For Minimum Collapse Failure in the Plastic Range with Minimum Yield Stress Limitations:
Applicable D/t Span
API Grade  D/t Ratio Span 

H40 casing and tubing  16.40 and less 
K55 casing and J55 tubing  14.81 and less 
C75 casing and tubing  13.60 and less 
L80 and N80 casing and tubing  13.38 and less 
C90 casing and tubing  13.01 and less 
C95 casing, X95 drill pipe, and T95 casing and tubing  12.85 and less 
P105 tubing  12.57 and less 
P110 casing  12.44 and less 
Q125 casing  12.11 and less 
For Minimum Failure in the Plastic Range:
Factors and Applicable D/t Span
API Grade  A  B  C  D/t Ratio Span 

H40  2.950  .0465  754  16.40 to 27.01 
J55 and K55  2.991  .0541  1206  14.81 to 25.01 
C75 and E  3.054  .0642  1806  13.60 to 22.91 
L80 and N80  3.071  .0667  1955  13.38 to 22.47 
C90  3.106  .0718  2254  13.01 to 21.69 
C95  3.124  .0743  2404  12.85 to 21.33 
P105  3.162  .0794  2702  12.57 to 20.70 
P110  3.181  .0819  2852  12.44 to 20.41 
Q125  3.239  .0895  3301  12.11 to 19.63 
For Minimum Failure in Transition Zone Between Plastic and Elastic Range:
Factors and Applicable D/t Ratio Span
API Grade  F  G  D/t Ratio Span 

H40  2.063  .0325  27.01 to 42.64 
J55 and K55  1.989  .0360  25.01 to 37.21 
C75  1.990  .0418  22.91 to 32.05 
L80 and N80  1.998  .0434  22.47 to 31.02 
C90  2.017  .0466  21.69 to 29.18 
C95  2.029  .0482  21.33 to 28.36 
P105  2.053  .0515  20.70 to 26.89 
P110  2.066  .0532  20.41 to 26.22 
Q125  2.106  .0582  19.63 to 24.46 
For Minimum Failure in the Elastic Range:
Applicable D/t Span
Grade  D/t Ratio Span 

H40  42.64 and greater 
J55 and K55  37.21 and greater 
C75  32.05 and greater 
L80 and N80  31.02 and greater 
C90  29.18 and greater 
C95  28.36 and greater 
P105  26.89 and greater 
P110  26.22 and greater 
Q125  24.46 and greater 
Yp = Minimum Yield Strength
Grade  Factors in psi 

H40  40,000 
J55 and K55  55,000 
C75  75,000 
L80 and N80  80,000 
C90  90,000 
C95  95,000 
P105  105,000 
P110  110,000 
Q125  125,000 
S135  135,000 
Where:
= Minimum collapse pressure in psi  
D = 
Nominal outside diameter in inches 
t = 
Nominal wall thickness in inches 
Collapse Pressure Under Axial Tension Stress:
Minimum collapse pressure under axial load can be determined from tables in API Bul 5C2, 20th Edition, 1987.
Internal Yield Pressure
(Casing, Tubing and Drill Pipe  Minimum Wall)
Pl = 
minimum internal yield pressure in psi 
D = 
nominal outside diameter of pipe in inches 
t = 
nominal wall thickness of pipe in inches 
Yp = 
specified minimum yield strength in psi 
Internal Pressure of pipe at 80% of Minimum Yield:
(Also: Maximum Formation Fracturing Pressure)
Internal Pressure of Couplings:
Where:S = 
80% of Yp for mill test pressure and for maximum fracturing pressure 
= 
100% of Yp for internal yield pressure 
W (W1 for special clearance) = outside diameter of coupling 

d1 = 
thread root diameter of coupling at end of pipe in powertight position 
For round thread casing and tubing couplings:
where:
T = included taper on diameter
Other symbols are listed in API Std. 5B.
For buttress thread casing couplings:
T = 
included taper on diameter 
l = 
0.400 in. for 41/2 in. OD 
= 
0.500 in. for 5 in. through 133/8 in. OD 
= 
0.375 in. for 16 in. OD and larger 
Other symbols are listed in API Std.5B.
Internal Pressure Leak Resistance:
Where:
P = 
internal pressure, psi 
E = 
30 x 106 
T = 
thread taper 
N = 
no. of thread turns makeup 
p = 
thread pitch 
W = 
coupling OD 
Es =  E1 for round thread 
=  E7 for buttress thread 
Round Thread Casing
The following equations apply to both short and long threads and couplings. One is for failure by fracture and the other is for failure by thread jumping or pullout; the lesser of the resulting values is listed as the joint strength.
Fracture Strength:
Pullout Strength:
Where:
Pj = 
minimum joint strength in pounds 
Ajp = 
crosssectional area of the pipe wall under the last perfect thread, in square inches 
= 
0.7854 [ ( D  0.1425)2 d2 ] 
D = 
nominal outside diameter of pipe in inches 
d = 
nominal inside diameter of pipe in inches 
L = 
engaged thread length in inches 
= 
L4  M for nominal makeup, as listed in API Std. 5B 
Yp = 
minimum yield strength of pipe in pounds per square inch 
Up = 
minimum ultimate strength of pipe in pounds per square inch 
D  *.74 D 0.59 

4 1/2  .30467 
5  .28631 
5 1/2  .27066 
6 5/8  .24251 
7  .23476 
7 5/8  .22321 
8 5/8  .20756 
9 5/8  .19455 
10 3/4  .18226 
11 3/4  .17295 
13 3/8  .16022 
16  .14415 
18 5/8  .13179 
20  .12636 
Buttress Thread Casing
For casing with buttress threads, two formulas are used; one is for the strength of the threaded pipe and the other, for the coupling. The lesser of the values obtained by the two formulas governs.
Pipe Strength:
Coupling Thread Strength:
Where:
Pj = 
minimum joint strength in pounds 
Yp = 
minimum yield strength of pipe, psi 
Up = 
minimum ultimate strength of pipe, psi 
Uc = 
minimum ultimate strength of coupling, psi 
Ap = 
cross sectional area of pipe, square inches 
.7854 ( D2  d2 )  
Ac = 
cross sectional area of coupling, square inches 
= 
.7854 ( W2  d12 ) 
D = 
outside diameter of pipe, inches 
d = 
inside diameter of pipe, inches 
W = 
outside diameter of coupling, inches 
d1 = 
diameter at the root of the coupling thread at the end of the pipe in the powertight position 
Drill Pipe and Round Thread Tubing
Yield strength values for round thread tubing and drill pipe are determined as the product of the specified minimum yield strength for the steel grade and the area of section under the root of the last perfect thread of the pipe or of the area of the body of the pipe, whichever is smaller.
NonUpset Tubing:
External Upset Tubing and Internal and External Upset Drill Pipe:
Where:
Pj = 
minimum joint strength in pounds 
D = 
nominal outside diameter of pipe in inches 
d = 
nominal inside diameter of pipe in inches 
hs = 
height of thread in inches 
= 
0.05560 in. for 10 threads per inch 
= 
0.07125 in. for 8 threads per inch 
Yp = 
specified minimum yield strength of pipe in pounds per square inch. Values are listed elsewhere. 