","calculator_id":"air-changes-per-hour","canonical_url":"https://tttkmbb.com/energy/air-changes-per-hour","name":"Air Changes per Hour Calculator","title":"Air Changes per Hour Calculator – ACH from Room Volume and Airflow (m³/h, L/s, CFM) and Required Airflow for a Target ACH","category":"energy","category_name":"Home Energy & Electrical","tool_name":"calculate_air_changes_per_hour","featured_mcp_tool":false,"description":"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.","use_when":"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).","inputs":[{"name":"room_volume_m3","label":"Room volume","type":"number","unit":"m³","required":false,"max":10000000,"exclusive_min":0,"description":"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).","example":50},{"name":"room_length_m","label":"Room length","type":"number","unit":"m","required":false,"max":1000,"exclusive_min":0,"description":"Optional room length in metres (used with width and height instead of room_volume_m3).","example":5},{"name":"room_width_m","label":"Room width","type":"number","unit":"m","required":false,"max":1000,"exclusive_min":0,"description":"Optional room width in metres.","example":4},{"name":"room_height_m","label":"Room height","type":"number","unit":"m","required":false,"max":100,"exclusive_min":0,"description":"Optional room height in metres.","example":2.5},{"name":"airflow_value","label":"Airflow","type":"number","required":true,"min":0,"max":100000000,"description":"Ventilation or fan airflow in the unit given by airflow_unit.","example":300},{"name":"airflow_unit","label":"Airflow unit","type":"enum","required":true,"values":[{"value":"m3_per_h","label":"m³/h","aliases":["m3/h","m³/h"]},{"value":"l_per_s","label":"L/s","aliases":["l/s","lps"]},{"value":"cfm","label":"CFM (ft³/min)","aliases":["ft3/min"]}],"description":"Unit of airflow_value (1 CFM = 1.699 m³/h, 1 L/s = 3.6 m³/h).","example":"m3_per_h"},{"name":"target_ach","label":"Target ACH","type":"number","unit":"1/h","required":false,"max":200,"exclusive_min":0,"description":"Optional target air changes per hour, to report the airflow needed to reach it.","example":8}],"outputs":[{"name":"ach","label":"Air changes per hour","type":"number","unit":"1/h","decimals":2,"description":"airflow_m3_per_h / room_volume_m3."},{"name":"room_volume_used_m3","label":"Room volume","type":"number","unit":"m³","decimals":2,"description":"Volume used (given, or length × width × height)."},{"name":"airflow_m3_per_h","label":"Airflow (m³/h)","type":"number","unit":"m³/h","decimals":1,"description":"Airflow converted to cubic metres per hour."},{"name":"airflow_l_per_s","label":"Airflow (L/s)","type":"number","unit":"L/s","decimals":1,"description":"airflow_m3_per_h / 3.6."},{"name":"airflow_cfm","label":"Airflow (CFM)","type":"number","unit":"CFM","decimals":1,"description":"airflow_m3_per_h / 1.699."},{"name":"required_airflow_for_target_m3_per_h","label":"Airflow for target ACH","type":"number","unit":"m³/h","decimals":1,"description":"target_ach × room_volume_m3; only when target_ach is given."},{"name":"required_airflow_for_target_cfm","label":"Airflow for target ACH (CFM)","type":"number","unit":"CFM","decimals":1,"description":"required_airflow_for_target_m3_per_h / 1.699; only when target_ach is given."},{"name":"guidance","label":"Guidance","type":"string","decimals":4,"description":"Typical ACH design values from ASHRAE standards and where the result falls."}],"input_schema":{"type":"object","properties":{"room_volume_m3":{"description":"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). Unit: m³.","type":"number","maximum":10000000,"exclusiveMinimum":0,"examples":[50],"x-unit":"m³"},"room_length_m":{"description":"Optional room length in metres (used with width and height instead of room_volume_m3). Unit: m.","type":"number","maximum":1000,"exclusiveMinimum":0,"examples":[5],"x-unit":"m"},"room_width_m":{"description":"Optional room width in metres. Unit: m.","type":"number","maximum":1000,"exclusiveMinimum":0,"examples":[4],"x-unit":"m"},"room_height_m":{"description":"Optional room height in metres. Unit: m.","type":"number","maximum":100,"exclusiveMinimum":0,"examples":[2.5],"x-unit":"m"},"airflow_value":{"description":"Ventilation