API 5CT Casing Sizes, Dimensions & Weight Tables

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API 5CT Casing Sizes, Dimensions & Weight Tables

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API 5CT Casing Sizes — Complete Dimension Tables & Grade Guide

Selecting the right casing size is one of the most consequential decisions in well design. Get it wrong and you either can't run the completion tools you need (too small a drift), or you've over-engineered the string and added unnecessary cost and hookload to the program. This page provides complete API 5CT casing dimensions — OD, nominal weight, wall thickness, drift diameter, and coupling OD — for all standard sizes from 4½ inch to 20 inch, along with the grade selection context engineers need alongside the numbers.

ZC Steel Pipe manufactures and exports API 5CT casing in grades J55, K55, N80, L80, T95, P110, and C110 from our Hai'an City mill. We have supplied casing strings for oil and gas wells across Africa, the Middle East, and South America, with full API monogram, dimensional inspection reports, and third-party inspection available on every order.

Casing String Design — A Brief Overview
How to Read a Casing Dimension Table
API 5CT Casing Sizes — Full Dimension Tables
Casing Grade Summary
Coupling Dimensions & Types
Casing vs Tubing — Key Differences
How to Specify Casing for Your Project
Frequently Asked Questions

1. Casing String Design — A Brief Overview

A modern oil and gas well uses multiple concentric casing strings, each serving a different function and set at a different depth. Understanding which size goes where is the starting point for any casing dimension query.

Conductor Casing

Typical OD: 16", 18-5/8", 20", 30"

Depth: Shallow — 30 to 300 ft

Purpose: Isolate unconsolidated near-surface formations, support wellhead

Common grades: K55, J55

Surface Casing

Typical OD: 10-3/4", 11-3/4", 13-3/8", 16"

Depth: 300 to 3,000 ft

Purpose: Protect freshwater aquifers, support BOP, isolate loss zones

Common grades: J55, K55, N80

Intermediate Casing

Typical OD: 7", 7-5/8", 8-5/8", 9-5/8"

Depth: 3,000 to 10,000+ ft

Purpose: Isolate problem zones, enable drilling to target depth

Common grades: N80, L80, P110

Production Casing

Typical OD: 4-1/2", 5", 5-1/2", 6-5/8", 7"

Depth: Full well depth — to TD

Purpose: Contain reservoir pressure, house completion equipment

Common grades: N80, L80, T95, P110, C110

Field Note — Casing Size Ladder
Well design works from the bottom up: pick your production casing size first (driven by tubing OD, perforation gun diameter, and completion tool ODs), then work upward selecting each outer string to accommodate the drift diameter needed to run the next string in. A typical land well might run 4½" production inside 7" intermediate inside 9⅝" surface inside 13⅜" conductor. Each jump must clear the drift of the outer string with adequate clearance for centralizers.

2. How to Read a Casing Dimension Table

KEY TERMS

OD (Outside Diameter) — The nominal outside diameter that identifies the casing size. This is the dimension used to order and specify casing (e.g., "9-5/8 inch casing").

Nominal Weight (lb/ft) — The theoretical weight per linear foot of the pipe body, used for hookload calculations. Multiple weights are available for each OD.

Wall Thickness (t) — The pipe body wall thickness in inches, directly derived from OD and weight.

ID (Inside Diameter) — Calculated as OD minus 2×t. The nominal bore before accounting for tolerances.

Drift Diameter — The minimum guaranteed bore through the casing, including through the coupling. Always smaller than nominal ID. This is what determines what tools can pass through.

Coupling OD — The outside diameter of the threaded coupling. Critical for calculating clearance in the borehole.

3. API 5CT Casing Sizes — Full Dimension Tables

For exact OD, wall thickness and plain-end weight on any size, use our API 5CT casing dimension lookup tool.

