Enter your airflow requirements. Results update automatically.
Use the arrows to step through standard duct sizes.
About this ductulator (duct sizing) calculator
This free ductulator sizes ventilation ductwork from an airflow quantity in litres per second, against your chosen maximum air velocity (m/s) and maximum static loss (Pa/m). It returns a recommended round duct diameter and matching rectangular equivalents, each with its actual velocity and friction loss and an OK / near-limit / over-limit badge. It is built for UK building-services engineers, HVAC designers and ductwork contractors who need a quick, defensible first-pass size before detailed design.
How the duct size is calculated
Friction loss is computed with the Darcy–Weisbach equation, using a Colebrook–White friction factor solved iteratively (with a Swamee–Jain starting estimate) for the selected duct material’s absolute roughness. Air properties are taken at roughly 20 °C at sea level (ρ = 1.2 kg/m³, μ = 1.81×10⁻⁵ Pa·s). The tool steps through commercial duct sizes and picks the smallest that keeps velocity and friction within your limits. Rectangular equivalents use the Huebscher circular-equivalent diameter De = 1.30·(wh)0.625/(w+h)0.25 from ASHRAE Fundamentals. Standard sizes are a commercial superset approximating BS EN 1505/1506.
Frequently asked questions
Which duct materials are supported?
You can pick galvanised steel, aluminium, PVC/plastic, flexible duct, fibrous glass or concrete. Each carries its own absolute roughness value, which feeds the Colebrook–White friction factor, so rougher materials show higher pressure loss for the same size and flow.
How are the rectangular equivalents chosen?
For the selected round size, the tool searches commercial rectangular dimensions whose Huebscher equivalent diameter falls within tolerance of that diameter, then screens them against your velocity and friction limits. You can also step any width or height up or down with the arrows to explore alternatives.
Can I rely on these sizes for final design?
No. The results are for design guidance only and the standard-size list includes some non-standard intermediate dimensions. Always verify the final selection with a qualified HVAC engineer before specifying or installing ductwork.