Skip to main content
Shops & Outbuildings Free Pro Features Available

Torque-Tension Calculator - Bolt Preload from Applied Torque Using K-Factor Method

Calculate bolt clamping force, preload, and proof load percentage for any fastener size and grade

Free torque-tension calculator using the short-form K-factor equation T = K × D × F. Enter bolt diameter, grade, and nut factor to calculate the clamp load produced by a given torque value. Supports SAE Grade 5 and Grade 8, ASTM A193 B7 and B16, A325, A490, metric Class 8.8 and 10.9 fasteners. Shows preload as a percentage of proof load and yield strength, with recommended torque ranges for dry, lubricated, and anti-seize conditions. Essential for flange assembly, structural connections, and rotating equipment bolting.

Pro Tip: The nut factor K accounts for roughly 90% of your applied torque being consumed by friction rather than generating clamp load. A K of 0.20 (dry steel-on-steel) versus 0.15 (lubricated) changes your achieved preload by 33% at the same torque. Always specify the lubricant condition in your bolting procedure - torque values without a stated K-factor are meaningless. For critical flanges, consider hydraulic tensioning to bypass friction entirely.

Look up ASME B16.5 bolt patterns for flanges

Flange Bolt Reference →

Check coupling alignment after bolting

Coupling Alignment Calculator →

Verify motor mounting bolt requirements

Motor Efficiency Calculator →

Calculate crane rigging loads for equipment lifts

Crane Rigging Calculator →
Start Using This Tool See How It Works

PREVIEW All Pro features are currently free for a limited time. No license key required.

Bolt Torque-Tension Calculator
Pop Out

How It Works

  1. Select Bolt Size and Grade

    Choose the nominal bolt diameter (1/4" through 4" or M6 through M100) and material grade. The calculator loads the proof strength, yield strength, and tensile strength for the selected grade automatically.

  2. Set the Nut Factor (K)

    Select the lubrication condition or enter a custom K-factor. Common values: 0.20 for dry steel, 0.18 for cadmium plated, 0.15 for moly paste, 0.12 for anti-seize compound. The K-factor is the single biggest variable in torque-tension relationships.

  3. Enter Target Torque or Preload

    Input either the applied torque (ft-lbs or N-m) to calculate resulting preload, or input a desired preload (lbs or kN) to calculate required torque. The calculator works in both directions.

  4. Review Preload Results

    See the calculated clamp force, percentage of proof load, percentage of yield, and recommended torque range. Color-coded indicators warn if preload exceeds 90% of yield or falls below 50% of proof load.

  5. Export Bolting Procedure

    Generate a bolt-up sequence table showing pass-by-pass torque values (typically 30%, 60%, 100%, then final check pass) for controlled flange assembly per ASME PCC-1 guidelines.

Built For

  • Millwrights assembling flanged piping connections per ASME PCC-1 bolting procedures
  • Structural ironworkers verifying A325 and A490 bolt tension for steel connections
  • Maintenance mechanics torquing rotating equipment hold-down bolts during installations
  • Pressure vessel inspectors checking bolt preload adequacy for ASME Section VIII joints
  • Wind turbine technicians verifying tower bolt torque during annual inspections
  • Pipeline crews calculating tie-in flange bolt torque for API 6A and ASME B16.5 connections
  • Quality engineers developing torque specification sheets for production assembly

Features & Capabilities

K-Factor Short-Form Equation

Uses the industry-standard T = K × D × F formula for quick torque-to-tension conversion. Includes preset K-factors for common lubrication conditions and allows custom input for specialty coatings or thread compounds.

Multi-Standard Fastener Database

Built-in material properties for SAE, ASTM, and ISO metric fastener grades including proof strength, yield strength, tensile strength, and tensile stress area for each bolt size.

Proof Load and Yield Warnings

Real-time indicators show preload as a percentage of proof load and yield strength. Visual warnings flag under-torqued bolts (below 50% proof) and over-torqued bolts (above 90% yield) to prevent joint failure.

