Bolt Torque Chart & Calculator

Bolt Torque Chart & Calculator

Calculate tightening torque for any bolt — SAE Grade 2/5/8, metric ISO 8.8/10.9/12.9, or stainless steel. Supports UNC, UNF, metric coarse, and metric fine threads. All values computed using T = K × F × d at 75% of proof load per Machinery’s Handbook, 29th Edition.

A standard bolt torque chart lists the recommended tightening torque specifications for common bolt sizes and grades. The torque values on this page are calculated from T = K × F × d, where K is the torque coefficient (0.20 dry, 0.18 lubricated), F is the target preload at 75% of proof load, and d is the nominal bolt diameter. Proof strengths and tensile stress areas are per SAE J429, ISO 898-1, ASTM F593, and ASME B1.1, sourced from Machinery’s Handbook 29th Ed.

Bolt Torque Calculator

Select your thread system, bolt size, grade, and condition to calculate torque and clamp load instantly.

Recommended Torque
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Clamp Load (Preload)
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SAE: MH29 p. 1534 (SAE J429). Metric: ISO 898-1. Stainless (inch): ASTM F593. Stainless (metric): ISO 3506-1. UNC/UNF stress areas: MH29 p. 1844. Metric stress area formula: MH29 p. 1529.

K value notes: K = 0.20 is standard for plain steel, zinc-plated, and dry black oxide bolts. K = 0.25 for hot-dip galvanized (thicker, rougher coating). K = 0.18 for lubricated (anti-seize, oil). Threadlockers vary by product — some have lubricity data on their TDS, others (like Loctite 243) do not. Check your threadlocker’s data sheet; if no K factor is specified, use dry K = 0.20. Use “Custom K” for other conditions.
Stainless steel: Austenitic stainless (304/A2, 316/A4) is highly susceptible to galling. Always use anti-seize — dry stainless K values can exceed 0.30, making torque-based preload unreliable without lubrication. Select “Lubricated” or enter a custom K per your anti-seize manufacturer’s data.
Aluminum: Aluminum threads are soft and prone to galling and stripping. Use anti-seize and reduce preload percentage. Consult the fastener manufacturer for K values.
Bolt torque chart per Machinery's Handbook 29th Ed.

Source: Machinery’s Handbook 29th Ed. — Torque formula (T=KFd): p. 1521; torque coefficients: p. 1521; SAE proof strengths (SAE J429): p. 1534; UNC/UNF tensile stress areas (ASME B1.1): p. 1844; metric stress area formula: p. 1529. Metric proof stress per ISO 898-1. K=0.20 applies to plain steel, zinc-plated, and dry black oxide bolts per industry convention. MH29’s K=0.30 for “nonplated, black finish” represents worst-case corroded/oxidized conditions and is omitted.

Formula Reference

T  =  K  ×  Fi  ×  d

where:
T  = tightening torque [force × length] — e.g. in-lbs, ft-lbs, N·m
K  = torque coefficient (nut factor) [dimensionless]
Fi  = target preload (clamp load) [force] — e.g. lbf, N, kN
d  = nominal bolt diameter [length] — e.g. in, mm

Machinery’s Handbook 29th Ed., p. 1521

Fi  =  p  ×  At  ×  Sp

where:
Fi  = target preload [force] — e.g. lbf, N, kN
p  = preload fraction (0.75 for reusable, 0.90 for permanent) [dimensionless]
At  = tensile stress area [length²] — e.g. in², mm²
Sp  = proof strength of bolt grade [force / length²] — e.g. psi, MPa

Machinery’s Handbook 29th Ed., p. 1521

Unified threads (UNC/UNF):

At  =  0.7854 × (D − 0.9743 / n)²

where:
At  = tensile stress area [length²] — in²
D  = basic major diameter [length] — in
n  = threads per inch [1/length] — TPI

Machinery’s Handbook 29th Ed., p. 1536, Eq. 2a. Values tabulated on pp. 1844–1845 (ASME B1.1). This page uses the tabulated values for UNC/UNF.

Metric threads (ISO):

At  =  0.7854 × (d − 0.9382P)²

where:
At  = tensile stress area [length²] — mm²
d  = nominal bolt diameter [length] — mm
P  = thread pitch [length] — mm

Machinery’s Handbook 29th Ed., p. 1529, Eq. 11 (JIS B 1082 / ISO 898-1). MH29 does not tabulate metric stress areas — this page computes them from this formula. Note: MH29 uses the symbol As for this formula; this page uses At consistently to match the torque equations on p. 1521.

How to Use This Page

Calculator (top): Select your thread system, bolt size, grade, and K condition. Results update instantly.

Reference table (below the calculator): Browse torque values across all sizes for a given thread series. Use the Thread Series dropdown to switch between UNC, UNF, metric coarse, and metric fine. SAE bolt grades are identified by radial lines on the hex head: Grade 2 has no marks, Grade 5 has three lines, Grade 8 has six lines. Metric bolts are stamped with the property class number directly on the head (8.8, 10.9, or 12.9).

For the correct tap drill size when tapping bolt holes, see our tap drill size chart.

