Choose a bolt size, grade and thread condition and get the tightening torque, the clamp load it produces and the bolt's tensile stress area, in N·m, lbf·ft and lbf·in. The chart below lists common coarse sizes for SAE grade 2, 5 and 8 and metric 8.8, 10.9 and 12.9.
Worked example
These are example values. Change any entry above and press Calculate to run your own numbers.
Bolt
1/2-13 UNC
Tensile stress area
0.1419 in² (91.55 mm²)
Proof load
12,061 lbf (53.65 kN)
Clamp load (preload)
9,046 lbf (40.24 kN)
Tightening torque
102.2 N·m = 75.38 lbf·ft (904.6 lbf·in)T = K × d × F with K = 0.2
The result is a calculated torque for the clamp load you chose, not a specification. The real clamp load for a given torque varies by 25% or more with friction (thread and under-head condition, plating, lubricant, reuse), so use the joint designer's or the manufacturer's torque where one is given, and a torque-angle or tension method for critical joints.
Proof strengths: SAE J429 for inch grades (grade 2 55 ksi, grade 5 85 ksi, grade 8 120 ksi) and ISO 898-1 for metric classes (8.8 580 MPa up to M16 and 600 MPa above, 10.9 830 MPa, 12.9 970 MPa). Stress area uses the standard thread formulas.
How it works
Tightening a bolt stretches it like a spring, and that tension is what clamps the joint. Torque is only an indirect way to get it. Most of the torque you apply is lost to friction under the head and in the threads, which is why the same torque can give very different clamp loads on dry, oiled and plated bolts. This calculator uses the standard short-form equation with a nut factor for the thread condition.
The formulas
Torque T = K × d × F (K nut factor, d nominal diameter, F clamp load)
Clamp load F = percent of proof load × proof strength × tensile stress area
Tensile stress area, inch threads: As = π ÷ 4 × (d − 0.9743 ÷ threads per inch)²
Torque chart: SAE bolts, lbf·ft (dry, 75% of proof load)
Bolt
Grade 2
Grade 5
Grade 8
1/4-20 UNC
5.47
8.45
11.9
5/16-18 UNC
11.3
17.4
24.6
3/8-16 UNC
20
30.9
43.6
7/16-14 UNC
32
49.4
69.8
1/2-13 UNC
48.8
75.4
106
5/8-11 UNC
97.1
150
212
3/4-10 UNC
172
267
376
1-8 UNC
–
644
909
Torque chart: metric bolts, N·m (dry, 75% of proof load)
Bolt
Class 8.8
Class 10.9
Class 12.9
M5 x 0.8
6.17
8.83
10.3
M6 x 1.0
10.5
15
17.6
M8 x 1.25
25.5
36.5
42.6
M10 x 1.5
50.5
72.2
84.4
M12 x 1.75
88
126
147
M14 x 2.0
141
201
235
M16 x 2.0
218
312
365
M20 x 2.5
441
610
712
M24 x 3.0
761
1,053
1,231
Calculated with K = 0.20 for plain dry steel; lightly oiled threads (K = 0.15) need about 25% less, and plated or well-lubricated ones (K = 0.10) about half. These are calculated values for carbon and alloy steel bolts, not a specification: friction varies, so use the joint designer’s torque where one is given.
Frequently asked questions
What is the torque for a 1/2-13 grade 5 bolt?
About 75 lbf·ft (102 N·m) for a plain dry bolt at 75% of proof load. A grade 8 bolt of the same size, with its higher proof strength, comes to about 106 lbf·ft. Lightly oiled threads need about 25% less torque for the same clamp load, 56.5 lbf·ft for the grade 5.
What is the torque for an M10 8.8 bolt?
About 50 N·m (37 lbf·ft) for a plain dry bolt at 75% of proof load. A class 10.9 bolt of the same size comes to about 72 N·m. An M12 class 8.8 is about 88 N·m.
Should I lubricate a bolt before tightening?
Lubrication lowers the friction, so a given torque produces more clamp load. If a torque value was set for dry threads and you oil the bolt without lowering the torque, you can stretch it past its proof load. Use the torque for the condition the joint is actually in, and use the thread condition setting here to see the difference.
Why are the torque values different from other charts?
Torque charts depend on the nut factor, the clamp load percentage and the thread area they assume, and none of these is fixed by a standard. This page states its assumptions: the factor you select, 75% of proof load by default, and the standard stress area. Charts built for 60% or 90%, or for a different friction, will show other numbers.
What does proof load mean?
Proof load is the tension a bolt can carry without any permanent stretch. It is the stress area multiplied by the proof strength of the grade. Reusable joints are normally tightened to about 75% of it, and critical joints up to about 90%.
Can I use this for stainless steel, flanges, gaskets or aluminium?
No. This is for carbon and alloy steel bolts of the SAE and ISO grades listed. Stainless steel, soft materials, gasketed joints and flange bolting have their own limits and friction values, so use the joint designer's or the manufacturer's torque for them.
