Duct Sizing Calculator helps you determine the correct duct dimensions from airflow and velocity. It provides circular and rectangular duct sizes for proper air distribution and energy efficiency in HVAC systems.
About airflow. Airflow is the volume of air moved per unit time. The calculator uses liters per second (L/s) or cubic meters per second (m³/s) in metric, and cubic feet per minute (CFM) in imperial. Correct airflow ensures sufficient ventilation while avoiding oversizing.
About velocity. Air velocity is the speed of air through a duct, expressed in meters per second (m/s) or feet per minute (fpm). It affects noise, pressure drop, and energy use — higher velocity means smaller ducts but more friction and noise; lower velocity means larger ducts and lower losses.
About area and diameter. The required cross-sectional area depends on the ratio of airflow to velocity: A = Q / V. From this area, the equivalent circular duct diameter is calculated as d = √(4A/π). Circular ducts minimize friction, while rectangular ducts are often used for space constraints.
About aspect ratio. The rectangular option uses a width-to-height aspect ratio (W:H) to determine proportions. For example, an aspect of 2:1 means width is twice the height. Both sides scale automatically to achieve the calculated area.
Calculation. The calculator finds the duct area from A = Q / V. Then it derives a circular diameter from d = √(4A/π), and rectangular dimensions from W = √(A·r) and H = W/r, where r is the aspect ratio.
- Example 1: Q = 250 L/s (0.25 m³/s), V = 4.0 m/s → A = 0.0625 m² → d ≈ 282 mm.
- Example 2: Same area with r = 2:1 → W ≈ 354 mm, H ≈ 177 mm.
- Example 3: 500 CFM at 800 fpm (imperial) → converts to ~0.236 m³/s and 4.06 m/s → d ≈ 273 mm.
Corresponding tools. Use the HVAC Load Calculator to estimate thermal loads, or the Duct Friction Calculator for pressure losses. For quick unit conversions, try Feet ⇄ Meters or CFM ⇄ L/s.