Torque Accuracy & Important Notes

Torque is an indirect measurement of bolt tension. A torque wrench does not measure bolt tension directly — it measures the rotational force applied, and the actual clamp load depends on friction, which varies with surface finish, lubrication, and bolt condition. Per Machinery’s Handbook, a torque wrench achieves ±25% accuracy on preload. More precise methods exist:

Method Accuracy
Tightening by feel ±35%
Torque wrench ±25%
Turn-of-nut method ±15%
Preload indicating washer ±10%
Bolt elongation measurement ±3–5%
Strain gauge / ultrasonic ±1%

Source: Machinery’s Handbook 29th Ed., p. 1526, Table 2.

For critical joints, MH29 recommends measuring bolt tension directly with a strain gauge or measuring bolt elongation with a micrometer. In most manufacturing and maintenance settings, these methods are not practical, and torque-based tensioning with a calibrated torque wrench is the standard approach — sufficient for the vast majority of bolted connections when the correct K factor and preload percentage are used.

About the values on this page:

  • Proof load is the maximum force a bolt can sustain without permanent deformation. Bolts are tested to this value during manufacturing. For specifications that do not publish a proof load, it is typically calculated at 92% of minimum yield strength.
  • Clamp load is calculated at 75% of proof load, providing a safety buffer so the bolt is not stressed too close to its proof load during tightening. Exceeding proof load risks permanent deformation or failure. An engineer may specify a different preload percentage for a particular application — use the Preload % input in the calculator to adjust.
  • All torque values use the formula T = K × F × d where F is the clamp load and d is the nominal diameter. Torque is computed in in-lbs and converted to ft-lbs (÷12) or N·m (×1.35582).
  • UNC and UNF stress areas are from MH29 p. 1844 (ASME B1.1). Metric stress areas are computed from At = 0.7854(d − 0.9382P)² per MH29 p. 1529 (MH29 uses As for this formula; see Formula Reference below).
  • SAE Grade 2 proof strengths apply to bolts 1/4"–3/4" in diameter up to 6" long. For longer Grade 2 fasteners, proof strength is reduced significantly — consult SAE J429 for the exact values.
  • Proof load, stress area, yield strength, and grade data are based on SAE J429, ISO 898-1, ASTM F593, and ISO 3506-1.

Example: Torquing a 1/2"-13 Grade 5 Bolt

You are assembling a bracket using 1/2"-13 UNC Grade 5 bolts with plain zinc-plated finish (dry). From the chart: the recommended torque is 75 ft‑lbs. This achieves a clamp load of approximately 9,046 lbf (toggle “More Columns” to see clamp loads). If you apply anti-seize lubricant, switch the Condition dropdown to “Lubricated” and the torque drops to 68 ft‑lbs for the same clamp load.

The underlying calculation: At = 0.1419 in², Sp = 85,000 psi, F = 0.75 × 0.1419 × 85,000 = 9,046 lbf. T = 0.20 × 9,046 × 0.50 = 905 in‑lbs = 75 ft‑lbs.

Bolt Torque Terminology

Proof Load
The maximum tensile force a bolt can sustain without permanent deformation. Proof load equals the tensile stress area (At) multiplied by the proof strength (Sp) for the bolt grade. Torque targets are set as a percentage of proof load — typically 75% for reusable connections and 90% for permanent connections.
Torque Coefficient (K)
A dimensionless factor that accounts for friction in the threads and under the bolt head. K = 0.20 is the standard value for plain steel, zinc-plated, and dry black oxide bolts; K = 0.18 for lubricated fasteners (anti-seize, oil). Threadlockers vary — some (like Loctite 2047) provide explicit lubricity data, while others (like Loctite 243) do not. The Loctite 243 TDS (April 2025) contains no K factor or torque adjustment guidance. Check your threadlocker’s data sheet for K factor data; if none is specified, use dry K = 0.20. These values are used by major fastener suppliers including Portland Bolt, AFT Fasteners, and Fuller Fasteners for off-the-shelf fasteners. Lower K means less friction and more efficient conversion of torque to clamp load. Note: Machinery’s Handbook lists K = 0.30 for “nonplated, black finish,” but this represents a worst-case condition (heavily oxidized or corroded surfaces) not typical of commercially available fasteners and is intentionally omitted from this chart.
Clamp Load (Preload)
The axial tensile force in the bolt after tightening, which creates the clamping force holding the joint together. For reusable connections, target preload is 75% of proof load: F = 0.75 × At × Sp.
Tensile Stress Area (At)
The effective cross-sectional area of a bolt at the thread root, used for strength calculations. At is smaller than the nominal shank area because threads reduce the cross-section. Values are defined by ASME B1.1 for UNC/UNF threads and computed from At = 0.7854(d − 0.9382P)² for metric threads (MH29 p. 1529; MH29 uses As for this formula).

Frequently Asked Questions

What torque should I use for a 3/8 bolt?