or fan airflow in the unit given by airflow_unit.","type":"number","minimum":0,"maximum":100000000,"examples":[300]},"airflow_unit":{"description":"Unit of airflow_value (1 CFM = 1.699 m³/h, 1 L/s = 3.6 m³/h).","type":"string","enum":["m3_per_h","l_per_s","cfm"],"examples":["m3_per_h"]},"target_ach":{"description":"Optional target air changes per hour, to report the airflow needed to reach it. Unit: 1/h.","type":"number","maximum":200,"exclusiveMinimum":0,"examples":[8],"x-unit":"1/h"}},"additionalProperties":false,"required":["airflow_value","airflow_unit"]},"output_schema":{"type":"object","properties":{"ach":{"description":"airflow_m3_per_h / room_volume_m3. Unit: 1/h.","type":"number","x-unit":"1/h"},"room_volume_used_m3":{"description":"Volume used (given, or length × width × height). Unit: m³.","type":"number","x-unit":"m³"},"airflow_m3_per_h":{"description":"Airflow converted to cubic metres per hour. Unit: m³/h.","type":"number","x-unit":"m³/h"},"airflow_l_per_s":{"description":"airflow_m3_per_h / 3.6. Unit: L/s.","type":"number","x-unit":"L/s"},"airflow_cfm":{"description":"airflow_m3_per_h / 1.699. Unit: CFM.","type":"number","x-unit":"CFM"},"required_airflow_for_target_m3_per_h":{"description":"target_ach × room_volume_m3; only when target_ach is given. Unit: m³/h.","type":"number","x-unit":"m³/h"},"required_airflow_for_target_cfm":{"description":"required_airflow_for_target_m3_per_h / 1.699; only when target_ach is given. Unit: CFM.","type":"number","x-unit":"CFM"},"guidance":{"description":"Typical ACH design values from ASHRAE standards and where the result falls.","type":"string"}}},"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","method":"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.","sources":[{"name":"Wikipedia – Air changes per hour","url":"https://en.wikipedia.org/wiki/Air_changes_per_hour","type":"reference","retrieved_at":"2026-09-24"},{"name":"ANSI/ASHRAE Standards 62.1 and 62.2 – Ventilation for Acceptable Indoor Air Quality","url":"https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2","type":"standard","retrieved_at":"2026-09-24"}],"freshness":{"type":"static","max_age_seconds":null,"note":"Deterministic formula with fixed constants; results never go stale. Inputs supplied by the caller determine the output."},"examples":[{"name":"50 m³ room, 300 m³/h, target 8 ACH","inputs":{"room_volume_m3":50,"airflow_value":300,"airflow_unit":"m3_per_h","target_ach":8},"expected":{"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},"url":"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"},{"name":"6 × 5 × 2.7 m room with a 500 CFM fan","inputs":{"room_length_m":6,"room_width_m":5,"room_height_m":2.7,"airflow_value":500,"airflow_unit":"cfm"},"expected":{"room_volume_used_m3":81,"airflow_m3_per_h":849.5,"ach":10.49},"url":"https://tttkmbb.com/api/v1/calculate/air-changes-per-hour?room_length_m=6&room_width_m=5&room_height_m=2.7&airflow_value=500&airflow_unit=cfm"}],"faq":[{"q":"How many air changes does a room need?","a":"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."},{"q":"Does an air purifier's CADR count as airflow?","a":"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."},{"q":"What is the difference between L/s, m³/h and CFM?","a":"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."}],"tags":["air changes per hour","ach calculator","ventilation rate","cfm to m3/h","room ventilation","air purifier cadr"],"related":[{"calculator_id":"hvac-btu-sizing","reason":"Heating and cooling capacity of the same room."},{"calculator_id":"heat-loss","reason":"Conductive losses through the room envelope."},{"calculator_id":"volume","reason":"Convert the room volume between m³ and ft³."}],"links":{"html":"https://tttkmbb.com/energy/air-changes-per-hour","markdown":"https://tttkmbb.com/energy/air-changes-per-hour.md","json":"https://tttkmbb.com/energy/air-changes-per-hour.json","api":"https://tttkmbb.com/api/v1/calculate/air-changes-per-hour","schema":"https://tttkmbb.com/api/v1/calculators/air-changes-per-hour","openapi":"https://tttkmbb.com/openapi.json","mcp":"https://tttkmbb.com/mcp"},"version":"v1","updated_at":"2026-09-24"},"timestamp":"2026-09-24T03:47:35Z"}