According to the API 5CT specification, the thickness is determined by the combination of the Outside Diameter (OD) and the Nominal Weight (Linear Weight) of the pipe.
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How to Determine API Casing Thickness
The wall thickness is calculated based on the steel's density and the pipe's dimensions to meet specific pressure requirements:
Heavier Weight = Thicker Wall: For the same Outside Diameter, a "heavier" pipe has a thicker wall, which increases its Collapse Resistance and Burst Strength.
Drift Diameter: This is a critical measurement slightly smaller than the Inside Diameter (ID). it guarantees that a cylindrical mandrel of a specific size can pass through the pipe.
API 5CT Casing Size Chart (Common Sizes)
Below is a chart showing the most frequently used API casing sizes, their weights, and corresponding wall thicknesses.
| Outside Diameter (OD) | Nominal Weight | Wall Thickness | Inside Diameter (ID) | Drift Diameter |
| inches (mm) | lb/ft ( kg/m ) | inches (mm) | inches (mm) | inches (mm) |
| 4-1/2" (114.3) | 9.50 | 0.205 (5.21) | 4.090 (103.89) | 3.965 (100.71) |
| 11.60 | 0.250 (6.35) | 4.000 (101.60) | 3.875 (98.43) | |
| 13.50 | 0.290 (7.37) | 3.920 (99.57) | 3.795 (96.39) | |
| 5-1/2" (139.7) | 15.50 | 0.275 (6.98) | 4.950 (125.73) | 4.825 (122.56) |
| 17.00 | 0.304 (7.72) | 4.892 (124.26) | 4.767 (121.08) | |
| 20.00 | 0.361 (9.17) | 4.778 (121.36) | 4.653 (118.19) | |
| 7" (177.8) | 23.00 | 0.317 (8.05) | 6.366 (161.70) | 6.241 (158.52) |
| 26.00 | 0.362 (9.19) | 6.276 (159.41) | 6.151 (156.24) | |
| 32.00 | 0.453 (11.51) | 6.094 (154.79) | 5.969 (151.61) | |
| 9-5/8" (244.5) | 36.00 | 0.352 (8.94) | 8.921 (226.59) | 8.765 (222.63) |
| 40.00 | 0.395 (10.03) | 8.835 (224.41) | 8.679 (220.45) | |
| 47.00 | 0.472 (11.99) | 8.681 (220.50) | 8.525 (216.54) | |
| 13-3/8" (339.7) | 48.00 | 0.330 (8.38) | 12.715 (322.96) | 12.559 (319.00) |
| 54.50 | 0.380 (9.65) | 12.615 (320.42) | 12.459 (316.46) | |
| 68.00 | 0.480 (12.19) | 12.415 (315.34) | 12.259 (311.38) | |
| 20" (508.0) | 94.00 | 0.438 (11.13) | 19.124 (485.75) | 18.936 (480.97) |
| 133.00 | 0.635 (16.13) | 18.730 (475.74) | 18.542 (470.97) |
Don't wait for production. We stock 4-1/2" to 20" casing in various wall thicknesses. Check Our Ready-to-Ship Inventory.
Casing Selection by Type
The thickness and size you choose depend on which part of the well the casing is used for:
Conductor Casing (20" - 30"): Usually the thinnest relative to its size; used to prevent the top of the well from caving in.
Surface Casing (13-3/8" - 20"): Protects freshwater zones and provides a base for the BOP (Blowout Preventer).
Intermediate Casing (7" - 9-5/8"): Used in deeper wells to isolate high-pressure zones or unstable formations. Requires higher wall thickness to resist collapse.
Production Casing (4-1/2" - 7"): The deepest string. It must withstand the highest internal pressures and often has the thickest walls relative to its diameter.
Important Considerations for You
Tolerance: API 5CT allows for a wall thickness tolerance (usually -12.5%). This means the actual pipe can be slightly thinner than the nominal thickness on the chart.
Grade Influence: While "Thickness" provides physical strength, the Steel Grade (J55, K55, N80, P110) determines the chemical strength. A thinner P110 pipe might be stronger than a thicker J55 pipe.
Coupling OD: Remember that the coupling (the connection piece) has a larger diameter than the pipe body, which must be considered when calculating clearance in the borehole.
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FAQ
What is the wall thickness tolerance for API 5CT?
The wall thickness tolerance for API 5CT casing and tubing is -12.5%.
This means:
The actual wall thickness at any point must not be less than 87.5% of the nominal wall thickness.
There is typically no specific plus (+) tolerance for thickness, but the maximum thickness is effectively limited by the pipe's overall weight tolerance (usually +6.5% for single lengths).
What are the different sizes of casings?
API 5CT casing sizes are standardized by their Outside Diameter (OD). They typically range from 4-1/2 inches to 20 inches.
Here are the most common sizes categorized by their use in a well:
Small (Production Casing): 4-1/2", 5", 5-1/2"
Medium (Intermediate Casing): 7", 7-5/8", 9-5/8"
Large (Surface/Conductor Casing): 13-3/8", 16", 20"
