HVAC ACH Calculator

Two calculations in one: find the air changes per hour a room is actually getting from its measured airflow, or work backwards to the CFM you need to hit a target ACH. Enter length × width × height, floor area × ceiling height, or a known volume — imperial or metric. Free, no signup.

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ACH Calculator (Air Changes Per Hour)

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Air changes per hour

6.00

1,680 ft³ · Living / family room target 6–8 ACH

Within the recommended range
Air changes per hour
6.00In the recommended 6–8 ACH range
One full air change
10.0 min60 ÷ ACH — how long the room takes to turn over once
Air moved per hour
10,080 ft³/hrAirflow × 60 minutes
Room volume
1,680 ft³168 sq ft of floor
Airflow per sq ft
1.00 CFM/ft²Residential supply runs near 1 CFM per sq ft
Recommended for a living / family room
168–224 CFM6–8 ACH in this room volume

ACH assumes the room mixes perfectly. It is a sizing and screening metric — ASHRAE 62.1 sizes commercial outdoor air per person and per square foot, and 62.2 sizes homes from floor area and bedroom count. Check the governing code before you commit to equipment.

Residential

Whole house (ventilation minimum)
0.35–0.6 ACH
Living / family room
6–8 ACH
Bedroom
5–6 ACH
Bathroom
6–7 ACH
Residential kitchen
7–8 ACH
Laundry room
8–9 ACH
Basement
3–4 ACH
Attached / parking garage
4–6 ACH

Commercial

Business office
6–8 ACH
Conference room
8–12 ACH
Hallway / corridor
3–5 ACH
Classroom
6–20 ACH
Retail store / mall
6–10 ACH
Restaurant dining area
8–10 ACH
Commercial kitchen
14–60 ACH
Bar / tavern
15–20 ACH
Gym / fitness room
6–10 ACH
Public restroom
10–12 ACH
Theatre / auditorium
8–15 ACH
Indoor swimming pool
10–15 ACH
Smoking lounge
15–20 ACH
Server / computer room
10–20 ACH

Industrial

Warehouse
6–30 ACH
Machine shop
6–12 ACH
Welding shop
10–20 ACH
Repair garage
20–30 ACH
Boiler / mechanical room
15–20 ACH
Paint / spray booth
20–50 ACH
Bakery / food production
20–30 ACH

Healthcare

Hospital patient room
6–8 ACH
Airborne isolation room (AII)
12–15 ACH
Operating room
20–25 ACH
Cleanroom (ISO 7)
60–90 ACH

Engineering Toolbox / Omni residential table — conditioned-air turnover, not outdoor air.

Air changes per hour
ACH = CFM × 60 ÷ ft³
Required airflow
CFM = ACH × ft³ ÷ 60
Whole house (ASHRAE 62.2)
0.35 ACH minimum
Patient room (ASHRAE 170)
6 ACH total, 2 outdoor
Isolation room (ASHRAE 170)
12 ACH total, negative
Operating room (ASHRAE 170)
20 ACH total, 4 outdoor
Enclosed garage (IMC §404)
0.75 CFM/ft² ≈ 4.5 ACH
Residential supply rule of thumb
≈ 1 CFM per sq ft

  • Pick the direction

    Find ACH turns measured airflow and room volume into air changes per hour. Find required CFM starts from a target ACH and tells you the airflow to deliver.

  • Describe the room

    Enter the three dimensions, the floor area plus ceiling height, or a volume straight off your takeoff. Choose the space type so the result is graded against the right range.

  • Read the verdict

    See ACH or required CFM, minutes per air change, airflow per square foot, and the recommended CFM window for that room — plus an optional outdoor-air split for ASHRAE 170 work.

How it works

  1. 1

    Pick the direction

    Find ACH turns measured airflow and room volume into air changes per hour. Find required CFM starts from a target ACH and tells you the airflow to deliver.

  2. 2

    Describe the room

    Enter the three dimensions, the floor area plus ceiling height, or a volume straight off your takeoff. Choose the space type so the result is graded against the right range.

  3. 3

    Read the verdict

    See ACH or required CFM, minutes per air change, airflow per square foot, and the recommended CFM window for that room — plus an optional outdoor-air split for ASHRAE 170 work.

How to calculate air changes per hour

Air changes per hour is one identity and its inverse. To find ACH, multiply airflow in CFM by 60 to get cubic feet of air moved per hour, then divide by the room volume in cubic feet: ACH = (CFM × 60) ÷ volume. A 12 ft × 14 ft × 10 ft room is 1,680 cubic feet, so 168 CFM moves 10,080 cubic feet per hour and delivers exactly 6 air changes. Flip it around when you are sizing equipment: CFM = (ACH × volume) ÷ 60, so the same room at a 6 ACH target needs 168 CFM of supply or exhaust.

The number only means something against a target. Residential living spaces are typically designed for 6–8 ACH and bedrooms for 5–6, while a commercial kitchen runs 14–60 and a repair garage 20–30 because the load is contaminant dilution rather than comfort. Healthcare is code-driven: ASHRAE 170 sets a minimum of 6 total air changes for a patient room, 12 for an airborne infection isolation room, and 20 for an operating room, each with a minimum outdoor-air share. ACH assumes perfect mixing and is a screening and sizing metric — ASHRAE 62.1 sizes commercial outdoor air per person and per square foot, and 62.2 sizes homes from floor area and bedroom count, so confirm against the governing code before you commit to equipment.

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ACH Calculator (Air Changes Per Hour) FAQs

How do you calculate air changes per hour?

Multiply the airflow in CFM by 60 to get cubic feet of air moved per hour, then divide by the room volume in cubic feet: ACH = (CFM × 60) ÷ volume. For example, 168 CFM in a 12 ft × 14 ft × 10 ft room is (168 × 60) ÷ 1,680 = 6 air changes per hour.

Rearrange the same formula: CFM = (ACH × room volume in cubic feet) ÷ 60. A 250 sq ft room with an 8 ft ceiling is 2,000 cubic feet, so hitting 8 ACH requires (8 × 2,000) ÷ 60 = 266.67 CFM.

It depends on the space. Residential living rooms are typically designed for 6–8 ACH, bedrooms 5–6, and bathrooms 6–7, while offices run 6–8 and classrooms 6–20. Commercial kitchens need 14–60 and repair garages 20–30 for contaminant dilution. ASHRAE 170 requires a minimum of 6 total ACH for a hospital patient room, 12 for an airborne isolation room, and 20 for an operating room.

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