What is CFM? How to Calculate CFM for a Room
CFM (cubic feet per minute) is the standard unit of measure for airflow — typically used for fans, HVAC systems, and ventilation equipment. Our CFM calculator helps you determine the required air flow rate for any room based on its square footage, ceiling height, and the number of air changes per hour (ACH) needed. You can also run it in reverse as an air changes per hour calculator: enter your existing airflow and room dimensions to see how many ACH you are achieving.
Air Change per Hour Formula
The relationship between CFM, room volume, and ACH is expressed in two equivalent ways:
Calculate Required Airflow (CFM)
CFM = Floor Area (ft²) × Ceiling Height (ft) × ACH / 60
Where:
- CFM — Required air volume flow rate in cubic feet per minute
- Floor Area — Room floor square footage (ft²)
- Ceiling Height — Average floor-to-ceiling height in feet
- ACH — Air changes per hour required for the room type
- / 60 — Converts from per-hour to per-minute
Calculate Air Changes per Hour (ACH)
ACH = 60 × CFM / Room Volume (ft³)
Metric Equivalents
m³/h (CMH) = Floor Area (m²) × Ceiling Height (m) × ACH ACH = m³/h / Room Volume (m³)
Unit conversions: 1 CFM = 1.699 m³/h; 1 m³/h = 0.589 CFM
How to Use Our CFM Calculator
- Select a unit system — Imperial (ft, CFM) or Metric (m, m³/h)
- Choose a calculation mode — Calculate required airflow (CFM/m³/h) or calculate ACH from existing airflow
- Enter room dimensions — Floor area and ceiling height
- Enter ACH or airflow — ACH target when calculating CFM, or current airflow when calculating ACH
- Click Calculate — Results show both CFM and m³/h, plus the formula used
Sample Calculation — How to Calculate Airflow in CFM
Consider a bedroom that is 15 ft × 12 ft with a 9 ft ceiling:
- Floor area = 15 × 12 = 180 ft²
- Room volume = 180 × 9 = 1,620 ft³
- Recommended ACH for a bedroom = 5–6 (using 6)
- Required CFM = 1,620 × 6 / 60 = 162 CFM
- Equivalent in m³/h = 162 × 1.699 ≈ 275 m³/h
Room Air Change per Hour Requirements
Different rooms require different ventilation rates to maintain air quality, control humidity, and remove pollutants. The table below shows typical ACH recommendations:
| Building / Room Type | ACH Range |
|---|---|
| Residential | |
| Basements | 3–4 |
| Bedrooms | 5–6 |
| Bathrooms | 6–7 |
| Living rooms | 6–8 |
| Kitchens | 7–8 |
| Laundry rooms | 8–9 |
| Offices & Commercial | |
| Business offices | 6–8 |
| Conference rooms | 8–12 |
| Canteens | 7–12 |
| Copy rooms | 10–12 |
| Main computer rooms | 10–14 |
| Smoking areas | 13–15 |
| Restaurants & Bars | |
| Dining areas | 8–10 |
| Food staging areas | 10–12 |
| Restaurant kitchens | 14–60 |
| Bars | 15–20 |
| Public Spaces | |
| Hallways | 3–5 |
| Waiting rooms | 4–8 |
| Banks | 4–8 |
| Hospital wards | 6–8 |
| Gymnasiums | 6–10 |
| Malls and retail stores | 6–10 |
| Classrooms | 6–20 |
| Churches | 8–12 |
| Theatres and auditoriums | 8–15 |
| Restrooms / Lavatories | 6–15 |
| Swimming pools | 10–15 |
| Hospital facilities | 10–20 |
| Smoking rooms | 15–20 |
FAQs
What does CFM stand for?
CFM stands for cubic feet per minute. It measures the volume of air that flows through a system — such as a fan or air handler — in one minute.
What is a good CFM for a room?
It depends on the room's size and purpose. Use the formula CFM = Volume × ACH / 60 and choose ACH from the table above. A standard bedroom (200 ft², 8 ft ceiling, 6 ACH) needs about 160 CFM.
What is one air change per hour (1 ACH)?
One ACH means the total air volume of the room is completely replaced once every hour. A 30 m³ room at 4 ACH would require 30 × 4 = 120 m³ of fresh air per hour.
How do I convert CFM to m³/h?
Multiply CFM by 1.699. For example, 100 CFM = 100 × 1.699 = 169.9 m³/h. To convert m³/h to CFM, divide by 1.699.
Imperial vs Metric: which should I use?
Use Imperial if your room dimensions are in feet and you're sizing equipment rated in CFM (common in the USA). Use Metric if you work in metres and need m³/h (common in Europe and most of the world).