4 Inch Duct Calculator
Calculate airflow capacity for 4 inch round ductwork. See CFM ratings at different velocities, recommended applications, and friction loss data for 4" HVAC ducts.
Airflow Capacity for 4" Round Duct
A 4 inch round duct has a cross sectional area of 12.6 square inches (0.087 sq ft). The table below shows the CFM airflow at different air velocities.
| Velocity (FPM) | Airflow (CFM) | Noise Level | Use Case |
|---|---|---|---|
| 600 FPM | 52 CFM | ✅ Quiet | Residential supply |
| 700 FPM | 61 CFM | ✅ Quiet | Residential supply |
| 800 FPM | 70 CFM | ⚠️ Moderate | Commercial supply |
| 900 FPM | 79 CFM | 🔊 May be noisy | Industrial |
Recommended Applications
Bathroom exhaust, small bedrooms, individual supply runs
At the standard residential friction rate of 0.08 in/wg per 100 feet, a 4 inch duct is ideal for delivering 61 CFM at a quiet 700 FPM velocity.
Quick Duct Sizing Formula
CFM for 4" Duct
CFM = (π/4) × (4/12)² × Velocity At 700 FPM: CFM = 0.0873 × 700 = 61 CFM