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Equal-Friction Air Duct Calculator
Round, Rectangular & Flat-Oval
Duct Sizing
Sizes ductwork by the equal-friction method (Darcy–Weisbach with the Colebrook–White friction factor) for standard air, then returns equivalent rectangular and flat-oval sizes, velocity checks and straight-duct pressure loss.
Air Flow & Duct Material
Steps 01 – 03 · Design flow, surface roughness, friction limit
01Air Flow Rate
L / s
02Friction Limit (max)
Pa / m · standard limit 1.0
03Duct Material
Absolute roughness ε used in Colebrook–White
Optional Constraints & Run Length
Steps 04 – 06 · Velocity cap, fixed rectangular side, total length
04Maximum Velocity OPTIONAL
m / s · upsizes duct if it governs
05Fixed Rectangular Side OPTIONAL
mm · holds one dimension, solves the other
06Total Straight-Duct Length OPTIONAL
m · for straight-duct friction pressure loss (excludes fittings)
Recommended Velocity Ranges
Typical comfort HVAC design velocities
Main ducts (supply / return)
4 – 8 m/s
Branch ducts
2.5 – 6 m/s
Final branches / grilles
1.5 – 3 m/s
Friction limit (max)
≤ 1.0 Pa/m
Design basis: Round duct sized by the equal-friction method — Darcy–Weisbach pressure gradient with the Colebrook–White friction factor, standard air (ρ = 1.20 kg/m³, ν = 1.51×10⁻⁵ m²/s, ~20 °C). Rectangular and flat-oval equivalents use the ASHRAE circular-equivalent-diameter relations. Results are indicative only and do not replace professional engineering judgement.
Results Ready
Round Duct Diameter
355
mm · standard spiral
5.05 m/s · 0.84 Pa/m at Ø 355 mm
Velocity
5.05
m / s
Friction Rate
0.84
Pa / m
Exact Min Ø
343
mm (continuous)
Straight-Duct Loss
8.4
Pa over 10 m
Duct Sized
Selected Ø 355 mm spiral (exact minimum 343 mm). Within main-duct range (4–8 m/s). Governing constraint: friction limit. Straight-duct friction loss ≈ 8.4 Pa (fittings excluded).
Equivalent Rectangular Sizes
Chosen side ± 50 mm, other side solved to 1 mm · meets friction & max velocity
Friction Ø 343 mm · ≤ 8 m/s
| Size (mm) | Aspect | Equiv. round (mm) | Velocity (m/s) | Friction (Pa/m) | Status |
|---|---|---|---|---|---|
| 400 × 250 | 1.60:1 | 343 | 5.00 | 1.00 | Option |
| 450 × 225 | 2.00:1 | 343 | 4.94 | 1.00 | Option |
| 500 × 206 | 2.43:1 | 343 | 4.85 | 1.01 | ✓ Fixed side |
| 550 × 191 | 2.88:1 | 343 | 4.76 | 1.00 | Option |
| 600 × 179 | 3.35:1 | 343 | 4.66 | 1.00 | Option |
Equivalent Flat-Oval Sizes
Chosen side ± 25 mm, other side solved to 1 mm · meets friction & max velocity
Friction Ø 343 mm · ≤ 8 m/s
| Size H×W (mm) | Aspect | Equiv. round (mm) | Velocity (m/s) | Friction (Pa/m) | Status |
|---|---|---|---|---|---|
| 239 × 450 | 1.88:1 | 343 | 5.25 | 1.00 | Option |
| 226 × 475 | 2.10:1 | 343 | 5.19 | 1.01 | Option |
| 215 × 500 | 2.33:1 | 343 | 5.12 | 1.01 | ✓ Fixed side |
| 206 × 525 | 2.55:1 | 343 | 5.05 | 1.00 | Option |
| 197 × 550 | 2.79:1 | 343 | 5.00 | 1.01 | Option |
Flow Visualisation
Indicative velocity profile & moving air stream · representative 1/7-power turbulent profile (illustrative, not a numerical CFD solve)
SizeØ 355 mm
Mean V5.05 m/s
Centreline6.19 m/s
Hyd. Ø355 mm
Re118,761
RegimeTurbulent
Cross-section velocity profile — flow toward you
Longitudinal air stream — flow left to right
Velocity
0
6.19
m/s
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