Bolt Torque Chart — Structural and Metric Fastener Tightening Values

Proper bolt torque ensures that fasteners achieve the required clamping force without yielding the bolt. This reference provides torque values for structural bolts (ASTM A325/F1852, A490/F2280, F3125), metric structural bolts (ISO 8.8, 10.9, 12.9), and general-purpose machine bolts (SAE grades). Includes the torque equation, nut factor (K) explanation, and when torque control is insufficient alone.

Safety note: Torque is an indirect measure of clamping force. Friction variation (K factor) can cause ±30% error in actual bolt tension from the same applied torque. For slip-critical connections, use direct tension indicators (DTI) or twist-off type bolts as the primary verification method.

Structural Bolt Torque Values — ASTM F3125 (A325/A490)

Values for turn-of-nut pretensioning verification only. AISC/RCSC uses rotation method, DTI, or calibrated wrench — torque alone is not an RCSC-approved tightening method for slip-critical connections.

Minimum bolt pretension per RCSC Table 8.2.1 (kips):

Approximate torque to achieve RCSC minimum pretension (T = K × P × d):

K = 0.20 (typical for clean, dry, unlubricated threads); d = nominal bolt diameter

These torque values are estimates. Actual required torque varies with lubrication, surface condition, washer type, and nut grade. Always calibrate the wrench on job-site fastener assemblies per RCSC Section 8.

Metric Structural Bolt Torque Values

ISO 898-1 metric structural bolts — recommended torque (Nm) for full pretension:

Values use K = 0.20, target pretension = 0.70 × fub × As (70% of minimum tensile strength, per EN 14399-1).

Grade 8.8 (Fu = 800 MPa)

Grade 10.9 (Fu = 1,040 MPa)

Grade 12.9 (Fu = 1,220 MPa)

SAE Grade Bolt Torque Chart (Imperial, UNC/UNF)

For general-purpose machine bolts. Values assume K = 0.20 (dry threads, no lubrication).

Torque in ft-lb (multiply by 12 for in-lb):

SAE Grade 2 (Fy = 57 ksi for d ≤ 3/4 in; 36 ksi for d > 3/4 in)

SAE Grade 5 (Fy = 92 ksi for d ≤ 1 in; 81 ksi for d > 1 in)

SAE Grade 8 (Fy = 130 ksi)

The Torque Equation and K Factor

The fundamental torque–tension relationship:

T = K × P × d

Where:
  T = Applied torque (ft-lb or Nm)
  K = Nut factor (dimensionless) — accounts for friction
  P = Target bolt tension (lb or N)
  d = Nominal bolt diameter (ft or m — must match torque units)

Nut factor K values by condition:

Practical implication: Switching from dry to greased threads reduces friction, so the same torque produces more tension. Using dry-based torque values with lubricated bolts can yield 30–40% higher tension than intended, potentially yielding the bolt.

Approved Tightening Methods per RCSC

The Research Council on Structural Connections (RCSC) specifies four approved methods for pretensioned and slip-critical connections:

Snug-tight only (no pretension) is permitted for bearing-type connections where slip is acceptable. Tightening just enough to bring plies into firm contact.

Torque-Tension Relationship

The relationship between applied torque and resulting bolt tension is governed by the torque-tension equation:

T = K * d * Fi

Where:

• T = applied torque (ft-lb or N-m)
• K = nut factor (dimensionless, depends on lubrication and surface condition)
• d = nominal bolt diameter (in. or mm)
• Fi = installed pretension (kips or kN)

The nut factor K is highly variable and depends on surface condition, lubrication, and thread condition. Per the RCSC Specification Commentary, typical K values are:

Because K can vary by 50% or more depending on surface condition, the RCSC Specification requires that torque values be calibrated for each bolt lot and surface condition using a calibrated tension-indicating device. Torque alone is not a reliable indicator of pretension without calibration.

The specified minimum pretension per AISC 360-22 Table J3.1 is:

Minimum pretension equals 70% of the specified minimum tensile strength of the bolt.

Nut Rotation Table per RCSC

The turn-of-nut method is the most reliable and widely used pretensioning method. Per RCSC Specification Section 8.2.1, the required nut rotation from snug-tight condition is:

Where d = nominal bolt diameter. For slopes greater than 1:20, beveled washers per ASTM F436 are required and the rotation must be verified by testing.

The turn-of-nut method is preferred because:

• It is largely independent of the K factor
• It provides consistent pretension across bolt lots
• It is fast to execute in the field
• Verification is visual (match-marking before and after rotation)

Torque Values by Grade and Diameter

The following table provides estimated torque values for standard conditions (K = 0.18, plain finish, as-received). These values must be calibrated on-site per RCSC requirements and should not be used directly for installation without verification.

Calculated using T = K _ d _ Fi with d in inches and Fi in kips, then converting to ft-lb (multiply by 1/12 conversion factor is already included).

