Deriving bore from OD minus twice the wall
This gives a nominal figure from nominal inputs. It is arithmetic, not a measurement.
Do not use it as a guaranteed bore. Where bore matters, state how it is accepted.
Tube size notation
Steel Tube Size Chart
An equipment schedule rarely arrives in the form a tube supplier can quote from. This page translates a schedule line into an orderable item: which notation is which, what a pipe-size label does and does not tell you, and how to state OD, wall and length so the offer matches the requirement.
This page publishes no dimensional table. Every figure used in a live enquiry must come from the standard edition the contract invokes or from your own schedule.
Notation
Four notations that get used as though they were one
A schedule can carry any of these, and they are not interchangeable. The single most common ordering error is treating a size label as though it were a measurement.
| Notation | What it actually is | How to carry it onto an order |
|---|---|---|
| Nominal bore (NB) / nominal size (NPS) | A pipe designation. It is a label, not a measurement. | Treat it as an identifier to be resolved, never as a dimension to be ordered. |
| Outside diameter (OD) | A real measurement, and the one that governs handling, supports and fittings. | Give OD in mm with the tolerance basis for it. |
| Inside diameter (ID) | A real measurement, but rarely a directly controlled one. | State whether ID is measured, or derived from OD and wall. |
| Wall thickness | The dimension that, with OD, most often determines the acceptance basis. | Give wall in mm and name minimum-wall or average-wall. |
The trap. A nominal bore or nominal size is a designation shared by items across a range of actual dimensions. Two items carrying the same label can differ in wall and therefore in bore, which is why the label alone never closes a purchase requirement.
Explore the full steel-tubes range to place the size decision inside the right product route.
Conversion
From a schedule line to an orderable item
Four steps take a schedule line to something a supplier can quote against. Skipping the last step is how a size label silently becomes a different product.
| Step | What has to be true | Action |
|---|---|---|
| Read the schedule line as written | An equipment schedule can carry pipe size, tube OD, or both, and the two are not the same system. | Identify which notation the line actually uses before converting anything. |
| Resolve the designation to a measurement | The pipe-size label has to be looked up; it cannot be computed. | Obtain the OD and wall the schedule intends, from the drawing or the governing standard. |
| Re-state the item as tube dimensions | A tube order is placed as OD, wall and length with tolerances — not as a size label. | Write OD, wall and length in mm with the tolerance pair that controls acceptance. |
| Confirm no substitution has occurred | A pipe item and a tube item of similar appearance can differ in both dimension and basis. | Have the design authority confirm the tube item is acceptable for the connection. |
A complete tube item states the product standard, grade, OD, wall, length and quantity with an explicit unit, names the tolerance pair that governs acceptance, and identifies the delivered condition. Anything missing from that list is an open question the supplier has to answer for you.
Wall and bore
Where the arithmetic stops being safe
Wall thickness interacts with every other dimension on the order. These four patterns account for most of the disputes on a size translation.
Assuming a wall from a size label
A designation such as a schedule number references a wall, but a tube order needs the value.
Convert the reference into an explicit millimetre wall on the order.
Mixing units within one schedule
Inches and millimetres can both appear, and a decimal point changes the item.
Fix one unit for the order and confirm every line against it.
Carrying a wall basis silently
Two offers of the same nominal size can sit on different acceptance bases.
Write minimum-wall or average-wall beside the wall on every relevant item.
The wall convention itself — minimum-wall against average-wall — is set out on the tube tolerance basis page, and it has to be agreed before wall figures are compared between offers.
Adjacent decisions
Finish the dimension basis before consolidating the package
Size translation is one step in a chain. These two pages settle the rest of it.
A213 T22 tube package
Apply the size basis inside a thermal-service grade requirement, straight or formed.
Steel tube tolerance guide
Resolve the minimum-wall or average-wall acceptance basis and the measurement method.
Steel tube dimensions
The OD, ID and wall relationships, and why nominal bore is not a guaranteed bore.
GCC markets
Carry the size basis to the destination
A size stated once has to survive the quotation, the packing list and the receiving check. Country flags identify the destination; quotation must confirm the supply route, origin and availability.
Identify the emirate, fabricator or site address and unloading restrictions; distinguish a domestic handover from an import leg.
Name the consignee, importer and final site; ask who confirms product classification and border documentation before dispatch.
State the nominated receiving point and project document approver; align certificate review with the required arrival window.
Specify the actual port, border point or inland site requested; separate the entry leg from onward transport and unloading.
Identify the contractor or fabrication yard receiving the tubes; agree import-document responsibility and the named delivery place.
Confirm whether the order is for local consumption or onward shipment; name the consignee and agree who handles each transport leg.
Enquiry handoff
Turn the resolved size into a reviewable enquiry
State OD, wall and length in one unit, name the tolerance pair that governs acceptance, and identify the delivered condition. That is what lets an offer be compared against the requirement rather than against another offer.
Open the steel-tubes RFQ and enter the resolved item: product standard, grade, OD, wall, length, quantity with its unit, the controlling tolerance and the destination. Add your name, company and reply email on that form. Supporting drawings can be shared through the destination’s verified contact process.
Opening the RFQ does not submit anything. A submission is complete only when the receiving form returns a genuine acknowledgement.