CFM Calculator
Calculate airflow requirements for HVAC, fans, and ventilation.
Duct Size vs. CFM Capacity
At recommended residential air velocity (700 FPM)
| Duct Diameter | Area (sq in) | CFM | Max CFM @900 FPM |
|---|---|---|---|
| 4" | 12.6 | 61 | 79 |
| 6" | 28.3 | 137 | 177 |
| 8" | 50.3 | 244 | 314 |
| 10" | 78.5 | 381 | 491 |
| 12" | 113.1 | 549 | 707 |
| 14" | 153.9 | 747 | 961 |
How We Calculate This
This cfm calculator uses established formulas and industry-standard data to provide accurate estimates.
- Enter your specific values into the calculator fields above
- Our algorithm applies the relevant formulas using your inputs
- Results are calculated instantly in your browser — nothing is sent to a server
- Review the detailed breakdown to understand how each factor affects your result
These calculations are estimates based on standard formulas. For critical decisions, always consult a qualified professional.
How to Convert Oven Recipes to Air Fryer
This calculator determines airflow (CFM) using two methods: the Air Changes per Hour (ACH) method for room ventilation, and the velocity method for duct sizing.
The basic rule:
- ACH method: CFM = (room volume in cu ft × air changes per hour) ÷ 60
- Duct method: CFM = duct area (sq ft) × air velocity (FPM)
- Duct area for round duct = π × (diameter/2)² ÷ 144 (convert sq in to sq ft)
- Bathroom fans: 1 CFM per sq ft of bathroom floor area (minimum 50 CFM)
- Range hoods: 100 CFM per linear foot of range width (minimum), 1 CFM per 100 BTU for gas ranges
Proper ductwork design also considers static pressure, friction loss, and equivalent duct length for fittings. For complex HVAC systems, use Manual D duct design methods. Always size fans with enough capacity to overcome duct static pressure.
When Would You Use This Calculator?
This cfm calculator is designed for anyone who needs quick, reliable estimates without complex spreadsheets or professional consultations.
- When you need a quick estimate before committing to a purchase or project
- When comparing different options or scenarios side by side
- When planning a budget and need to understand potential costs
- When you want to verify a quote or estimate you've received from a professional
- When teaching or learning about the concepts behind these calculations
Frequently Asked Questions
How do I calculate CFM for a room?
Calculate the room volume (length × width × height), multiply by the recommended air changes per hour (ACH), and divide by 60. For a 15×12×8 room with 6 ACH: (1,440 cu ft × 6) ÷ 60 = 144 CFM. Different room types need different ACH rates.
What size bathroom exhaust fan do I need?
For bathrooms up to 100 sq ft: 1 CFM per sq ft (50 CFM minimum). For larger bathrooms: add up 50 CFM per toilet, 50 per shower, 100 per jetted tub. A standard 8×10 bathroom needs 80 CFM. Install a fan rated at the next standard size up (80, 110, 150 CFM).
What CFM range hood do I need?
Electric range: minimum 100 CFM per linear foot of range width (300 CFM for 30" range). Gas range: 1 CFM per 100 BTU of burner output. A 60,000 BTU gas range needs at least 600 CFM. For island installations, increase CFM by 50%.
What duct size do I need for a given CFM?
At typical residential velocities (600-900 FPM): 6" duct carries 100-150 CFM, 8" carries 200-300 CFM, 10" carries 350-500 CFM, 12" carries 500-700 CFM. Keep velocity under 900 FPM in residential systems to minimize noise.
What are Air Changes per Hour (ACH)?
ACH is how many times the entire volume of air in a room is replaced per hour. Standard rates: bedrooms 4-6 ACH, living rooms 6-8 ACH, kitchens 15-20 ACH, bathrooms 8-10 ACH, garages 6-8 ACH, workshops 10-15 ACH.
How do I convert CFM to duct size?
Use the formula: duct area = CFM ÷ velocity. For 300 CFM at 900 FPM: area = 0.33 sq ft = 48 sq in, which is about an 8" round duct (50.3 sq in). Always round up to the next standard duct size for safety margin.