Size a duct the way most designers do it in practice: pick an airflow, then either a friction rate (equal friction) or a target velocity. The calculator returns the exact round diameter, the next standard round size with its real velocity and friction, and a rectangular duct for the ceiling height you have available.
How to use it
- Enter the airflow the duct section carries (m³/h).
- Use equal friction around 0.8–1.0 Pa/m for most supply and extract ducts, 0.6 Pa/m where noise matters.
- Or size by velocity: mains in the ceiling 5–7 m/s, branches 4–5 m/s, run-outs to diffusers 2.5–3.5 m/s.
- Set the height you can fit above the ceiling to get a rectangular size; keep the aspect ratio under about 4:1.
Step-by-step guides
- Duct Sizing Guide: Equal Friction, Fittings and the Index Run
- Psychrometric Chart for AHU Design: Mixing, Coils and Reheat
- Cooling Load Calculation Basics: Solar, Internal and Latent Loads
More free tools
- Ventilation & Air Change Calculator (ACH ↔ m³/h)
- MEP Unit Converter — kW, BTU/h, TR, CFM, L/s, bar, psi
- All free MEP calculators
- MEP Library
Results are for preliminary design and checking. Always confirm against the standard edition and local regulations that apply to your project.
Online course · certificate included
HVAC Systems Design — Heating, Cooling & Ventilation
28 modules from heat loss to plant room: boilers, heat pumps, cooling loads, psychrometrics, AHUs and ductwork. The VRF course is included.
Modules 1–3 are free, no account needed · full course €15,00 one-time