UK Conduit Sizes & Cable Fill
20 mm, 25 mm and 32 mm conduit in PVC and galvanised steel with cable fill calculations and bend radius requirements.
Standard Conduit Sizes
| Nominal Size (mm) | External OD (mm) | Internal Bore (mm) | Internal Area (mm²) | 45% Fill (mm²) |
|---|---|---|---|---|
| 16 | 16 | ~12.6 | ~125 | ~56 |
| 20 | 20 | ~16.1 | ~204 | ~92 |
| 25 | 25 | ~21.1 | ~350 | ~158 |
| 32 | 32 | ~27.3 | ~585 | ~263 |
Cable Fill — Number of Singles per Conduit
Based on single-core PVC-insulated cables (6491X) at 45% fill:
| Cable Size | Cable OD (mm²) | 20 mm conduit | 25 mm conduit | 32 mm conduit |
|---|---|---|---|---|
| 1.0 mm² | ~5.8 | 3 | 5 | 9 |
| 1.5 mm² | ~6.5 | 2 | 4 | 7 |
| 2.5 mm² | ~7.8 | 1 | 3 | 5 |
| 4.0 mm² | ~9.0 | 1 | 2 | 4 |
| 6.0 mm² | ~10.5 | 1 | 1 | 3 |
Values are approximate — always verify with manufacturer data and BS 7671 tables.
Conduit Material Types
| Type | Material | Application | Notes |
|---|---|---|---|
| Round PVC | Rigid uPVC | Concealed domestic, surface light commercial | Light, cheap, easy to work |
| Oval PVC | Rigid uPVC (oval section) | Concealed in plaster (shallower chase) | Reduces chase depth in walls |
| Flexible PVC | Corrugated PVC | Awkward routes, ceiling voids | Easy routing, not for surface mounting |
| Galvanised steel | Hot-dip galvanised steel | Industrial, commercial, fire-rated areas | Can serve as CPC, high mechanical protection |
| Stainless steel | 304 / 316 stainless | Corrosive environments, food processing | Expensive, specialist use |
Minimum Bend Radius
| Conduit Size (mm) | Min. Internal Bend Radius (mm) |
|---|---|
| 16 | 40 |
| 20 | 50 |
| 25 | 63 |
| 32 | 80 |
Related Calculators
See the Cable Trunking Sizes for trunking options, or the UK Cable Sizes for cable specifications.
How We Calculate This
Frequently Asked Questions
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Last updated: April 2026
All calculations are estimates. Verify with your supplier.