A bolt torque chart gives a reference tightening torque for a specified thread size, strength grade and assembly condition. In the calculated tables below, M10 × 1.5 Class 8.8 is 50.5 N·m (about 37 lbf·ft), and 1/2-13 SAE Grade 5 is 75.4 lbf·ft (about 102 N·m). Both assume a nut factor K = 0.20 and a target preload of 75% of proof load. These assumptions are a calculation basis, not a verified setting for every dry steel joint.

How Tightening Torque Works

Tightening stretches the bolt and compresses the joint. The resulting tensile force in the bolt is the preload. A torque wrench measures applied torque, not bolt tension. Friction at the threads and under the rotating head or nut absorbs much of the applied torque, so changes in oil, coating or washers can change the preload at the same wrench setting.

How We Calculated the Tables

Proof load Fp = Sp × As
Target preload F = 0.75 × Fp
Reference torque T = K × F × d

  • Sp: proof stress for the grade and size.

  • As: tensile stress area of the threaded section (not the plain shank area).

  • d: nominal bolt diameter.

  • K: empirical nut factor that bundles friction and geometry. It is not the thread friction coefficient.

Use F in newtons and d in meters to obtain T in N·m. Use F in lbf and d in inches to obtain lbf·in, then divide by 12 for lbf·ft. The 75% proof-load target is the selected basis for these reference calculations. The appropriate assembly target depends on the joint design and tightening procedure.

Metric Bolt Torque Chart: Coarse Threads

Calculation inputs: ISO 898-1:2013 nominal tensile stress areas and stresses under proof load. For the sizes shown, Class 8.8 uses 580 MPa through M16 and 600 MPa above M16; Classes 10.9 and 12.9 use 830 MPa and 970 MPa. These are calculated reference torques, not torque specifications issued by ISO. Multiply N·m by 0.7376 to obtain lbf·ft.

Metric Fine Threads

At the same nominal diameter, property class, preload fraction and assumed K, the larger tensile stress area of a fine thread gives a higher calculated torque. Actual torque also depends on thread geometry and friction. These tables do not establish the capacity of reduced-loadability head designs or tapped holes in softer materials. Check the product specification when selecting hex head screws.

SAE Bolt Torque Chart: UNC Threads

Calculation inputs: SAE J429_201405 proof stresses of 85,000 psi for Grade 5 and 120,000 psi for Grade 8 over the sizes shown. Grade 5 uses 74,000 psi above 1 inch; do not extrapolate this table to larger diameters. For a small fastener, a wrench graduated in lbf·in may be more convenient: the calculated 1/4-20 Grade 5 value is approximately 101 lbf·in. Select a tool with a suitable working range.

Confirm the inch thread and material grade when selecting Grade 5 hex head screws or Grade 8 hex head screws before you pick a value. Similar head sizes do not mean the same torque.

Worked Example: M10 × 1.5, Class 8.8

Proof load: 580 N/mm² × 58.0 mm² = 33,640 N. Target preload: 0.75 × 33,640 = 25,230 N. With d = 0.010 m:

T = 0.20 × 25,230 × 0.010 = 50.46 N·m
Table value: 50.5 N·m

This calculation estimates the torque corresponding to the selected target preload. It does not verify the capacity of an actual bolt, housing, gasket or tapped hole. The joint design must establish an acceptable preload range, allowing for tightening variation, external loads and the capacity of the mating parts.

Dry vs Lubricated Torque

For the same target preload, a lower nut factor gives a lower calculated torque. However, K cannot be assigned reliably from a label such as “dry,” “zinc-plated” or “anti-seize” alone. Coating systems, lubricant formulations, contact surfaces and assembly conditions all matter. Use supplier data for the specific fastener system or a validated torque–clamp-force test.

The K values in this table are illustrative inputs, not recommended values for particular coatings or lubricants. The torque changes apply to a fixed target preload; they are not universal lubricant correction factors. Torque–clamp-force testing, such as testing under ISO 16047, can help establish the relationship for a defined fastener system.

If the wrench setting remains unchanged while K falls from 0.20 to 0.15, the simplified model predicts about 33% more preload. Starting from 75% of proof load, that reaches approximately 100% of proof load. Proof load is not the same as yield load, but this change can consume the intended margin and increase the risk of overloading the bolt or internal thread. Do not add lubricant to a dry-specified assembly without an approved procedure.

Checks Before You Set the Wrench

For preload-critical joints, validate the tightening procedure on representative parts. Options include a validated torque–angle procedure, bolt-elongation measurement or a suitable load-indicating device. Torque–angle control does not directly measure bolt tension. Account for tool variation, friction variation and preload loss after seating.

  • Identify the fastener. Confirm diameter, pitch, grade or class, length and product standard.

  • Check the mating parts. Nut strength, washer hardness, thread engagement and clamped-part strength can limit the joint.

  • Match the surface condition. Know the coating, lubricant, washer arrangement and which part turns.

  • Follow the procedure. Use its tightening sequence, stages, tool requirements and reuse rules.

  • Allow for running torque. A prevailing-torque nut resists turning before it seats. Use the assembly procedure’s specified measurement and allowance; the reference tables do not include this additional resistance.

For socket head screws in tapped holes, check engagement length and the strength of the internal-thread material. A stronger screw alone does not make that internal thread stronger.

Why Published Torque Charts Disagree

Charts differ because they use different preload targets, friction values, stress areas or strength limits. Some add running torque, others use a full thread-and-bearing friction model. Compare the assumptions first, then the numbers.

NASA’s Fastener Design Manual, RP-1228 explains torque calculation and the limitations of assumed torque coefficients. The tables in this article are our calculations using the stated inputs, not copied NASA torque values. Mechanical-property inputs are identified with each table; the nut factor and preload fraction are separate calculation assumptions.

Need replacement fasteners or help matching a spec? Contact us with the thread size, grade or class, coating, mating thread and washer arrangement, plus any torque requirement on the drawing. We can then confirm the hardware before you set a wrench value.

Frequently Asked Questions

How much higher is Grade 8 torque than Grade 5?

For the sizes in the UNC table, Grade 8 has about 41% higher calculated torque than Grade 5 when diameter, pitch, K and the percentage of proof load are held constant. The ratio follows from 120,000 ÷ 85,000. It is not a general instruction to increase an existing assembly torque when replacing a bolt with a stronger grade.

Can I use these tables for stainless steel bolts?

No. These tables use the carbon- and alloy-steel property classes and SAE grades stated above. Stainless fasteners require values for their own material and property class. Their tightening procedure should also address galling and the specified lubricant or coating.

Does a crowfoot adapter change the wrench setting?

Yes, if the adapter changes the effective lever arm. For an adapter extending straight beyond the square drive in line with the wrench, the basic setting is T × L ÷ (L + E). Here T is the required fastener torque, L is the effective distance from the drive center to the specified hand-load point, and E is the distance from the drive center to the fastener center. Use the same length units and follow the tool manufacturer’s instructions for the actual setup.