Standard casing sizes are governed by the API 5CT specification (from the American Petroleum Institute). In the oil and gas industry, casing is categorized by its Outer Diameter (OD).
While there are many niche sizes, the industry typically relies on a set of "standard" diameters used for different sections of the well (Conductor, Surface, Intermediate, and Production).
Download our Comprehensive API 5CT Casing Size & Weight Chart for your project planning.
API 5CT Standard Casing Sizes Table
Casing & Small Diameter Casing (4-1/2" - 7")
| OD (inch/mm) | Nominal Weight | Wall (inch) | Wall (mm) | ID (mm) | Drift (mm) |
| 4-1/2" | 9.50 # | 0.205 | 5.21 | 103.89 | 100.71 |
| (114.3 mm) | 11.60 # | 0.250 | 6.35 | 101.60 | 98.43 |
| 13.50 # | 0.290 | 7.37 | 99.57 | 96.39 | |
| 5-1/2" | 14.00 # | 0.244 | 6.20 | 127.30 | 124.13 |
| (139.7 mm) | 15.50 # | 0.275 | 6.98 | 125.73 | 122.56 |
| 17.00 # | 0.304 | 7.72 | 124.26 | 121.08 | |
| 20.00 # | 0.361 | 9.17 | 121.36 | 118.19 | |
| 23.00 # | 0.415 | 10.54 | 118.62 | 115.44 | |
| 7" | 20.00 # | 0.272 | 6.91 | 163.98 | 160.81 |
| (177.8 mm) | 23.00 # | 0.317 | 8.05 | 161.70 | 158.52 |
| 26.00 # | 0.362 | 9.19 | 159.41 | 156.23 | |
| 29.00 # | 0.408 | 10.36 | 157.07 | 153.90 | |
| 32.00 # | 0.453 | 11.51 | 154.79 | 151.61 |
Intermediate & Technical Casing (7-5/8" - 10-3/4")
| OD (inch/mm) | Nominal Weight | Wall (inch) | Wall (mm) | ID (mm) | Drift (mm) |
| 7-5/8" | 26.40 # | 0.328 | 8.33 | 177.01 | 173.84 |
| (193.68 mm) | 29.70 # | 0.375 | 9.53 | 174.63 | 171.45 |
| 33.70 # | 0.430 | 10.92 | 171.83 | 168.66 | |
| 9-5/8" | 36.00 # | 0.352 | 8.94 | 226.59 | 222.63 |
| (244.48 mm) | 40.00 # | 0.395 | 10.03 | 224.41 | 220.45 |
| 43.50 # | 0.435 | 11.05 | 222.38 | 218.41 | |
| 47.00 # | 0.472 | 11.99 | 220.50 | 216.53 | |
| 53.50 # | 0.545 | 13.84 | 216.79 | 212.83 | |
| 10-3/4" | 40.50 # | 0.350 | 8.89 | 254.66 | 250.69 |
| (273.05 mm) | 45.50 # | 0.400 | 10.16 | 252.12 | 248.16 |
| 51.00 # | 0.450 | 11.43 | 249.58 | 245.62 |
Surface Casing & Conductor (13-3/8" - 20")
| OD (inch/mm) | Nominal Weight | Wall (inch) | Wall (mm) | ID (mm) | Drift (mm) |
| 13-3/8" | 48.00 # | 0.330 | 8.38 | 322.96 | 319.00 |
| (339.72 mm) | 54.50 # | 0.380 | 9.65 | 320.42 | 316.46 |
| 61.00 # | 0.430 | 10.92 | 317.88 | 313.92 | |
| 68.00 # | 0.480 | 12.19 | 315.34 | 311.38 | |
| 16" | 65.00 # | 0.375 | 9.53 | 387.35 | 382.57 |
| (406.4 mm) | 75.00 # | 0.438 | 11.13 | 384.15 | 379.37 |
| 20" | 94.00 # | 0.438 | 11.13 | 485.75 | 480.97 |
| (508 mm) | 106.50 # | 0.500 | 12.70 | 482.60 | 477.82 |
| 133.00 # | 0.635 | 16.13 | 475.74 | 470.97 |
Casing Length Ranges
API also standardizes the length of each individual joint of pipe. When buying casing, you must specify the "Range":
Range 1 (R1): 16.0 to 25.0 ft (4.88 to 7.62 m)
Range 2 (R2): 25.0 to 34.0 ft (7.62 to 10.36 m)
Range 3 (R3): 34.0 to 48.0 ft (10.36 to 14.63 m) - Most common for casing strings.
End Connections:
STC (Short Thread Coupled)
LTC (Long Thread Coupled)
BTC (Buttress Thread Coupled)
Premium Connections (Gas-tight seals for high-pressure or deep wells)
API 5CT OCTG Factory
API 5CT Oil well casing pipe test equipments
FAQ
What are the common casing sizes?
The most common casing sizes (Outer Diameter) used in the oil and gas industry according to API 5CT standards are:
Production Casing: 4-1/2", 5-1/2", 7"
Intermediate Casing: 7-5/8", 8-5/8", 9-5/8"
Surface Casing: 10-3/4", 13-3/8"
Conductor Pipe: 16", 20"
How thick should casing be?
The thickness of casing (wall thickness) is not a single fixed number; it is determined by the well depth, internal pressure, and environmental loads.
Generally, casing thickness follows these guidelines:
Typical Range: Most common casings have a wall thickness between 5 mm and 16 mm (approx. 0.200" to 0.625").
Selection Criteria: Thicker walls are used in deeper sections of the well to prevent collapse (from external ground pressure) and burst (from internal fluid pressure).
Measurement: In the industry, thickness is usually chosen by selecting a specific Nominal Weight (lb/ft) for a given outer diameter. For example, a 5-1/2" casing can come in different thicknesses resulting in weights of 15.5 lb/ft, 17 lb/ft, or 20 lb/ft.
