HVAC Airflow Calculator – Calculate Required CFM

Calculate the required airflow for HVAC systems based on room volume and air changes per hour (ACH).

Room Volume (m³)

Air Changes/Hour (ACH)

How to Use This HVAC Airflow Calculator

Enter room volume in cubic meters

Calculate volume by multiplying length × width × height. For a 5m × 4m × 2.5m room, enter 50 m³.

Select air changes per hour (ACH)

Choose ACH based on room type. Offices need 4-6, kitchens need 15-20, bathrooms need 8-10.

Get required airflow in CFM and m³/h

Results show airflow in both cubic feet per minute and cubic meters per hour for HVAC sizing.

Recommended Air Changes Per Hour by Room Type

Room Type	Air Changes/Hour (ACH)	Purpose
Living room / Bedroom	4-6	General comfort and air quality
Office / Study	4-6	Maintain CO2 levels for concentration
Kitchen (residential)	15-20	Remove cooking odors, heat, and moisture
Bathroom	8-10	Control humidity and prevent mold
Hospital room	12-20	Infection control and air purity
Laboratory	10-15	Remove fumes and contaminants
Gym / Exercise room	8-12	Handle increased CO2 from exertion

Source: ASHRAE Standard 62.1 and building code requirements

Understanding HVAC Airflow Requirements

What Is Air Changes Per Hour?

Air changes per hour (ACH) tells you how many times the air in a room gets replaced each hour. An ACH of 6 means the room's entire air volume is exchanged 6 times per hour, or once every 10 minutes. Higher ACH means better ventilation but also higher energy costs.

CFM vs m³/h

CFM (cubic feet per minute) is the standard airflow unit in the US. m³/h (cubic meters per hour) is used internationally. To convert: 1 CFM equals about 1.7 m³/h. HVAC equipment specs often list both units.

Why Airflow Matters

Proper airflow removes stale air, excess moisture, cooking odors, and indoor pollutants. It brings in fresh oxygen and maintains comfortable temperatures. Too little airflow causes stuffiness and mold. Too much wastes energy and creates drafts.

The Airflow Formula

CFM = (Volume in ft³ × ACH) / 60. This calculator converts your metric volume to cubic feet, multiplies by air changes per hour, then divides by 60 to get flow per minute. The m³/h result is simply volume × ACH.

Tips for Proper Room Ventilation

Size ducts correctly

Undersized ducts restrict airflow and create noise. Use a duct calculator to match duct size to required CFM.

Balance supply and return

Every room needs both supply and return vents. Closed doors can block return airflow and cause pressure problems.

Clean filters regularly

Clogged filters reduce airflow by 50% or more. Check monthly and replace every 1-3 months depending on use.

Consider energy recovery ventilators

ERVs bring in fresh air while recovering heat from exhaust air. They maintain ventilation without wasting energy.

Frequently Asked Questions

How do I calculate room volume?

Multiply length × width × height. For a room that is 5m long, 4m wide, and 2.5m high: 5 × 4 × 2.5 = 50 m³. For irregular rooms, break them into rectangular sections and add the volumes together.

What ACH should I use for my home?

Most homes target 0.35 to 0.5 ACH for whole-house ventilation under modern energy codes. Individual rooms need higher rates: bathrooms 8-10 ACH, kitchens 15-20 ACH, living spaces 4-6 ACH.

Is higher airflow always better?

No. Excessive airflow wastes energy, creates drafts, and can make HVAC systems noisy. It may also prevent proper dehumidification because air moves too fast over cooling coils. Match airflow to the room's actual needs.

How do I measure actual airflow?

Use an anemometer to measure air velocity at vents, then multiply by vent area. Or use a flow hood that captures all air from a diffuser. Professional HVAC techs have specialized equipment for accurate measurements.

What affects required ventilation rates?

Occupancy is the biggest factor — more people need more fresh air. Cooking, cleaning chemicals, pets, and smoking all increase ventilation needs. Local building codes set minimum rates that must be followed.

Other Free Tools

Search tools