Understanding Gasket Dimensions and Flange Details

• Nominal size (DN or NPS): the pipe size designation, not an exact measurement.
• Inside diameter (ID): the bore of the gasket.
• Outside diameter (OD): the outer edge of a ring gasket, or the sealing face edge for a full-face gasket.
• Bolt circle diameter (PCD): the diameter of the circle through the bolt-hole centres.
• Number and size of bolt holes.
• Thickness: the gasket thickness before compression.
• Pressure class or rating (PN or Class): part of the flange standard.

Ring gaskets (flat ring) sit inside the bolt circle on the raised face; full-face gaskets cover the whole face including bolt holes. See ring and full-face gaskets. Spiral wound gaskets add inner and outer rings and are made to fit specific flange standards; see spiral wound gaskets. Cut and special shapes follow your drawing; see cut and custom gaskets.

Flanges are made to different standards, and common families include metric PN-rated, ASME or ANSI inch-class, and older national or local standards. Table-based flanges also exist in some regions. Two flanges of the same nominal size from different standards usually have different bolt circles and hole counts and cannot share a gasket. If you know the standard and rating, give it. If you do not, measure the flange.

Flat face, raised face and ring-type joint (RTJ) faces are common. The face type changes the gasket style and the load needed. A raised-face flange takes a ring gasket inside the bolts, while a flat-face flange often takes a full-face gasket. RTJ flanges use metal ring joints, which are a special item.

1. Measure the pipe bore or required gasket ID.
2. Measure the raised-face OD or the OD of the flange face.
3. Measure the bolt circle diameter from the centre of one hole to the centre of the opposite hole (for an even number of holes).
4. Count the bolt holes and measure their diameter.
5. Measure the overall flange OD if a full-face gasket is needed.
6. Note the flange thickness and face condition: pitting, scoring and warping.
7. Take a dimensioned sketch or a photograph with a ruler visible.

Thickness depends on the flange finish, bolt load and material. Material suitability depends on the medium and conditions; see the gasket material selection guide.

• Nominal sizes are not measurements, so do not order from nominal size alone when the standard is uncertain.
• Measurements of worn or corroded flanges may not reflect the original standard.
• Dimensions do not decide pressure rating or suitability for the service.
• Critical or regulated service requires technical review and the correct certification documents.

Provide the dimensions above, the standard if known, material or medium details and quantity. Upload a drawing, or use custom gasket manufacturing for non-standard shapes.