18 Inch Duct Calculator
Calculate airflow capacity for 18 inch round ductwork. See CFM ratings at different velocities, recommended applications, and friction loss data for 18" HVAC ducts.
Airflow Capacity for 18" Round Duct
A 18 inch round duct has a cross sectional area of 254.5 square inches (1.767 sq ft). The table below shows the CFM airflow at different air velocities.
| Velocity (FPM) | Airflow (CFM) | Noise Level | Use Case |
|---|---|---|---|
| 600 FPM | 1060 CFM | ✅ Quiet | Residential supply |
| 700 FPM | 1237 CFM | ✅ Quiet | Residential supply |
| 800 FPM | 1414 CFM | ⚠️ Moderate | Commercial supply |
| 900 FPM | 1590 CFM | 🔊 May be noisy | Industrial |
Recommended Applications
Main trunk lines, large open areas, return air
At the standard residential friction rate of 0.08 in/wg per 100 feet, a 18 inch duct is ideal for delivering 1237 CFM at a quiet 700 FPM velocity.
Quick Duct Sizing Formula
CFM for 18" Duct
CFM = (π/4) × (18/12)² × Velocity At 700 FPM: CFM = 1.7671 × 700 = 1237 CFM