ToolnozaStart with airflow and a velocity target
A velocity-based duct size calculation answers: what flow area carries my airflow at the chosen average velocity? Establish the required airflow independently and choose a velocity appropriate to the design constraints. This calculator does not set ventilation requirements or select a fan.
Duct sizing formula and unit conversion
Area A = Q / v, where Q is m³/s and v is m/s. For a round duct, D = √(4A / π). For a rectangular duct with chosen width w, height h = A / w. These dimensions are internal flow dimensions.
Divide m³/h by 3600 to get m³/s. Multiply CFM by 0.0004719474432 to get m³/s. Convert a width entered in millimeters to meters before dividing; multiply the resulting diameter or height by 1000 to return millimeters.
Worked example: 1000 m³/h at 5 m/s
Q = 1000 / 3600 = 0.27778 m³/s. A = 0.27778 / 5 = 0.05556 m². The round internal diameter is approximately 265.96 mm.
With a rectangular width of 300 mm, h = 0.05556 / 0.300 = 0.18519 m, or 185.19 mm. The round and rectangular results have the same area; this does not imply equal friction loss.
Checking an available duct size
If an available round duct has 280 mm internal diameter, its area is π × 0.280² / 4 = 0.06158 m². The same airflow gives velocity 0.27778 / 0.06158 ≈ 4.51 m/s. Recalculate velocity for the actual chosen size instead of assuming the original target remains unchanged.
Use the Duct Size Calculator
Select m³/h or CFM, enter airflow and target velocity in m/s, then enter a rectangular width in mm if needed. Compare the required area, round diameter and rectangular height. The Kitchen Hood CFM Calculator can provide a separate width-based residential hood estimate; it does not replace the duct pressure-loss check.
What area sizing does not include
An equal-friction calculation additionally considers length, roughness, shape and fittings. Noise, available fan pressure, leakage and local design rules also matter. Two rectangles with equal area can have different perimeter and friction. Use actual internal dimensions, accounting for any lining that reduces flow area.