Work out how tight to torque a bolt. Choose a metric (class 4.6 to 12.9) or inch (SAE grade 2, 5 or 8) bolt, the thread condition and the preload you want, and get the tightening torque in N·m, ft-lb and in-lb, plus a torque chart for every size.
Last updated:
Bolt standard
Target preload
% of proof
75% for reusable joints, up to 90% for permanent ones
kN
lbf
Enter your values to see the result.
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How to use the Bolt Torque Calculator
Choose metric or inch bolts, then the size and the class or grade.
Pick the thread condition: dry, oiled, lubricated or a custom K.
Set the target preload as a percentage of proof load (75% is typical) or as a force.
Read the tightening torque and the clamp load.
Use the chart for other sizes of the same grade.
The torque equation
T = K × d × F, with F = preload % × proof strength × tensile stress area (As).
Example: a 1/2-13 UNC grade 5 bolt has As = 0.1419 in² and a proof strength of 85,000 psi, so its proof load is 12,060 lbf. At 75% the preload is 9,046 lbf. Dry (K = 0.20): T = 0.20 × 0.5 in × 9,046 lbf = 905 in-lb ≈ 75 ft-lb.
Proof strength by class and grade
Bolt
Proof strength
Min. tensile strength
ISO class 4.6
225 MPa
400 MPa
ISO class 5.8
380 MPa
500 MPa
ISO class 8.8 (≤ M16 / > M16)
580 / 600 MPa
800 / 830 MPa
ISO class 10.9
830 MPa
1,040 MPa
ISO class 12.9
970 MPa
1,220 MPa
SAE grade 2 (1/4–3/4 in)
55 ksi
74 ksi
SAE grade 5 (1/4–1 in)
85 ksi
120 ksi
SAE grade 8
120 ksi
150 ksi
Typical nut factors
Condition
K
Plain steel or zinc plated, dry (as received)
0.20
Lightly oiled
0.18
Anti-seize, grease
0.13–0.16
Molybdenum disulfide paste, wax
0.10–0.12
Frequently asked questions
How is bolt torque calculated?
The short-form torque equation is T = K × d × F, where K is the nut factor, d the nominal bolt diameter and F the desired preload (clamp force). An M10 class 8.8 bolt (As 58 mm², proof 580 MPa) at 75% of proof has F ≈ 25.2 kN; with K = 0.2, T = 0.2 × 0.010 m × 25,230 N ≈ 50 N·m.
What is the nut factor K?
K is an experimental factor that lumps together thread and under-head friction and geometry. Typical values are about 0.20 for dry steel, 0.18 lightly oiled, 0.15 with anti-seize or grease and 0.10–0.12 with moly paste. Real values scatter by ±25% or more.
Why does lubricated torque have to be lower?
With less friction, more of the applied torque turns into stretch. Using a dry torque value on a lubricated bolt can overload and yield it, so reduce the torque in proportion to K.
What preload should I use?
A common target is 75% of proof load for joints that will be taken apart again and up to 90% for permanent joints. Proof load is the tensile stress area multiplied by the proof strength of the class or grade.
What do 8.8 and 10.9 mean on metric bolts?
The first number is one hundredth of the tensile strength in MPa and the second is the ratio of yield to tensile strength. Class 8.8 has 800 MPa tensile and about 640 MPa yield; class 10.9 has 1,000 MPa and about 900 MPa.
Should I use this instead of the manufacturer's torque?
No. If an engine, wheel, vehicle or equipment manufacturer specifies a torque, use it; it accounts for gaskets, joint materials and fastener coatings. This calculator is for general fasteners where no specification exists.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.