API 5CT casing is made in 14 standard outside diameters from 4½ to 20 in (114.30–508.00 mm). Each diameter is sold in several nominal weights, and the weight, not the diameter, sets the wall thickness, the inside diameter and the drift. This page lists every standard size, shows how to read a designation such as 9-5/8″ 47 lb/ft, and covers the drift, length range, connections and grade limits that decide whether the string can be run.
The table below is the API casing size chart in its shortest usable form: the diameter, the nominal weights the standard tabulates for it, and where the size normally appears in a well. Weights are pounds per foot of plain-end pipe; the count of weights per diameter ranges from one at 18⅝ in to thirteen at 7 in.
| OD (in) | OD (mm) | Nominal weights tabulated (lb/ft) | Where it normally appears |
|---|---|---|---|
| 4½ | 114.30 | 9.50, 10.50, 11.60, 13.50, 15.10 | Production liner, slim-hole completion |
| 5 | 127.00 | 11.50, 13.00, 15.00, 18.00, 21.40, 23.20, 24.10 | Production liner, tie-back string |
| 5½ | 139.70 | 14.00, 15.50, 17.00, 20.00, 23.00, 26.80, 29.70, 32.60, 35.30, 38.00, 40.50, 43.10 | Production casing, workover strings |
| 6⅝ | 168.28 | 20.00, 24.00, 28.00, 32.00 | Intermediate or production liner |
| 7 | 177.80 | 17.00, 20.00, 23.00, 26.00, 29.00, 32.00, 35.00, 38.00, 42.70, 46.40, 50.10, 53.60, 57.10 | Production casing, the most common production size |
| 7⅝ | 193.68 | 24.00, 26.40, 29.70, 33.70, 39.00, 42.80, 45.30, 47.10, 51.20, 55.30 | Intermediate casing in deeper wells |
| 8⅝ | 219.08 | 24.00, 28.00, 32.00, 36.00, 40.00, 44.00, 49.00 | Intermediate casing |
| 9⅝ | 244.48 | 32.30, 36.00, 40.00, 43.50, 47.00, 53.50, 58.40, 59.40, 64.90, 70.30, 75.60 | Intermediate casing, the highest-volume casing OD worldwide |
| 10¾ | 273.05 | 32.75, 40.50, 45.50, 51.00, 55.50, 60.70, 65.70 | Surface or intermediate casing in high-pressure wells |
| 11¾ | 298.45 | 42.00, 47.00, 54.00, 60.00, 65.00, 71.00 | Surface casing |
| 13⅜ | 339.72 | 48.00, 54.50, 61.00, 68.00, 72.00 | Surface casing, the standard surface size on most land wells |
| 16 | 406.40 | 65.00, 75.00, 84.00, 109.00 | Surface or conductor casing |
| 18⅝ | 473.08 | 87.50 | Conductor casing |
| 20 | 508.00 | 94.00, 106.50, 133.00 | Conductor or surface casing |
Weights are the nominal values tabulated in API 5CT, Annex C. Not every listed weight is stocked in every grade, and not every weight is available from every mill. The confirmed scope appears on the order acknowledgement.
Reading down that column of weights explains most of the disagreement between quotations. A 5½ in string can be ordered anywhere from 14.00 to 43.10 lb/ft, and the wall behind those two numbers differs by a factor of more than three. For the sizes themselves, the mill offers them as API 5CT casing in seamless or electric-welded form.
Nominal weight is the mass per foot of plain-end pipe, and for a fixed outside diameter there is only one free variable: the wall. The relationship is fixed enough to check a quotation by hand:
W (lb/ft) = 10.69 × t (in) × [OD (in) − t (in)]
Work it backwards from a 9⅝ in pipe at 47.00 lb/ft and the wall comes out at 0.472 in, or 11.99 mm. Nothing else in the designation can move it. The table below applies that relationship to four of the most ordered sizes, so a buyer can see what each weight step actually buys: heavier is not a cosmetic upgrade, it is a smaller hole and a stronger wall at the same time.