4-1/2" Casing

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
4-1/2	9.50	0.205	4.090	3.965	4.750	J55, K55
4-1/2	10.50	0.224	4.052	3.927	4.750	J55, K55, N80
4-1/2	11.60	0.250	4.000	3.875	4.750	N80, L80, P110
4-1/2	13.50	0.290	3.920	3.795	4.750	N80, L80, P110
4-1/2	15.10	0.337	3.826	3.701	4.750	N80, P110, C110

5" Casing

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
5	11.50	0.220	4.560	4.435	5.200	J55, K55
5	13.00	0.253	4.494	4.369	5.200	J55, N80, L80
5	15.00	0.296	4.408	4.283	5.200	N80, L80, P110
5	18.00	0.362	4.276	4.151	5.200	N80, P110, C110

5-1/2" Casing

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
5-1/2	14.00	0.244	5.012	4.887	5.750	J55, K55
5-1/2	15.50	0.275	4.950	4.825	5.750	J55, N80
5-1/2	17.00	0.304	4.892	4.767	5.750	N80, L80, P110
5-1/2	20.00	0.361	4.778	4.653	5.750	N80, L80, P110, C110
5-1/2	23.00	0.415	4.670	4.545	5.750	P110, C110, Q125

6-5/8" Casing

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
6-5/8	20.00	0.288	6.049	5.924	7.390	J55, K55, N80
6-5/8	24.00	0.352	5.921	5.796	7.390	N80, L80, P110
6-5/8	28.00	0.417	5.791	5.666	7.390	N80, L80, P110, C110
6-5/8	32.00	0.475	5.676	5.550	7.390	P110, C110, Q125

7" Casing

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
7	17.00	0.231	6.538	6.413	7.656	J55, K55
7	20.00	0.272	6.456	6.331	7.656	J55, N80, L80
7	23.00	0.317	6.366	6.241	7.656	N80, L80, P110
7	26.00	0.362	6.276	6.151	7.656	N80, L80, P110
7	29.00	0.408	6.184	6.059	7.656	N80, P110, T95, C110
7	32.00	0.453	6.094	5.969	7.656	P110, T95, C110, Q125
7	35.00	0.498	6.004	5.879	7.656	P110, C110, Q125

7-5/8" Casing

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
7-5/8	24.00	0.300	7.025	6.900	8.500	J55, N80
7-5/8	26.40	0.328	6.969	6.844	8.500	N80, L80
7-5/8	29.70	0.375	6.875	6.750	8.500	N80, L80, P110
7-5/8	33.70	0.430	6.765	6.640	8.500	N80, P110, T95
7-5/8	39.00	0.500	6.625	6.500	8.500	P110, C110, Q125

8-5/8" Casing

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
8-5/8	24.00	0.264	8.097	7.972	9.625	J55, K55
8-5/8	28.00	0.304	8.017	7.892	9.625	J55, K55, N80
8-5/8	32.00	0.352	7.921	7.796	9.625	N80, L80
8-5/8	36.00	0.400	7.825	7.700	9.625	N80, L80, P110
8-5/8	40.00	0.450	7.725	7.600	9.625	N80, L80, P110
8-5/8	44.00	0.500	7.625	7.500	9.625	P110, C110
8-5/8	49.00	0.557	7.511	7.386	9.625	P110, C110, Q125

9-5/8" Casing

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
9-5/8	32.30	0.312	9.001	8.845	10.625	J55, K55
9-5/8	36.00	0.352	8.921	8.765	10.625	J55, N80, L80
9-5/8	40.00	0.395	8.835	8.679	10.625	N80, L80, P110
9-5/8	43.50	0.435	8.755	8.599	10.625	N80, L80, P110
9-5/8	47.00	0.472	8.681	8.525	10.625	N80, P110, T95, C110
9-5/8	53.50	0.545	8.535	8.379	10.625	P110, T95, C110, Q125

10-3/4" Casing

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
10-3/4	32.75	0.279	10.192	10.036	11.750	J55, K55
10-3/4	40.50	0.350	10.050	9.894	11.750	J55, N80
10-3/4	45.50	0.400	9.950	9.794	11.750	N80, L80, P110
10-3/4	51.00	0.450	9.850	9.694	11.750	N80, P110
10-3/4	55.50	0.495	9.760	9.604	11.750	N80, P110

11-3/4" Casing

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
11-3/4	42.00	0.333	11.083	10.925	12.750	J55, K55
11-3/4	47.00	0.375	11.000	10.844	12.750	J55, K55, N80
11-3/4	54.00	0.435	10.882	10.724	12.750	N80, L80, P110
11-3/4	60.00	0.489	10.772	10.616	12.750	N80, L80, P110
11-3/4	65.00	0.534	10.681	10.526	12.750	P110, C110
11-3/4	71.00	0.582	10.587	10.430	12.750	P110, C110, Q125

