# How much CFM does my room need?

Works out the airflow in cubic feet per minute (CFM) a room needs for a number of air changes per hour (ACH), or the air changes a fan’s CFM gives, from the room’s length, width and height.

- Page: https://www.acalculator.org/construction/cfm-calculator
- JSON spec: https://www.acalculator.org/construction/cfm-calculator.json
- Version: 668d2b13c15d

## Default answer

Example with the default inputs (Find CFM needed, Room length 15 ft, Room width 12 ft, Ceiling height 8 ft, Air changes per hour (ACH) 5): A 1,440 ft³ room needs 120 CFM for 5 air changes per hour.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| find | Find | What to work out: the CFM for a number of air changes, or the air changes from a CFM. |
| l | Room length | The length of the room. |
| w | Room width | The width of the room. |
| h | Ceiling height | The height of the ceiling. |
| ach | Air changes per hour (ACH) | How many times an hour the room’s whole volume of air is replaced. CDC suggests 5 or more. |
| cfm | Airflow (CFM) | The clean airflow into the room in cubic feet per minute, from a fan, air cleaner or HVAC system. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| airflow | Airflow (CFM) | Cubic feet of air per minute: room volume in ft³ × ACH ÷ 60. |
| m3h | Airflow (m³/h) | The same airflow in cubic meters per hour. |
| ls | Airflow (L/s) | The same airflow in liters per second. |
| changes | Air changes per hour | How many times an hour the room’s air is replaced: CFM × 60 ÷ volume in ft³. |
| volume | Room volume | Length × width × height. |
| clear99 | Minutes to remove 99% of airborne particles | In a well-mixed room with no new source: ln(100) ÷ ACH × 60 minutes. |

## Method

Volume = length × width × height (ft³); CFM = volume × ACH ÷ 60; ACH = CFM × 60 ÷ volume; minutes to remove 99% = ln(100) ÷ ACH × 60.

## Assumptions

- The room is a box: length × width × height. Subtract large built-in volumes yourself.
- ACH counts clean air (outdoor air or filtered air), and the air in the room is well mixed.
- 1 ft = 0.3048 m exactly, so 1 CFM = 1.699 m³/h = 0.4719 L/s.

## Worked examples

1. find = cfm, l = 4.572, w = 3.6576, h = 2.4384, ach = 5 gives airflow = 120, changes = 5, clear99 = 55.262042. Source: CDC NIOSH, Aim for 5 or More Air Changes per Hour (ACH = CFM × 60 ÷ (area × height)). https://www.cdc.gov/niosh/ventilation/prevention/Aim-for-5.html, retrieved 2026-10-02.
2. find = ach, l = 6.096, w = 4.572, h = 3.048, cfm = 300 gives changes = 6, airflow = 300, clear99 = 46.051702. Source: Ln(100) ÷ 6 × 60 = 46 minutes, as in CDC, Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B: Air (ACH = Q ÷ V; Table B.1: 6 ACH removes 99% in 46 minutes). https://www.cdc.gov/infection-control/hcp/environmental-control/appendix-b-air.html, retrieved 2026-10-02.
3. find = cfm, l = 5, w = 4, h = 2.5, ach = 12 gives m3h = 600, airflow = 353.146667, ls = 166.666667. Source: CDC, Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B: Air (ACH = Q ÷ V; Table B.1: 6 ACH removes 99% in 46 minutes). https://www.cdc.gov/infection-control/hcp/environmental-control/appendix-b-air.html, retrieved 2026-10-02.

## FAQ

### How do I calculate CFM for a room?

Multiply the room’s volume in cubic feet by the air changes per hour, then divide by 60. A 15 × 12 ft room with an 8 ft ceiling holds 1,440 ft³; for 5 air changes an hour it needs 1,440 × 5 ÷ 60 = 120 CFM.

### How do I calculate air changes per hour from CFM?

ACH = CFM × 60 ÷ room volume in ft³. A 300 CFM air cleaner in a 20 × 15 ft room with a 10 ft ceiling (3,000 ft³) gives 300 × 60 ÷ 3,000 = 6 air changes per hour.

### How many air changes per hour should a room have?

CDC suggests aiming for 5 or more air changes per hour of clean air in rooms people share. Health-care rooms have their own minimums, for example 12 ACH for airborne infection isolation rooms and 6 ACH for standard patient rooms.

### How long does it take to clear the air?

In a well-mixed room with no new source, removing 99% of airborne particles takes ln(100) ÷ ACH hours. At 6 ACH that is about 46 minutes, the figure in CDC’s Table B.1.

### Do fans that only move air around count?

No. Air changes count clean air: outdoor air from ventilation, or air passed through a filter. A ceiling fan mixes the air but adds no clean air. If you have several sources, add their clean-air CFM together.

### How do I convert CFM to m³/h or L/s?

1 CFM = 1.699 m³/h = 0.4719 L/s, from 1 ft = 0.3048 m exactly. The page shows all three.

## Sources

- CDC NIOSH, Aim for 5 or More Air Changes per Hour (ACH = CFM × 60 ÷ (floor area × ceiling height); aim for 5 or more air changes per hour of clean air). https://www.cdc.gov/niosh/ventilation/prevention/Aim-for-5.html (retrieved 2026-10-02)
- CDC, Guidelines for Environmental Infection Control in Health-Care Facilities, Appendix B: Air (ACH = Q ÷ V with Q in ft³ per hour; removal time t = −ln(C₂ ÷ C₁) ÷ (Q ÷ V) × 60; Table B.1: 6 ACH removes 99% in 46 minutes; Table B.2 room minimums). https://www.cdc.gov/infection-control/hcp/environmental-control/appendix-b-air.html (retrieved 2026-10-02)