If you’ve ever tried to work out the right pipe size, you’ll know it can get confusing. Nominal Bore (NB) doesn’t actually tell you the exact inside diameter—it’s just an industry term for the approximate bore size. The outside diameter (OD) is fixed for each NB, but the inside diameter (ID) changes depending on the wall thickness.
On this page you’ll find a chart covering common NB pipe sizes, their Outside Diameters (O.D), Internal Diameters (I.D), and Wall Thickness. It’s all laid out in millimetres so you can quickly check the measurements you need when you’re cutting, ordering, or quoting.
The calculation for Internal Diameter isn’t official, we used the following formula to work it out:
Internal Diameter = Outside Diameter – (2 × Wall Thickness)
| Nominal Bore (NB) | Outside Diameter (O.D) | Internal Diameter (ID) | Wall Thickness (mm) | Section |
|---|---|---|---|---|
| 20NB | 27mm | 22.7mm 22.1mm 21.5mm 20.3mm | 2.0 2.3 2.6 3.2 | Extra Light Light Medium Heavy |
| 25NB | 34mm | 29.7mm 28.5mm 27.3mm 25.7mm | 2.0 2.6 3.2 4.0 | Extra Light Light Medium Heavy |
| 32NB | 42mm | 38.4mm 37.2mm 36.0mm 34.4mm | 2.0 2.6 3.2 4.0 | Extra Light Light Medium Heavy |
| 40NB | 48mm | 43.7mm 42.5mm 41.9mm 40.3mm | 2.3 2.9 3.2 4.0 | Extra Light Light Medium Heavy |
| 50NB | 60mm | 55.7mm 55.1mm 53.1mm 51.3mm | 2.3 2.9 3.6 4.5 | Extra Light Light Medium Heavy |
| 65NB | 76mm | 71.5mm 3.2mm 68.9mm 67.1mm | 2.3 3.2 3.6 4.5 | Extra Light Light Medium Heavy |
| 80NB | 89mm | 83.7mm 82.5mm 80.9mm 78.9mm | 2.6 3.2 4.0 5.0 | Extra Light Light Medium Heavy |
| 90NB | 102mm | 96.4mm 94.2mm 93.6mm 91.6mm | 2.6 3.2 4.0 5.0 | Extra Light Light Medium Heavy |
| 100NB | 114mm | 107.9mm 107.1mm 105.3mm 103.5mm | 3.2 3.6 4.5 5.4 | Extra Light Light Medium Heavy |
| 125NB | 140mm | 135.3mm 134.3mm 131.3mm 130.5mm | 3.0 3.5 5.0 5.4 | Extra Light Light Medium Heavy |
| 150NB | 165mm | 162.3mm 161.3mm 158.3mm 157.5mm | 3.0 3.5 5.0 5.4 | Extra Light Light Medium Heavy |
If you’re planning a project and need NB pipe cut, supplied, or fabricated to suit, we can help. Our team works with these sizes every day and can make sure you get exactly what you need without the hassle. Use the enquiry form below to tell us what you’re looking for, and we’ll come back to you with pricing and lead times.
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