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Bolt Torque Calculator - Torque Values by Size, Grade & Lubrication Condition

Look up proper tightening torque for standard bolts in dry, oiled, and anti-seize conditions

Field guide: What the Markings on a Bolt Head Mean

Torque application kit

Torque tools and thread products used with bolt torque values:

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Free bolt torque calculator for maintenance mechanics, millwrights, assemblers, and engineers. Enter your bolt size, thread pitch, grade or class, and lubrication condition to get the recommended tightening torque and the resulting clamp load in pounds. Getting bolt torque right matters more than most people realize. An under-torqued bolt does not develop enough clamp force to resist joint separation and will loosen under vibration. An over-torqued bolt exceeds the proof load and either yields immediately or fails prematurely under cyclic loading. This calculator covers SAE Grade 2, 5, and 8 bolts in inch sizes from 1/4-20 through 1-1/2-6, plus Metric Class 8.8, 10.9, and 12.9 bolts from M6 through M36. For each combination, it provides torque values at three lubrication conditions: dry (K=0.20), lightly oiled (K=0.15), and anti-seize or moly paste (K=0.10). The difference is dramatic - the same bolt torqued to 75 ft-lbs develops 40% more clamp force when lubricated with anti-seize than when dry, because less torque is wasted overcoming thread friction. Results include proof load, yield clamp load, recommended torque at 75% of proof load, and a warning if your specified torque exceeds the bolt's proof load.

Pro Tip: The biggest mistake in bolt torquing is ignoring the lubrication factor. A 3/4-10 Grade 8 bolt torqued to 200 ft-lbs dry develops about 18,000 lbs of clamp force. The same bolt at 200 ft-lbs with anti-seize develops about 25,000 lbs - enough to yield the bolt and potentially strip the threads or crack the flange. When switching from dry assembly to lubricated, you must reduce the torque value to hit the same clamp load. Conversely, torque specs written for oiled bolts will under-clamp if applied to dry, rusty bolts.

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How It Works

Select Bolt Size and Thread

Choose your bolt diameter and thread pitch. For inch bolts, select from standard UNC (coarse) and UNF (fine) sizes. For Metric, select the diameter and pitch (e.g., M12×1.75 coarse or M12×1.25 fine). Fine threads develop higher clamp loads at the same torque because of lower thread lead angle.
Select Bolt Grade or Class

Choose the bolt grade: SAE Grade 2 (low carbon, marked with no lines), Grade 5 (medium carbon, 3 lines), or Grade 8 (alloy, 6 lines). For Metric: Class 4.6, 8.8, 10.9, or 12.9. The grade determines proof load and yield strength, which directly set the maximum safe torque.
Select Lubrication Condition

Choose dry (as-received, no lubricant), lightly oiled (machine oil on threads), or anti-seize/moly paste. This sets the torque coefficient (K-factor): 0.20 for dry, 0.15 for oiled, 0.10 for anti-seize. The K-factor has a larger effect on clamp force than most people expect.
Review Torque Value and Clamp Load

See the recommended torque in ft-lbs and N-m, the resulting clamp load in pounds and kN, and the proof load of the bolt. The standard recommendation is to torque to 75% of proof load for static applications, which provides a safety margin against yield while developing adequate clamp force.
Check Against Your Application

Compare the calculated torque against your assembly specification. If your spec calls for a different target (e.g., 90% of proof for high-vibration flange bolting), adjust the target percentage. The calculator warns if the resulting clamp load exceeds the bolt's proof load at any setting.

Built For

Maintenance mechanics torquing flange bolts during pump and valve reassembly
Millwrights setting foundation bolts and equipment hold-down bolts to spec
Automotive technicians verifying torque values for suspension and drivetrain fasteners
Structural steel erectors torquing high-strength bolts in moment connections
Machine assemblers establishing torque standards for production bolting operations

Assumptions

Torque-tension relationship uses T = K x D x F, where K is the nut factor, D is nominal diameter, and F is clamp load.
Nut factor (K) is 0.20 for dry steel, 0.15 for lubricated, and varies by plating and surface condition.
Bolt proof load and yield strength are per the specified grade (SAE, ASTM, or ISO property class).
Joint surfaces are flat, parallel, and the bolt is not bending during tightening.