13-3/8" Casing (Surface Casing)

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
13-3/8	48.00	0.330	12.715	12.559	14.375	J55, K55
13-3/8	54.50	0.380	12.615	12.459	14.375	J55, K55, N80
13-3/8	61.00	0.430	12.515	12.359	14.375	K55, N80, L80
13-3/8	68.00	0.480	12.415	12.259	14.375	N80, L80, P110
13-3/8	72.00	0.514	12.347	12.191	14.375	N80, P110

16" Casing (Conductor / Intermediate)

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	STC Coupling OD (in)	Common Grades
16	65.00	0.375	15.250	15.062	17.000	J55, K55
16	75.00	0.438	15.124	14.936	17.000	K55, N80
16	84.00	0.495	15.010	14.822	17.000	K55, N80

18-5/8" & 20" Casing (Conductor)

OD (in)	Nom. Wt (lb/ft)	Wall t (in)	ID (in)	Drift Dia. (in)	Common Grades
18-5/8	87.50	0.435	17.755	17.567	J55, K55
20	94.00	0.438	19.124	18.936	J55, K55
20	106.50	0.500	19.000	18.812	J55, K55
20	133.00	0.635	18.730	18.542	K55, N80

Critical Engineering Point — Drift vs. ID
Never use nominal ID to size your completion tools. Always use drift diameter. The drift mandrel test is the contractual guarantee — nominal ID is a calculated figure and can have −0.5% OD tolerance and −12.5% wall tolerance stacked against it. When tool clearance is tight (e.g., wireline perforating guns in 4½" casing), specify and confirm drift diameter before ordering the string.

View our interactive API 5CT casing specification tables with filterable dimensions, grade properties and pressure ratings.

4. Casing Grade Summary

Grade	Min Yield (psi)	Min Tensile (psi)	Hardness Max	Typical Use	Sour Service
H40	40,000	60,000	—	Shallow, low-pressure wells	Not recommended
J55	55,000	75,000	—	Surface casing, conductor	Not recommended
K55	55,000	95,000	—	Surface / conductor, higher tensile	Not recommended
N80 (Type 1 / Q)	80,000	100,000	—	Intermediate / production casing	Not recommended
L80	80,000	95,000	HRC 23	Sour service production casing	Yes (ISO 15156)
T95	95,000	105,000	HRC 25.4	Sour service, higher strength	Yes (ISO 15156)
P110	110,000	125,000	—	Deep high-pressure wells	Not for sour service
C110	110,000	115,000	HRC 29	Sour service at P110 strength	Yes (ISO 15156)
Q125	125,000	135,000	—	Ultra-deep, HPHT wells	Not for sour service

For detailed specifications on individual grades, see our dedicated grade articles:

5. Coupling Dimensions & Types

API 5CT defines three standard thread and coupling types for casing. The coupling OD determines borehole clearance and the ability to run the string through the BOP rams.

Thread Type	Full Name	Sealing	Coupling OD vs Pipe OD	Notes
STC	Short Thread Coupling	Thread compound only	Larger than pipe OD	Standard for surface / conductor casing
LTC	Long Thread Coupling	Thread compound only	Larger than pipe OD	Higher tensile strength than STC, common for production
BTC	Buttress Thread Coupling	Thread compound only	Larger than pipe OD	Higher compression resistance, deep wells

For critical service applications — particularly sour service, HPHT, and thermal wells — premium connections replace API threads entirely. ZC holds independent patents on premium connections with metal-to-metal gas-tight sealing. See our Premium Connection product page →

Engineering Insight — Coupling OD and BOP Clearance
The coupling OD is the widest point on a casing joint. On wells with tight wellbore-to-casing clearances, confirm the coupling OD clears the BOP ram bore and the next outer casing drift diameter. A 9⅝" LTC coupling at 10.625" OD must pass through a 10¾" casing string whose drift diameter may be as small as 9.894" — which it cannot. This is how incorrect casing size selections get caught at the wellsite rather than on paper.

