How do I calculate CFM for a room?

Use the formula CFM = (Room Volume in ft³ × ACH) / 60. Multiply room area (sq ft) by ceiling height (ft) to get volume, then multiply by the recommended ACH for your room type (e.g., 5 for bedrooms, 15 for kitchens), and divide by 60. For example, a 200 sq ft bedroom with 8 ft ceilings: (200 × 8 × 5) / 60 = 133 CFM.

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

1 CFM equals 0.000471947 m³/s (cubic meters per second) and 0.471947 L/s (liters per second). To convert CFM to m³/h (cubic meters per hour), multiply by 1.699. These conversions are essential when comparing equipment rated in different unit systems.

How many ACH do I need for different room types?

According to ASHRAE 62.1, typical ACH requirements are: bedrooms 5, living rooms 6, kitchens 15, bathrooms 8, laundry rooms 10, garages 6, basements 5, offices 6, and workshops 10. Higher ACH values are needed for rooms with more pollutants, moisture, or heat generation.

Should I use more CFM than calculated for better ventilation?

Slightly above (10-15%) is acceptable and can improve air quality. However, significantly oversizing ventilation causes excessive energy consumption, noise, and in humid climates can introduce too much moisture. It can also create negative pressure problems in tightly sealed buildings. Proper sizing based on actual requirements is the best approach.

What is the difference between CFM and ACH?

CFM (Cubic Feet per Minute) measures the actual volume of air flowing per minute — it's the equipment specification you need. ACH (Air Changes per Hour) measures how many times the room's entire air volume is replaced per hour — it's the ventilation standard that varies by room type. CFM = (Volume × ACH) / 60 connects the two.

CFM Calculator Guide: How to Calculate Airflow for Any Room (ASHRAE 62.1)

Whether you're designing a residential ventilation system, selecting an exhaust fan, or sizing ductwork, understanding CFM is the foundation of proper HVAC airflow design. This guide explains what CFM means, how to calculate it for any room, and how to convert between CFM, m³/s, and liters per second.

Try the CFM Calculator →

What Is CFM?

CFM stands for Cubic Feet per Minute — the standard unit for measuring airflow volume in North America. It represents how many cubic feet of air pass through a point (or fill a space) each minute.

A higher CFM rating means more airflow capacity. But proper sizing is critical — too little airflow results in poor ventilation and indoor air quality, while too much wastes energy and creates noise.

Key CFM Conversions

From	To	Formula
CFM → m³/s	m³/s	CFM × 0.000471947
m³/s → CFM	CFM	m³/s ÷ 0.000471947
CFM → L/s	L/s	CFM × 0.471947
L/s → CFM	CFM	L/s ÷ 0.471947
CFM → m³/h	m³/h	CFM × 1.699
m³/h → CFM	CFM	m³/h ÷ 1.699

Example: A bathroom exhaust fan rated at 80 CFM moves 80 × 0.471947 = 37.8 L/s of air, which equals 80 × 1.699 = 135.9 m³/h.

Understanding ACH (Air Changes per Hour)

ACH stands for Air Changes per Hour — the number of times the entire volume of air in a room is replaced with fresh (or conditioned) air in one hour. It is the key ventilation metric that links room size to required airflow.

Recommended ACH by Room Type (ASHRAE 62.1)

Room Type	Recommended ACH	Notes
Bedroom	5	Low occupancy, minimal pollutant sources
Living Room	6	Moderate occupancy
Kitchen	15	Cooking fumes, heat, grease, odors
Bathroom	8	Moisture, odors
Laundry Room	10	Moisture, lint, detergent fumes
Garage	6	Vehicle exhaust, chemical fumes
Basement	5	Moisture, radon mitigation
Office	6	Occupant comfort, CO₂ control
Workshop	10	Dust, fumes, solvent vapors

How to Calculate CFM

The CFM Formula

The fundamental equation for ventilation airflow is:

CFM = (Room Volume in ft³ × ACH) / 60

Where:

Room Volume = Floor Area (sq ft) × Ceiling Height (ft)
ACH = Air Changes per Hour (from the table above)
60 = Minutes per hour (conversion factor)

Step-by-Step Calculation

Measure or estimate the room floor area in square feet
Measure the ceiling height in feet (default 8 ft for residential)
Multiply area × height to get room volume in cubic feet
Look up the recommended ACH for your room type
Multiply volume × ACH, then divide by 60
The result is your required CFM

Worked Example: 200 sq ft Kitchen

Let's calculate the CFM for a 200 sq ft kitchen with standard 8 ft ceilings:

Room area: 200 sq ft
Ceiling height: 8 ft
Volume: 200 × 8 = 1,600 ft³
ACH for kitchen: 15
CFM: (1,600 × 15) / 60 = 400 CFM
In L/s: 400 × 0.471947 = 188.8 L/s
In m³/s: 400 × 0.000471947 = 0.189 m³/s

Worked Example: 15 m² Bathroom (Metric)

For a 15 m² bathroom with 2.4 m ceiling:

Room area: 15 m² → 15 × 10.7639 = 161.5 sq ft
Ceiling height: 2.4 m → 2.4 × 3.28084 = 7.87 ft
Volume: 161.5 × 7.87 = 1,271 ft³
ACH for bathroom: 8
CFM: (1,271 × 8) / 60 = 169.5 CFM
In L/s: 169.5 × 0.471947 = 80.0 L/s

CFM Quick Reference Table

Room Size (sq ft)	Bedroom (5 ACH)	Living (6 ACH)	Kitchen (15 ACH)	Bathroom (8 ACH)
100	67 CFM	80 CFM	200 CFM	107 CFM
150	100 CFM	120 CFM	300 CFM	160 CFM
200	133 CFM	160 CFM	400 CFM	213 CFM
250	167 CFM	200 CFM	500 CFM	267 CFM
300	200 CFM	240 CFM	600 CFM	320 CFM
400	267 CFM	320 CFM	800 CFM	427 CFM
500	333 CFM	400 CFM	1,000 CFM	533 CFM

Based on standard 8 ft ceiling height.

Common Sizing Mistakes to Avoid

Ignoring room type: Using the same ACH for all rooms ignores the vastly different ventilation needs. A kitchen needs 3× the airflow of a bedroom.
Forgetting ceiling height: Rooms with vaulted or high ceilings have more volume, requiring proportionally more CFM. Always use actual ceiling height.
Oversizing exhaust fans: Too much exhaust in a tight home can create negative pressure, back-drafting gas appliances, and pulling in unconditioned air from gaps.
Ignoring makeup air: High-CFM exhaust systems need makeup air supply. Without it, the fan can't move its rated airflow and performance drops significantly.

Calculate Your CFM Now →

Related Resources

Duct Sizing Calculator — determine proper duct diameter from CFM
BTU Calculator — estimate cooling and heating capacity
Cooling Load Calculator — the engineering version of cooling calculations
Pipe Flow Calculator — for hydronic system design