Ventilation Duct Sizing Calculator

Last updated: 2026-08-24

Ventilation Duct Sizing Calculator — Calculates the required duct diameter and cross-sectional area for a ventilation duct based on airflow and pressure drop, following typical German residential HVAC guidelines.

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How to Use This Calculator

This ventilation duct sizing calculator helps you determine the necessary duct diameter for a given airflow and pressure drop, commonly used in German residential HVAC systems (Luftkanal). Start by entering the airflow rate in cubic meters per hour (m³/h) – typical values for a single room range from 50 to 200 m³/h, while whole-house systems may require 200–600 m³/h. Then enter the permissible pressure drop in Pascals (Pa); for residential ducts, 10–20 Pa is common. Click calculate to see the required duct diameter in millimeters and the cross-sectional area in square centimeters.

Formula and Methodology

The calculation is based on the continuity equation: Q = A × v, where Q is the airflow in m³/s, A is the duct cross-sectional area in m², and v is the air velocity in m/s. We convert your input airflow from m³/h to m³/s by dividing by 3600. A fixed velocity of 3.0 m/s is used, which is a standard for low-noise residential ventilation in Germany (DIN 18017-3 / VDI 6022). From the area, we compute the equivalent circular duct diameter using D = sqrt((4 × A) / π). The result is given in millimeters for easy selection of standard duct sizes. Note: This calculator assumes circular ducts and does not account for duct roughness, bends, or fittings – always consult a professional for detailed pressure drop calculations.

Practical Examples

Example 1: A kitchen exhaust requires 150 m³/h airflow with a 12 Pa pressure drop. Airflow in m³/s = 150 / 3600 = 0.04167 m³/s. Area = 0.04167 / 3.0 = 0.01389 m² = 138.9 cm². Diameter = sqrt((4 × 0.01389)/π) × 1000 ≈ 133 mm. So a 133 mm (approx. 5.2 inch) duct is needed.

Example 2: A central ventilation unit delivers 400 m³/h at 15 Pa. Flow in m³/s = 0.1111. Area = 0.1111 / 3 = 0.03704 m² = 370.4 cm². Diameter = sqrt((4 × 0.03704)/π) × 1000 ≈ 217 mm. A 217 mm (8.5 inch) duct would be required.

Tips and Best Practices

Always round up to the next commercially available duct size to ensure adequate airflow and lower pressure loss. For rectangular ducts, use the equivalent circular diameter formula. Ensure duct joints are sealed to prevent leakage. Consider using flexible ducts only for short runs, as they have higher friction. For multi-branch systems, balance the airflow with dampers. Keep duct runs as straight as possible and avoid sharp bends to minimize pressure drop.

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FAQ

What velocity is used in this calculator?

The calculator assumes a standard residential duct velocity of 3 m/s, which is typical for low-noise ventilation systems in German homes.

Can I use this for commercial ducts?

This tool is optimized for residential HVAC. Commercial systems often require lower velocities (1–2 m/s) to reduce noise and pressure loss.

What if my pressure drop is very low?

A very low pressure drop may result in a very large duct diameter. The calculator will still give a mathematically correct value, but you should verify if such a large duct is practical.