20 Inch Duct Calculator
Calculate airflow capacity for 20 inch round ductwork. See CFM ratings at different velocities, recommended applications, and friction loss data for 20" HVAC ducts.
Airflow Capacity for 20" Round Duct
A 20 inch round duct has a cross sectional area of 314.2 square inches (2.182 sq ft). The table below shows the CFM airflow at different air velocities.
| Velocity (FPM) | Airflow (CFM) | Noise Level | Use Case |
|---|---|---|---|
| 600 FPM | 1309 CFM | ✅ Quiet | Residential supply |
| 700 FPM | 1527 CFM | ✅ Quiet | Residential supply |
| 800 FPM | 1745 CFM | ⚠️ Moderate | Commercial supply |
| 900 FPM | 1963 CFM | 🔊 May be noisy | Industrial |
Recommended Applications
Commercial trunk lines, large return plenums, industrial applications
At the standard residential friction rate of 0.08 in/wg per 100 feet, a 20 inch duct is ideal for delivering 1527 CFM at a quiet 700 FPM velocity.
Quick Duct Sizing Formula
CFM for 20" Duct
CFM = (π/4) × (20/12)² × Velocity At 700 FPM: CFM = 2.1817 × 700 = 1527 CFM