Bolt Pretension Verification Methods

Per AISC 360-22 Section N5.6 and RCSC Section 9, the following methods are accepted for verifying that the specified minimum pretension has been achieved:

For structural joints designated as slip-critical in the design documents, the Engineer of Record must specify the inspection method. DTI washers are increasingly preferred because they provide a permanent, inspectable record of pretension without requiring the inspector to witness the tightening operation.

Structural vs. Machine Bolts: Key Differences

Frequently Asked Questions

Can I use torque alone to tighten structural A325/A490 bolts? For snug-tight bearing connections: yes, snugging by impact wrench is acceptable — no torque target needed. For pretensioned or slip-critical connections: torque alone is only acceptable via the calibrated wrench method (RCSC Section 8.2.4), which requires pre-calibration of the wrench on job-site bolt assemblies in a bolt tension calibrator before each shift.

What happens if I over-torque a structural bolt? Excessive torque can yield the bolt shank (stretch beyond elastic limit), reducing clamping force and potentially cracking the bolt. For A325 bolts, proof load = ~85% of tensile strength. Over-tensioning beyond 1.05× minimum pretension should be investigated; if bolt yields, replace it.

Should I use Loctite on structural bolts? No. RCSC and AISC specifications do not permit thread-locking compounds on structural connections. Bolt vibration loosening is prevented by achieving adequate pretension (friction grip), not by adhesives. For machine applications (equipment fasteners), follow the manufacturer's specification.

How does grip length affect required torque? Grip length (thickness of connected material) affects how many threads are engaged and the elastic elongation of the bolt. Longer grip → more elastic elongation → higher energy storage → more reliable pretension. RCSC turn-of-nut tables provide different rotation amounts for different grip-to-diameter ratios to account for this.

What is a calibrated wrench job-site test? Before each shift using the calibrated wrench method, test at least three complete bolt assemblies (bolt, nut, washer) of the same diameter, grade, and lot in a bolt tension calibrator (Skidmore-Wilhelm). Apply the job-site tightening method and verify the resulting tension meets RCSC minimums. Adjust the torque setting if needed.

What torque is needed for a 3/4 in A325 bolt in a snug-tight-only connection? Snug-tight is defined as the full effort of a worker using a spud wrench to bring plies into firm contact, or a few impacts of an impact wrench. There is no specific torque target — the objective is metal-to-metal contact without pretensioning. For bearing-type connections (snug-tight permitted by AISC 360), shear capacity is based on bolt shear strength (φRn = 17.9 kips for 3/4 in A325 N-condition), not pretension. The turn-of-nut and calibrated wrench methods only apply when a pretensioned or slip-critical connection is required.

How does galvanizing affect bolt torque requirements? Hot-dip galvanizing significantly increases thread friction, raising the K factor from ~0.20 (as-received) to 0.25 or higher. The same torque produces less bolt tension in galvanized assemblies. For galvanized A325 bolts, RCSC requires lubricating the bolt threads after galvanizing and performing job-site calibration tests. ASTM F3125 prohibits galvanizing A490 bolts entirely due to hydrogen embrittlement risk in high-strength steel.

Run This Calculation

→ Bolt Torque Calculator — calculate required tightening torque for structural and metric bolts from bolt grade, diameter, and nut factor K.

→ Bolted Connections Calculator — bolt shear and bearing capacity per AISC 360, AS 4100, EN 1993, CSA S16.

Related pages

• Bolt Hole Sizes Reference — standard, oversized, slotted hole diameters per AISC Table J3.3
• Bolt Capacity Table — A325 and A490 shear and tension capacity by bolt size
• Minimum Bolt Spacing — AISC edge distance and pitch requirements
• Steel Material Properties — A325, A490 bolt material grades and strengths
• Steel Connection Types — Shear Tab, End Plate, Moment — connection type selection guide
• Bolted Connections Calculator — full bolt group capacity per AISC 360
• Gusset Plate Calculator — braced frame gusset plate and bolt group design
• AISC Bolt Hole Sizes
• Steel Connection Design
• Bolt Group Capacity
• ASTM A36 Steel Properties

Torque values are for reference and preliminary estimation only. Final tightening procedures must comply with RCSC Specification requirements and be verified by a qualified inspector. Thread condition, lubrication, surface coatings, and washer type all significantly affect actual bolt tension produced by a given torque.

Disclaimer (educational use only)

This page is provided for general technical information and educational use only. It does not constitute professional engineering advice, a design service, or a substitute for an independent review by a qualified structural engineer. Any calculations, outputs, examples, and workflows discussed here are simplified descriptions intended to support understanding and preliminary estimation.

All real-world structural design depends on project-specific factors (loads, combinations, stability, detailing, fabrication, erection, tolerances, site conditions, and the governing standard and project specification). You are responsible for verifying inputs, validating results with an independent method, checking constructability and code compliance, and obtaining professional sign-off where required.

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