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Duct Sizing Tables — Domestic Ventilation
Duct sizes for domestic ventilation at recommended velocities. Round and rectangular equivalents, pressure drop per metre, flexible vs rigid duct comparison.
Round Duct — Flow Rates by Velocity
Flow rate Q = Area × Velocity. Area = pi × (d/2)².
| Duct Diameter | Area (mm²) | At 3 m/s (l/s) | At 4 m/s (l/s) | At 5 m/s (l/s) |
|---|---|---|---|---|
| 75mm | 4,418 | 13.3 | 17.7 | 22.1 |
| 100mm (4") | 7,854 | 23.6 | 31.4 | 39.3 |
| 125mm (5") | 12,272 | 36.8 | 49.1 | 61.4 |
| 150mm (6") | 17,671 | 53.0 | 70.7 | 88.4 |
| 200mm (8") | 31,416 | 94.2 | 125.7 | 157.1 |
Round to Rectangular Duct Equivalents
Common flat channel duct equivalents for domestic installations.
| Round Diameter (mm) | Equivalent Flat Channel (mm) | Flat Channel Area (mm²) | Typical Use |
|---|---|---|---|
| 100 | 110 x 54 | 5,940 | Bathroom extract |
| 125 | 204 x 60 | 12,240 | Kitchen extract |
| 150 | 220 x 90 | 19,800 | Cooker hood, main runs |
Pressure Drop Comparison
Approximate pressure drop per metre of straight duct at typical domestic velocities.
| Duct Type | 100mm at 4 m/s (Pa/m) | 150mm at 4 m/s (Pa/m) | Notes |
|---|---|---|---|
| Galvanised steel (rigid) | ~1.0 | ~0.5 | Best performance |
| Rigid plastic (PVC) | ~0.8 | ~0.4 | Smooth bore, good performance |
| Semi-rigid plastic | ~1.5 | ~0.7 | Common for MVHR branches |
| Flexible aluminium duct | ~3-5 | ~2-3 | 3-5x higher than rigid — minimise use |
Fitting Equivalent Lengths
Add these equivalent lengths to the straight duct run for total effective length.
| Fitting | Equivalent Length (m) |
|---|---|
| 90° bend (rigid) | 1.0 – 1.5 |
| 90° bend (flexible) | 2.0 – 3.0 |
| 45° bend | 0.5 – 0.7 |
| Tee junction | 2.0 – 3.0 |
| Grille/louvre terminal | 2.0 – 4.0 |
| In-line backdraught shutter | 1.0 – 2.0 |
Note: Values are approximate guidance. Always refer to specific duct and fan manufacturer data for system design. MVHR systems require balanced supply and extract flows — commissioning with measurement at every terminal is essential.
Related Pages
See the Ventilation Rates by Room, the Part F Ventilation Rates, and the Flow Rate Conversions.
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Last updated: April 2026
Verified against UK standards · estimates only, confirm with your supplier.