| OD (in) | Nominal weight (lb/ft) | Wall mm (in) | Inside diameter mm (in) | Drift mm (in) |
|---|---|---|---|---|
| 5½ | 14.00 | 6.20 (0.244) | 127.30 (5.012) | 124.12 (4.887) |
| 5½ | 17.00 | 7.72 (0.304) | 124.26 (4.892) | 121.08 (4.767) |
| 5½ | 20.00 | 9.17 (0.361) | 121.36 (4.778) | 118.18 (4.653) |
| 5½ | 23.00 | 10.54 (0.415) | 118.62 (4.670) | 115.44 (4.545) |
| 7 | 20.00 | 6.91 (0.272) | 163.98 (6.456) | 160.80 (6.331) |
| 7 | 23.00 | 8.05 (0.317) | 161.70 (6.366) | 158.52 (6.241) |
| 7 | 26.00 | 9.19 (0.362) | 159.42 (6.276) | 156.24 (6.151) |
| 7 | 29.00 | 10.36 (0.408) | 157.08 (6.184) | 153.90 (6.059) |
| 7 | 32.00 | 11.51 (0.453) | 154.78 (6.094) | 151.60 (5.969) |
| 9⅝ | 36.00 | 8.94 (0.352) | 226.60 (8.921) | 222.63 (8.765) |
| 9⅝ | 40.00 | 10.03 (0.395) | 224.42 (8.835) | 220.45 (8.679) |
| 9⅝ | 43.50 | 11.05 (0.435) | 222.38 (8.755) | 218.41 (8.599) |
| 9⅝ | 47.00 | 11.99 (0.472) | 220.50 (8.681) | 216.53 (8.525) |
| 9⅝ | 53.50 | 13.84 (0.545) | 216.80 (8.535) | 212.83 (8.379) |
| 13⅜ | 48.00 | 8.38 (0.330) | 322.96 (12.715) | 318.99 (12.559) |
| 13⅜ | 54.50 | 9.65 (0.380) | 320.42 (12.615) | 316.45 (12.459) |
| 13⅜ | 61.00 | 10.92 (0.430) | 317.88 (12.515) | 313.91 (12.359) |
| 13⅜ | 68.00 | 12.19 (0.480) | 315.34 (12.415) | 311.37 (12.259) |
| 13⅜ | 72.00 | 13.06 (0.514) | 313.60 (12.347) | 309.63 (12.190) |
Inside diameter and drift are calculated from the tabulated wall. Nominal weight labels are rounded, so back-calculating a wall from the label alone drifts by a few hundredths of an inch — read the wall from the standard table and use the formula only to sanity-check a quotation.
One practical consequence: a design that needs a specific drift cannot be met by ordering “9-5/8 in casing” and hoping. The weight has to be chosen first, because the drift moves 6 mm between 36.00 and 53.50 lb/ft.
Drift diameter is the guaranteed internal clearance of a joint. It is proven by pulling a cylindrical drift mandrel through the full length of every pipe at the mill, and it is always smaller than the calculated inside diameter because it has to absorb wall tolerance and ovality.
The deduction is not a single value. API 5CT sets it in three classes by size, and this is where a common shortcut fails: taking 1/8 in off the ID understates the deduction on 9⅝ in and above, which is exactly the range where clearance-critical completions are designed.
| Casing OD | Deduction from calculated ID | Mandrel length | Example |
|---|---|---|---|
| Below 9⅝ in | 3.18 mm (1/8 in) | 152 mm (6 in) | 7 in 26.00 lb/ft: ID 159.42 mm, drift 156.24 mm |
| 9⅝ in to 13⅜ in | 3.97 mm (5/32 in) | 305 mm (12 in) | 9⅝ in 47.00 lb/ft: ID 220.50 mm, drift 216.53 mm |
| Above 13⅜ in | 4.76 mm (3/16 in) | 305 mm (12 in) | 20 in 133.00 lb/ft: ID 475.74 mm, drift 470.98 mm |
Two things follow for procurement. First, if a completion tool is designed to pass with a few millimetres to spare, drift is the number to quote against the tool drawing, not the calculated ID. Second, a string that needs more clearance than standard drift allows is ordered to full drift or special drift, which is a deviation stated on the purchase order and not something a mill applies by default.