For a 3/8"-16 UNC bolt in dry condition (K=0.20) tightened to 75% of proof load: Grade 2 requires 20 ft-lbs, Grade 5 requires 31 ft-lbs, and Grade 8 requires 44 ft-lbs. If the bolt is lubricated (K=0.18), use the Condition dropdown above the chart to switch to “Lubricated” and see the adjusted values — lubricated torque is approximately 10% lower than dry. Always verify the bolt grade by checking the head markings before selecting a torque value.

What is the difference between Grade 2, Grade 5, and Grade 8 bolts?

SAE Grade 2, 5, and 8 bolts differ in material strength, identified by head markings per SAE J429. Grade 2 (no markings) is low-carbon steel with 55,000 psi proof strength for sizes up to 3/4". Grade 5 (3 radial lines) is medium-carbon quenched and tempered steel with 85,000 psi proof strength for sizes up to 1". Grade 8 (6 radial lines) is medium-carbon alloy quenched and tempered steel with 120,000 psi proof strength. Higher grade allows a higher target preload before yielding, so the recommended torque is higher for the same bolt size.

How do I convert ft-lbs to Nm for bolt torque?

Multiply the torque in ft-lbs by 1.35582 to get Newton-meters (Nm). For example, 75 ft-lbs × 1.3558 = 101.7 Nm. Toggle the “More Columns” button on this chart to see both ft-lb and Nm values side by side for every bolt size and grade.

Does lubrication affect bolt torque?

Yes. Lubrication reduces friction in the bolt joint — both in the threads and at the bearing surface under the head or nut — which means more of the applied torque converts to clamp load. The torque coefficient K drops from approximately 0.20 (dry/zinc-plated) to 0.18 (lubricated) per Machinery’s Handbook. This means a lubricated bolt achieves the same clamp load at 90% of the dry torque value. Using dry torque values on a lubricated bolt risks overloading the fastener. Use the Condition dropdown on this chart to switch between dry and lubricated torque values.

How is bolt torque calculated?

Bolt torque is calculated using T = K × F × d, where T is torque, K is the torque coefficient (0.20 for dry zinc-plated, 0.18 for lubricated), F is the target preload (clamp load), and d is the nominal bolt diameter. The target preload is typically 75% of the bolt’s proof load for reusable connections: F = 0.75 × At × Sp, where At is the tensile stress area and Sp is the proof strength for the bolt grade. All values on this page use this method with data from Machinery’s Handbook, 29th Edition.

Where do I apply lubricant on a bolt for proper torque?

For anti-seize or oil, apply to the threads and the bearing surface under the bolt head or nut. About 50% of applied torque is consumed by friction at the bearing surface and about 40% by thread friction — only roughly 10% actually produces clamp load. If you lubricate the threads but leave the bearing surface dry, the effective K factor will be somewhere between the dry and lubricated values, making your actual preload unpredictable. Threadlockers vary by product. Some (like Loctite 2047) are explicitly designed with high lubricity and will affect torque. Others (like Loctite 243) make no lubricity claims — the Loctite 243 TDS (April 2025) contains no K factor or torque adjustment guidance; the assembly instruction is simply “assemble and tighten as required.” Always check your specific threadlocker’s technical data sheet for lubricity or K factor data. If none is provided, use dry torque values (K = 0.20).

Can I download this bolt torque chart as a PDF?

Yes. Click the Print / PDF button above the table to open your browser’s print dialog, then select Save as PDF as the destination. The printable version hides interactive controls and formats the table for paper. All bolt sizes, grades, and torque values are included in the PDF output.

Does this chart include stainless steel and metric bolts?

Yes. Use the calculator at the top for any bolt — SAE, metric, or stainless steel. The reference table below supports UNC coarse, UNF fine, metric coarse, and metric fine threads via the Thread Series dropdown. Stainless steel grades (304, 316, A2-70, A4-70) are available in the calculator’s Grade dropdown. Stainless bolts are susceptible to galling — always use anti-seize lubricant and select the Lubricated condition.

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References

  • Oberg, E. et al. Machinery’s Handbook, 29th Edition, Industrial Press, 2012, pp. 1521–1534. Torque-preload relationships (T=KFd, p. 1521), torque coefficients (K values, p. 1521), SAE grade identification marks and mechanical properties (SAE J429, p. 1534).
  • Oberg, E. et al. Machinery’s Handbook, 29th Edition, pp. 1528–1529, 1844. Tensile stress areas for UNC/UNF threads (ASME B1.1, p. 1844) and metric stress area formula (p. 1529).
  • ISO 898-1 — Mechanical Properties of Fasteners Made of Carbon Steel and Alloy Steel, Part 1: Bolts, Screws and Studs. Defines proof stress for property classes 8.8, 10.9, 12.9.
  • ASTM F593 — Standard Specification for Stainless Steel Bolts, Hex Cap Screws, and Studs. Proof load stress for 18-8 (304) and 316 grades.
  • ISO 3506-1 — Mechanical Properties of Corrosion-Resistant Stainless Steel Fasteners. Defines property classes A2-70, A4-70 for metric stainless bolts.
  • SAE J429 — Mechanical and Material Requirements for Externally Threaded Fasteners. Grade markings and proof strengths for SAE Grade 2, 5, and 8 bolts.

Data last verified: March 2026

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