UOM Converter
Conduit & knockouts
Inch trade size ↔ metric designator (CEC / NEC), with the typical knockout punch diameter.
Try an example
Convert from
Trade size 1/2"
How it's calculated
- Trade sizes and metric designators pair one-to-one in the CEC and NEC tables.
- Knockout punch sizes: American Fittings / Greenlee published punch diameters.
Punch diameters vary by a few thousandths between makers; check the punch or box spec for a tight fit.
All trade sizes
| Inch | Metric | Knockout punch |
|---|---|---|
| 3/8" | 12 | — |
| 1/2" | 16 | 0.885" · 22.5 mm |
| 3/4" | 21 | 1.115" · 28.3 mm |
| 1" | 27 | 1.362" · 34.6 mm |
| 1-1/4" | 35 | 1.701" · 43.2 mm |
| 1-1/2" | 41 | 1.951" · 49.6 mm |
| 2" | 53 | 2.416" · 61.4 mm |
| 2-1/2" | 63 | 2.914" · 74.0 mm |
| 3" | 78 | 3.539" · 89.9 mm |
| 3-1/2" | 91 | 4.044" · 102.7 mm |
| 4" | 103 | 4.544" · 115.4 mm |
| 5" | 129 | — |
| 6" | 155 | — |
Worked examples
Trade size 1/2"
- Metric designator: 16
- Inch trade size: 1/2"
- Knockout punch: 0.885"
Trade size 1-1/4"
- Metric designator: 35
- Inch trade size: 1-1/4"
- Knockout punch: 1.701"
Metric designator 53
- Inch trade size: 2"
- Metric designator: 53
- Knockout punch: 2.416"
