# CFM Airflow Calculator

Calculate required CFM airflow for any room based on size and air changes per hour. Get recommended duct size for HVAC, ventilation, and exhaust fan.

## What this calculates

CFM (cubic feet per minute) is the standard measure of airflow in HVAC systems. This calculator determines the required CFM for any room based on its dimensions and the appropriate number of air changes per hour (ACH), then recommends a round duct diameter to deliver that airflow.

## Inputs

- **Room Length** (ft) — min 0 — Length of the room
- **Room Width** (ft) — min 0 — Width of the room
- **Ceiling Height** (ft) — min 0 — Floor to ceiling height
- **Air Changes per Hour** — options: 4 ACH - Residential (bedrooms), 6 ACH - Residential (living areas), 8 ACH - Commercial (offices), 10 ACH - Commercial (retail), 12 ACH - Industrial (light), 15 ACH - Industrial (heavy), 20 ACH - Labs / Clean rooms — Number of complete air volume replacements per hour

## Outputs

- **Room Volume** (ft³) — Total air volume of the room
- **Required CFM** (CFM) — Airflow needed
- **Round Duct Size** (in) — Recommended round duct diameter at 700 FPM velocity
- **Duct Cross-Section** (sq in) — Required duct cross-sectional area

## Details

The airflow calculation is: CFM = Room Volume (ft³) x Air Changes per Hour / 60. The ACH value depends on the room's purpose: bedrooms need fewer changes (4 ACH) than commercial kitchens (15-20 ACH) or laboratory spaces (20+ ACH).

Duct sizing uses the velocity method: Duct Area = CFM / Velocity. The recommended velocity for residential ductwork is 600-900 FPM (feet per minute). This calculator uses 700 FPM as a balanced target that minimizes noise while maintaining efficient airflow. Higher velocities create more noise; lower velocities require larger (more expensive) ducts.

For rectangular ducts, match the cross-sectional area shown. For example, if the calculator shows 78 sq in, you could use a 6x13 inch or 8x10 inch rectangular duct. When reducing or branching ducts, the total cross-sectional area of branches should equal or exceed the main trunk area.

## Frequently Asked Questions

**Q: How many air changes per hour does my room need?**

A: Bedrooms and living rooms: 4-6 ACH. Offices and retail: 6-10 ACH. Restaurants and gyms: 8-12 ACH. Kitchens and bathrooms: 10-15 ACH. Workshops and garages: 10-15 ACH. Labs, paint booths, and clean rooms: 15-25 ACH. Always check local building codes for minimum requirements.

**Q: What CFM exhaust fan do I need for my bathroom?**

A: The standard rule is 1 CFM per square foot of bathroom floor area, with a minimum of 50 CFM. For bathrooms with a jetted tub or steam shower, use 1.5 CFM per square foot. An 8x10 bathroom needs at least 80 CFM.

**Q: What is the difference between CFM and ACH?**

A: CFM (cubic feet per minute) measures actual airflow volume. ACH (air changes per hour) measures how many times the room's entire air volume is replaced in one hour. They're related by the formula: CFM = (Room Volume x ACH) / 60. ACH is used for design standards; CFM is used for equipment selection.

**Q: Can I use a smaller duct with higher velocity?**

A: Yes, but higher velocity means more noise and increased friction loss. Residential systems should stay below 900 FPM in main ducts and 600 FPM in branch runs. Commercial systems can tolerate up to 1,200-1,500 FPM. Going above these limits creates whistling and rumbling sounds.

**Q: How do I size a range hood exhaust fan?**

A: For a standard 30-inch range: 250-400 CFM. For a 36-inch professional range: 400-600 CFM. For a 48-inch commercial-style range: 600-1,200 CFM. The general rule is 100 CFM per linear foot of range width, or 1 CFM per 100 BTU of burner output for gas ranges.

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Source: https://vastcalc.com/calculators/construction/cfm
Category: Construction
Last updated: 2026-04-21