6. Casing vs Tubing — Key Differences

Parameter	Casing	Tubing
Purpose	Stabilize wellbore, isolate formations, support wellhead	Convey produced fluids to surface; inject fluids down-hole
Typical OD range	4-1/2" to 20"	1.050" to 4-1/2"
Cemented?	Yes — cemented to borehole wall	No — run inside casing, retrievable
Wall thickness	Heavier — designed for collapse and burst	Lighter — designed for internal pressure and tensile
API standard	API 5CT	API 5CT
Common grades	J55, K55, N80, L80, P110, T95, C110, Q125	H40, J55, N80, L80, P110, C110, Q125
ZC product page	Casing & Tubing →	Casing & Tubing →

7. How to Specify Casing for Your Project

Once you've fixed the size, check whether the grade survives downhole with our casing collapse-pressure calculator.

A complete casing purchase order should include the following information to avoid ambiguity and ensure the pipe delivered matches your design:

Standard: API 5CT (or ISO 11960 — the international equivalent)
Grade: e.g., N80 Type 1, or L80 (specify Type 1, 9Cr, or 13Cr for L80)
OD and nominal weight: e.g., 9-5/8" × 47.00 lb/ft
Thread type: STC, LTC, or BTC (or premium connection designation)
Range: R1 (16–25 ft), R2 (25–34 ft), or R3 (34–48 ft) — most casing is R3
Quantity: In linear feet or number of joints (specify average joint length if critical)
Mill test requirements: API monogram, third-party inspection, supplementary requirements (SR)
Delivery condition: Thread protectors, pipe dope, stenciling requirements

Procurement Note — Grade Substitution Traps
N80 Type 1 and N80Q are both called "N80" but are manufactured differently — N80 Type 1 is normalized or normalized-and-tempered; N80Q is quenched and tempered. Mixing them in a string is technically permissible under API 5CT but can create confusion in traceability documentation and is discouraged on sour service wells. Always specify "N80 Type 1" or "N80Q" explicitly, not just "N80."

8. Frequently Asked Questions

What are the standard sizes of oil well casing?

API 5CT casing is available in standard OD sizes from 4-1/2 inch through 20 inch. The most commonly used sizes are 4-1/2" and 5" (production casing), 5-1/2", 6-5/8", 7", and 7-5/8" (intermediate and production), 8-5/8", 9-5/8", 10-3/4", and 11-3/4" (intermediate), and 13-3/8" and 20" (surface and conductor). Each size is available in multiple weights (lb/ft) corresponding to different wall thicknesses.

What is the difference between casing size and casing weight?

Casing size is the nominal outside diameter (OD) — for example, 9-5/8 inch. Casing weight is the nominal weight per linear foot in pounds (lb/ft) — for example, 36 lb/ft or 47 lb/ft. For any given OD, multiple weights are available, each corresponding to a different wall thickness. Heavier weight means thicker wall, higher collapse and burst resistance, and greater hookload on the rig.

What is drift diameter in casing?

Drift diameter is the minimum guaranteed internal diameter through a casing joint, including through the coupling. It is smaller than nominal ID to account for manufacturing tolerances and upset geometry. Drift diameter determines what tools, perforating guns, and completion equipment can pass through the casing string. Always use drift diameter — not nominal ID — when sizing completion hardware.

What grades of steel are used in API 5CT casing?

API 5CT casing grades range from H40, J55, and K55 (surface casing) through N80 and L80 (intermediate/production) to P110, C110, and Q125 (high-strength and HPHT applications). For sour service (H₂S environments), only L80, T95, and C110 are approved under NACE MR0175 / ISO 15156. P110, N80, and Q125 are not acceptable for sour service.

What is OCTG casing and tubing?

OCTG stands for Oil Country Tubular Goods — the collective term for casing, tubing, and drill pipe used in oil and gas well construction. Casing is the large-diameter pipe cemented into the wellbore to stabilize the hole and isolate formations. Tubing is the smaller pipe run inside the casing through which oil and gas flow to surface. Both are governed by API 5CT. See our overview: Steel Casing Pipe — Grades, Specifications & API 5CT Applications →

Request API 5CT Casing — ZC Steel Pipe

ZC Steel Pipe manufactures and exports API 5CT casing in grades J55, K55, N80, L80, T95, P110, and C110 from our Hai'an City, China mill. All standard sizes 4-1/2" through 20", STC/LTC/BTC threads, and premium connections available. Full API monogram, dimensional inspection, and third-party certification. Active casing supply to projects in Africa, the Middle East, and South America.

Contact us: mandy.w@zcsteelpipe.com | WhatsApp: +86-139-1579-1813

→ Request a Quote

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