API 5CT Casing Pipe — Complete Size & Dimension Guide

Every casing purchase order starts with three numbers: outside diameter, nominal weight, and grade. Get any one of these wrong and you face the wrong drift clearance for your completion tools, a failed pressure test, or material that does not meet your well design. This page provides complete API 5CT casing dimension tables for all standard sizes from 4½ inch to 20 inch — organised by string type — with the procurement context EPC teams need to write a correct purchase order first time.

ZC Steel Pipe supplies API 5CT casing in grades N80, L80, T95, P110 and C110 to oil and gas projects across Africa, South America and Southeast Asia. Full MTC documentation, third-party inspection, and dimensional inspection reports available on every order.

Which String Uses Which Size

A modern oil well uses multiple concentric casing strings, each set at a different depth to serve a different purpose. Casing selection always works from the bottom up — choose your production casing size first, then work upward so each outer string can accommodate the drift diameter needed to run the next string through it.

A typical land well in West Africa or South America runs 4½ inch production inside 7 inch intermediate inside 9⅝ inch surface inside 13⅜ inch conductor. Each step must clear the drift of the outer string with enough margin for centralisers and running tools.

How to Read a Casing Dimension Table

Five dimensions appear in every API 5CT casing table. Each has a specific procurement use:

OD (Outside Diameter) — the nominal outside diameter in inches. This is what you order by and what determines borehole clearance and BOP compatibility.

Nominal Weight (lb/ft) — the theoretical weight per linear foot used for hookload calculations. Multiple weights exist for each OD, each corresponding to a specific wall thickness.

Wall Thickness — derived from OD and nominal weight. API 5CT allows minus 12.5 percent tolerance on delivered wall thickness. Use minimum wall for design calculations.

ID (Inside Diameter) — calculated as OD minus 2 times wall thickness. This is the nominal bore before tolerances are applied. Do not use ID to size completion tools.

Drift Diameter — the minimum guaranteed bore through the joint including through the coupling. Always use drift diameter for sizing perforating guns, packers, and tubing. This is the contractual guarantee — nominal ID is not.

API 5CT Casing Dimension Tables

4½ inch Casing

5 inch Casing

5½ inch Casing

7 inch Casing

7⅝ inch Casing

9⅝ inch Casing

10¾ inch Casing

13⅜ inch Casing (Surface)

16 inch Casing (Conductor / Intermediate)

18⅝ inch and 20 inch Casing (Conductor)

Grade Selection — What the Numbers Mean for Procurement

Selecting a grade is a mechanical and environmental decision, not just a price decision. The table below covers the grades ZC supplies and where each is used. For sour service projects — common in West Africa and parts of South America — grade selection must comply with NACE MR0175 / ISO 15156.

N80 is the most common production casing grade on non-sour wells in Africa and South America. Always specify N80-1 or N80Q on the purchase order — not just N80. They are manufactured differently and mixing sub-types in a string creates traceability problems on projects with strict documentation requirements.

For detailed grade specifications see the individual grade guides: API 5CT N80 Casing Pipe — N80-1 vs N80Q API 5CT L80 Casing Pipe — Sour Service Guide API 5CT P110 Casing Pipe — Specifications and Applications

Connection Types

API 5CT defines three standard thread and coupling types. Connection choice affects coupling OD, which determines borehole clearance and BOP ram compatibility.

For gas wells, HPHT applications, and any service requiring a gas-tight seal, premium connections replace API threads. ZC Steel Pipe holds independent patents on premium connections qualified to API 5C5 CAL IV for all standard casing sizes.

Length Ranges — R1, R2 and R3

API 5CT defines three standard length ranges. Most international EPC projects specify Range 3 to minimise the number of connections in the string and reduce make-up time on the rig floor.

Always specify the range on your purchase order. If not stated, mills may supply R2 by default on some sizes.

PSL-1 vs PSL-2

API 5CT defines two Product Specification Levels. Most international oil and gas projects in Africa and the Middle East require PSL-2 as a minimum.

PSL-1 material cannot be retroactively upgraded to PSL-2 after delivery. Confirm the PSL requirement in your project specification before placing the order.

Casing vs Tubing

Casing and tubing are both governed by API 5CT but serve different functions in the well.

How to Write a Complete Casing Purchase Order

An incomplete purchase order is the most common cause of incorrect casing delivery and project delays. Every casing order to ZC Steel Pipe should include:

1. Standard — API 5CT or ISO 11960
2. Grade — with sub-type (N80-1 or N80Q, L80 Type 1 or L80-13Cr)
3. OD and nominal weight — e.g. 9⅝ inch x 47.00 lb/ft
4. Thread type — STC, LTC, BTC, or premium connection designation
5. Range — R1, R2, or R3 (default R3 for most strings)
6. PSL level — PSL-1 or PSL-2
7. Quantity — in joints or metric tonnes with average joint length
8. Delivery port — for freight planning and lead time
9. Inspection scope — third-party witness, mill visit, supplementary requirements
10. MTC level — EN 10204 3.1 or 3.2