The drift test is done throughout the length, and it is repeated over the first 0.6 m (24 in) from the coupled end after power-tight make-up on casing, so the threaded end does not escape inspection.
Range is the length band a joint is delivered in, and it is the field that decides how many connections a string contains. It is also the field most often copied wrongly: the 20 to 24 ft figures that appear on several casing pages belong to tubing, not casing.
| Product | Range 1 | Range 2 | Range 3 |
|---|---|---|---|
| Casing | 16.0–25.0 ft (4.88–7.62 m) | 25.0–34.0 ft (7.62–10.36 m) | 34.0–48.0 ft (10.36–14.63 m) |
| Threaded and coupled tubing | 20–24 ft (6.10–7.32 m) | 28–32 ft (8.53–9.75 m) | 38–42 ft (11.58–12.80 m) |
| Integral tubing | 20–26 ft | 28–34 ft | 38–45 ft |
| Pup joints | 2, 3, 4, 6, 8, 10 and 12 ft, tolerance ±3 in | ||
Within a carload, casing is not free to sit anywhere in the band. For 95 % or more of the footage, Range 1 joints may vary by no more than 6.0 ft and must be at least 18.0 ft long; the equivalent figures are 5.0 ft and 28.0 ft for Range 2, and 6.0 ft and 36.0 ft for Range 3. Tubing is held tighter, with the whole carload inside a 2 ft variation. The carload tolerance does not apply to order items below 40,000 lb.
Range 3 is the usual choice on deep and offshore wells because fewer joints mean fewer connections to make up and fewer places for a leak to start. Range 2 remains the default in many land programmes, and Range 1 mostly appears in shallow wells, workover strings and surplus stock.
A string is shorter than the sum of its joint lengths. Every connection consumes a standoff as the pin enters the coupling, and tally sheets have to account for it; on a 200-joint string that allowance is measured in tens of metres, not centimetres.
The size on the order does not fix the connection. API 5CT covers the dimensional and delivery requirements for the connections below but leaves thread geometry and gauging to API 5B, so the two standards are normally quoted together.
| Connection | Designation in API 5CT | Thread form | Sealing | Typical use |
|---|---|---|---|---|
| STC | SC — short round thread casing | Round, 8 TPI | Thread compound | Shallow, low-pressure strings |
| LTC | LC — long round thread casing | Round, 8 TPI, longer thread | Thread compound | Moderate tensile loads |
| BTC | BC — buttress thread casing | Buttress, 5 TPI | Thread compound, higher torque capacity | Deep and heavy strings |
| Premium | Not an API connection | Proprietary, metal-to-metal | Metal-to-metal seal | Gas wells, HPHT, horizontal and sour service |
The naming matters when a quotation is checked. API 5CT uses SC, LC and BC; the trade uses STC, LTC and BTC. A quotation that says “BTC” and a certificate that says “BC” describe the same connection, and neither is a premium connection however it is marketed.
Coupling outside diameter is larger than pipe outside diameter, and that matters twice: it reduces the annular clearance in the hole, and it is the diameter the elevators and the spider actually handle. The value is tabulated per size and weight in API 5B, and it is the reason a “special clearance” coupling is sometimes specified on tight-annulus wells.
Size and grade are separate choices, but not independent ones: the standard does not tabulate every grade in every weight, and heavy walls at large diameters are usually restricted to the lower-strength grades. The grade ladder itself is short.
| Grade | Yield strength (MPa) | Min. tensile (MPa) | Max. hardness | Service |
|---|---|---|---|---|
| H40 | 276–552 | 414 | — | General, shallow |
| J55 | 379–552 | 517 | — | General |
| K55 | 379–552 | 655 | — | General, higher tensile than J55 |
| N80 Type 1 / N80Q | 552–758 | 689 | — | General; Q is quenched and tempered |
| R95 | 655–758 | 724 | — | General |
| L80 Type 1 / 3Cr / 9Cr / 13Cr | 552–655 | 655 | 23 HRC | Type 1 sour; Cr types for CO2 |
| C90 | 621–724 | 689 | 25.4 HRC | Sour |
| T95 | 655–758 | 724 | 25.4 HRC | Sour |
| C110 | 758–828 | 793 | 29 HRC | Sour, HPHT |
| P110 | 758–965 | 862 | — | Sweet, HPHT |
| C125 | 862–931 | 896 | 34 HRC | Ultra-deep, added by Addendum 1 |
| Q125 | 862–1034 | 931 | No fixed maximum | Ultra-deep HPHT |
Three points on this table are worth more than the numbers. L80 alone no longer identifies one steel: since the 11th edition, the symbol covers Type 1, 3Cr, 9Cr and 13Cr, and only Type 1 carries the sour-service qualification, so “L80” on a quotation has to be pinned to a type. M65 and C95 are not in the current list, and a supplier offering them is either quoting an obsolete edition or old stock. And the hardness column, not the yield column, decides sour service: P110 and Q125 have no hardness ceiling, which is why they carry no NACE qualification regardless of strength.
