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.
CONTENTS
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", 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", 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", 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 TERMSOD (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
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
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
7.390
J55, K55
7
20.00
0.272
6.456
6.331
7.390
J55, N80, L80
7
23.00
0.317
6.366
6.241
7.390
N80, L80, P110
7
26.00
0.362
6.276
6.151
7.390
N80, L80, P110
7
29.00
0.408
6.184
6.059
7.390
N80, P110, T95, C110
7
32.00
0.453
6.094
5.969
7.390
P110, T95, C110, Q125
7
35.00
0.498
6.004
5.879
7.390
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
8.000
J55, N80
7-5/8
26.40
0.328
6.969
6.844
8.000
N80, L80
7-5/8
29.70
0.375
6.875
6.750
8.000
N80, L80, P110
7-5/8
33.70
0.430
6.765
6.640
8.000
N80, P110, T95
7-5/8
39.00
0.500
6.625
6.500
8.000
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
10.125
J55, K55
9-5/8
36.00
0.352
8.921
8.765
10.125
J55, N80, L80
9-5/8
40.00
0.395
8.835
8.679
10.125
N80, L80, P110
9-5/8
43.50
0.435
8.755
8.599
10.125
N80, L80, P110
9-5/8
47.00
0.472
8.681
8.525
10.125
N80, P110, T95, C110
9-5/8
53.50
0.545
8.535
8.379
10.125
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
11.375
J55, K55
10-3/4
40.50
0.350
10.050
9.894
11.375
J55, N80
10-3/4
45.50
0.400
9.950
9.794
11.375
N80, L80, P110
10-3/4
51.00
0.450
9.850
9.694
11.375
N80, P110
10-3/4
55.50
0.495
9.760
9.604
11.375
N80, P110
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.
4. Casing Grade Summary
Grade
Min Yield (psi)
Min Tensile (psi)
Hardness Max
Typical Use
Sour Service
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
110,000
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
125,000
HRC 30
Sour service at P110 strength
Yes (ISO 15156)
Q125
125,000
135,000
HRC 34
Ultra-deep, HPHT wells
Conditional
For detailed specifications on individual grades, see our dedicated grade articles:
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
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", 7", and 7-5/8" (intermediate and production), 9-5/8" and 10-3/4" (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 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 and N80 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.
