Ventilation Duct Calculator

How to Use This Calculator

This ventilation duct calculator helps you determine the necessary duct dimensions for your ventilation system. Simply input the total airflow you need (in cubic meters per hour) and the desired air velocity (in meters per second). The calculator will instantly output the required diameter in millimeters and the cross-sectional area in square meters.

To get started, enter your values in the input fields above and click 'Calculate'. The result will appear below. Make sure to use consistent units: airflow in m³/h and velocity in m/s.

Formula and Methodology

The formula used is derived from the continuity equation for incompressible flow: Q = A × V, where Q is the volumetric flow rate, A is the cross-sectional area, and V is the velocity. Rearranging and solving for diameter D (in meters) when Q is in m³/h and V in m/s:

D = 2 × √(Q / (π × V × 60))

The factor 60 converts hours to minutes because we use m³/h for flow and m/s for velocity. The cross-sectional area is then A = π × (D/2)². This method is standard in HVAC design to ensure efficient airflow with minimal pressure loss.

Practical Examples

Example 1: A ventilation system needs to move 300 m³/h at a velocity of 4 m/s. Using the formula: D = 2 × √(300 / (π × 4 × 60)) = 2 × √(300 / (753.98)) = 2 × √(0.398) ≈ 2 × 0.631 ≈ 1.262 m = 1262 mm. The cross-sectional area would be π × (0.631)² ≈ 1.25 m².

Example 2: For a residential bathroom exhaust with 80 m³/h and a velocity of 2 m/s: D = 2 × √(80 / (π × 2 × 60)) = 2 × √(80 / 376.99) ≈ 2 × 0.462 ≈ 0.924 m = 924 mm. This seems large; in practice, you'd select a standard duct size close to this, such as 250 mm round, but then the velocity would be higher. Actually, let's recalc: 80 m³/h at 250 mm diameter (0.25 m) gives velocity = (80/3600) / (π × (0.125)²) ≈ 0.022 / 0.049 ≈ 0.45 m/s, which is too low. You'd typically increase velocity by reducing duct size. Anyway, the calculator gives you the theoretical minimum size.

Tips and Best Practices

Always consider the noise and pressure drop implications: higher velocity means more noise and higher pressure loss, so choose velocities in the recommended range for your application. For residential systems, keep velocity below 4 m/s. For industrial, up to 8 m/s is acceptable. Also, account for duct friction using equivalent lengths and branching.

If you have a rectangular duct, use the equivalent diameter formula: D_eq = 1.3 × (a × b)^0.625 / (a + b)^0.25, where a and b are the dimensions. This ensures the same airflow characteristics.

Finally, always check local building codes and standards (e.g., DIN 1946, ASHRAE) to ensure compliance.