The 11th edition removed the PSL1/PSL2/PSL3 structure that the 10th edition used. Requirements that were under PSL2 and PSL3 now sit in the main text or in the Annex A supplementary requirements, and NDE, impact testing or SSC testing that a project needs must be specified as supplementary requirements rather than assumed from a PSL label.
API 5CT tabulates 14 standard casing outside diameters: 4½, 5, 5½, 6⅝, 7, 7⅝, 8⅝, 9⅝, 10¾, 11¾, 13⅜, 16, 18⅝ and 20 in. Each diameter is listed with several nominal weights, so the standard contains dozens of size combinations. A casing size is therefore always an outside diameter plus a weight, never the diameter alone.
No. The dimensional tables in API 5CT stop at 20 in. Conductor larger than 20 in, including 24, 26, 30 and 36 in, is ordered against a project specification or a structural pipe standard such as ASTM A252. API 5CT does allow the specification to be applied to other plain-end sizes and wall thicknesses by agreement between purchaser and manufacturer, so large conductor can be certified to 5CT chemistry and mechanical properties, but the size itself is not tabulated.
It identifies the outside diameter and the nominal weight per foot of plain-end pipe. The 47 lb/ft fixes the wall thickness at 0.472 in (11.99 mm), which in turn fixes the inside diameter at 220.50 mm (8.681 in) and the drift at 216.53 mm (8.525 in). Two joints of the same 9-5/8 in diameter but different weights are different pipes with different pressure capacity and different clearance.
Drift is the diameter of a cylindrical mandrel that must pass through the full length of every joint, so it has to absorb wall tolerance and ovality. API 5CT deducts 3.18 mm (1/8 in) from the calculated inside diameter below 9⅝ in, 3.97 mm (5/32 in) from 9⅝ to 13⅜ in, and 4.76 mm (3/16 in) above 13⅜ in. The wall itself may run 12.5 % under nominal, which is why drift rather than calculated ID decides whether a tool will pass.
For casing, Range 1 is 16.0 to 25.0 ft, Range 2 is 25.0 to 34.0 ft and Range 3 is 34.0 to 48.0 ft. Tubing uses a different set: 20 to 24 ft, 28 to 32 ft and 38 to 42 ft. The 20 to 24 ft figure that appears on some casing pages is the tubing range, not the casing range. Range 3 reduces the number of connections in a string and is common on deep and offshore wells.
The 10th edition used the PSL1, PSL2 and PSL3 product specification levels. The 11th edition, published in December 2023, removed that structure: requirements that previously sat under PSL2 and PSL3 are now in the main text or in the Annex A supplementary requirements. Quotations that still quote PSL2 for casing are written in 10th-edition language, which is not wrong as a description of the older edition but no longer matches the current standard.
The grades listed in the 11th edition are H40, J55, K55, N80 (all types), L80 (Type 1, 3Cr, 9Cr and 13Cr), C90, R95, T95, P110, C110 and Q125, with C125 added by Addendum 1 published in May 2025. M65 and C95 do not appear in the current list, so a quotation that offers them is working from an obsolete edition or from old stock.
It is both. 4½ in is the smallest casing outside diameter in API 5CT and also the largest tubing outside diameter, so the diameter alone does not settle the question. The deciding factors are the connection, the wall range offered and the function in the well: casing is cemented in place and carries the wellbore, tubing is run inside casing and can be pulled.
