Duct sizing is one of the most common calculations in building services engineering. Whether you're sizing a main supply riser or a branch to a diffuser, the method is the same: flow rate, velocity, and cross-sectional area. Here's how it works.
The basic formula
For circular ducts: d = √(4Q / πV)
Where: d = diameter (m), Q = flow rate (m³/s), V = velocity (m/s)
Worked example: Given 500 L/s at 5 m/s. Convert: 500 L/s = 0.5 m³/s. Calculate: d = √(4 × 0.5 / π × 5) = √(0.1273) = 0.357 m = 357 mm. Nearest standard size (DW144): 400 mm. Actual velocity at 400 mm: V = 0.5 / (π × 0.2²) = 3.98 m/s.
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Standard circular duct sizes
Per DW144 (BESA specification for sheet metal ductwork): 80, 100, 125, 150, 160, 180, 200, 224, 250, 280, 300, 315, 350, 355, 400, 450, 500, 560, 600, 630, 710, 800, 900, 1000, 1120, 1250 mm.
Always round UP to the next standard size. Rounding down increases velocity and pressure drop.
Rectangular duct sizing
When space constraints require rectangular ducts, start with: A = Q / V. Choose width and height that give this area while respecting aspect ratio constraints. The recommended maximum aspect ratio is 4:1, though 2:1 is preferred for lower pressure drop.
Calculate equivalent circular diameter: deq = 1.3 × (a × b)0.625 / (a + b)0.25
Source: Equivalent diameter formula from CIBSE Guide C, Section 4.
Quick example: Same 500 L/s at 5 m/s → A = 0.1 m². At 2:1 aspect ratio: W = √(2A) = √(0.2) = 0.447 m → round to 450 mm, H = 225 mm. Equivalent diameter: ≈ 358 mm.
Typical duct velocities
| Application | Typical velocity (m/s) |
|---|---|
| Supply duct (main) | 5–8 |
| Supply duct (branch) | 3–5 |
| Extract duct (main) | 5–9 |
| Extract duct (branch) | 3–5 |
| Fresh air intake | 2.5–4 |
| Kitchen extract | 7–10 |
| Residential | 3–5 |
Source: Typical values from CIBSE Guide B, Table B2.1
Lower velocities mean lower pressure drop but larger ducts. Higher velocities save space but increase noise and fan energy. For occupied spaces, keep branch velocities below 5 m/s to control noise regeneration. For typical ventilation rates by room type, see our reference tables.
What comes after sizing
Once you have duct sizes, the next steps are pressure drop calculation (straight runs + fittings) and fan selection. The total system resistance determines what fan you need, and the fan power determines your SFP — which must meet Part L limits.
Related free tools
Our free Duct Sizing Calculator handles circular and rectangular sizing instantly — with standard size snap per DW144 and velocity reference from CIBSE Guide B.
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