Limitations

Does not account for prevailing-torque locknut resistance, which adds to the wrench torque without adding clamp load.
Bolt relaxation and embedment after initial tightening are not modeled.
Thermal cycling effects on clamp load (differential expansion between bolt and joint) are not calculated.
Joint stiffness ratio and external load effects on bolt fatigue are not analyzed (requires full VDI 2230 analysis).

References

VDI 2230 - Systematic Calculation of Highly Stressed Bolted Joints
SAE J429 - Mechanical and Material Requirements for Externally Threaded Fasteners
ASTM A325/A490 - Structural Bolt Standards (now ASTM F3125)
Machinery's Handbook, 31st Edition - Torque and Tension Relationships for Threaded Fasteners
ISO 898-1 - Mechanical Properties of Fasteners Made of Carbon Steel and Alloy Steel

Frequently Asked Questions

How do I determine the correct torque for a bolt?

Torque = K × D × F, where K is the friction coefficient (0.20 dry, 0.15 oiled, 0.10 anti-seize), D is the bolt diameter in inches, and F is the target clamp force in pounds. The standard target is 75% of the bolt's proof load. For a 1/2-13 Grade 5 bolt (proof load 12,050 lbs), target clamp = 9,038 lbs. Dry torque = 0.20 × 0.50 × 9,038 = 904 in-lbs = 75 ft-lbs. With anti-seize: 0.10 × 0.50 × 9,038 = 452 in-lbs = 38 ft-lbs.

Does lubrication change the torque spec?

Yes, dramatically. Lubrication reduces the friction coefficient, which means more of the applied torque goes into stretching the bolt (clamp force) rather than overcoming friction. A bolt torqued to the same ft-lbs develops 30-50% more clamp force when lubricated compared to dry. If a torque spec is written for dry bolts and you apply anti-seize, you must reduce the torque by about 35-50% to achieve the same clamp load. Always know whether a torque spec assumes dry or lubricated conditions.

What is the difference between Grade 5 and Grade 8 bolts?

Grade 5 bolts are medium-carbon steel with a proof load of about 85,000 PSI and tensile strength of 120,000 PSI. Grade 8 bolts are alloy steel with a proof load of 120,000 PSI and tensile strength of 150,000 PSI. Grade 8 develops about 40% more clamp force than Grade 5 at the same bolt size. However, Grade 8 is more brittle and less forgiving of over-torque. Use Grade 5 for general applications and Grade 8 when higher strength is specifically required by design.

Should I use fine or coarse thread bolts?

Coarse threads (UNC) are the default for general assembly, structural connections, and anywhere bolts are frequently removed. They are more tolerant of cross-threading and debris. Fine threads (UNF) develop 10-15% more clamp force at the same torque due to their lower helix angle and have a larger tensile stress area. Use fine threads for precision applications, vibration-prone connections, and where maximum clamp force per torque is needed. Avoid fine threads in field conditions where dirt and damage are common.

What happens if I over-torque a bolt?

Over-torquing stretches the bolt beyond its proof load into the yield zone. A bolt that has yielded does not return to its original length when loosened - it is permanently elongated and weakened. Under cyclic loading, a yielded bolt fails much sooner than a properly torqued one. Severe over-torque can break the bolt during tightening (usually in the thread run-out zone), strip the nut threads, or crack the flanged component being clamped. Always use a calibrated torque wrench and know your target value before you start tightening.

Disclaimer: This calculator provides torque values based on standard friction coefficients and published bolt proof loads. Actual required torque depends on surface finish, plating, prevailing-torque locking features, and joint design. Critical applications (pressure vessels, structural connections, safety-critical assemblies) require torque specifications from the equipment manufacturer or design engineer. Use a calibrated torque wrench for all torqued fasteners.

Bolt Torque Calculator - Torque Values by Size, Grade & Lubrication Condition

Torque application kit

How It Works

Select Bolt Size and Thread

Select Bolt Grade or Class

Select Lubrication Condition

Review Torque Value and Clamp Load

Check Against Your Application

Built For

Assumptions

Limitations

References

Frequently Asked Questions

Guides for this job

What the Markings on a Bolt Head Mean

Tap Drill Sizes: Why 75% Thread Is Usually Wrong

Fillet Weld Strength Planning

Bolt Torque: Why Lubrication Changes Everything

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