Multi-Pass Torque Sequence

Generates a pass-by-pass torque table following ASME PCC-1 recommendations: typically four passes at 30%, 60%, 100%, and a final verification pass. Includes cross-pattern bolt numbering for even load distribution.

Bidirectional Calculation

Enter torque to find preload, or enter a target preload to find the required torque. Useful for working backward from a gasket manufacturer's recommended seating stress to the wrench setting.

Comparison

Fastener Grade Proof Strength (ksi) Yield (ksi) Tensile (ksi) Typical K (Dry) Common Use
SAE Grade 5 85 92 120 0.20 General structural
SAE Grade 8 120 130 150 0.20 High-strength applications
ASTM A193 B7 105 105 125 0.20 Flanges, pressure vessels
ASTM A325 85 92 120 0.18 Structural steel connections
ASTM A490 120 130 150 0.18 Heavy structural steel
ISO 8.8 88 92 116 0.20 Metric general purpose
ISO 10.9 120 130 145 0.20 Metric high-strength

Frequently Asked Questions

The K-factor (also called the nut factor or torque coefficient) is a dimensionless number that accounts for all friction losses in the bolted joint. In the equation T = K × D × F, K typically ranges from 0.10 to 0.25. About 50% of applied torque is consumed by thread friction, 40% by bearing face friction, and only 10% actually generates clamp load. Lubrication reduces K, meaning less torque is needed for the same preload. The K-factor varies with surface finish, thread condition, hardness, and lubricant type.
For most industrial applications, target bolt preload is 60-75% of the bolt's proof load, which corresponds to roughly 50-65% of yield strength. ASME PCC-1 recommends a target of 50% of yield for controlled bolting with verified procedures. Going above 90% of yield risks permanent bolt stretch, while going below 40% of proof load risks joint loosening from vibration, thermal cycling, or gasket relaxation. Critical joints like reactor flanges may target higher preloads with controlled methods.
Approximately 85-90% of applied torque overcomes friction rather than generating bolt tension. Changing from dry steel (K = 0.20) to moly-based anti-seize (K = 0.12) reduces the torque required for the same preload by 40%. This means applying the same torque to a lubricated bolt produces 67% more preload than a dry bolt - enough to yield or break the fastener if the torque value is not adjusted. Always match the torque specification to the actual lubrication condition used.
Torque-controlled bolting (using a torque wrench) is the most common method but has an accuracy of roughly plus or minus 25% due to friction variability. Hydraulic bolt tensioning applies axial load directly, bypassing friction for plus or minus 5% accuracy, but requires specialized equipment and access to bolt ends. Other methods include turn-of-nut (counting rotation past snug-tight), stretch measurement (micrometer or ultrasonic), and load-indicating washers. Critical flanges often use tensioning or ultrasonics.
Start with the gasket manufacturer's recommended seating stress (psi), multiply by the gasket contact area to get total required bolt load, then divide by the number of bolts to get per-bolt preload. Add any hydrostatic end force from operating pressure. Use T = K × D × F to convert per-bolt preload to torque. Always verify that the calculated preload does not exceed 85-90% of the bolt's proof load. If it does, increase bolt size or quantity.
Calibrated click-type torque wrenches should be accurate to plus or minus 4% at the calibration point per ASME B107.300. However, the overall torque-tension accuracy is typically plus or minus 25% due to friction variability. For critical joints, use torque wrenches calibrated within the last 12 months and apply torque smoothly without jerking. Hydraulic torque wrenches offer better repeatability at plus or minus 3%. Digital torque wrenches provide data logging but similar friction-limited accuracy.
Standard pneumatic impact wrenches are not suitable for final torque because their impacting action does not produce consistent, measurable torque values. However, they are commonly used for run-down (snugging bolts before final torque). Calibrated hydraulic torque wrenches and electronic pulse tools with torque feedback can be used for final torque in production and structural bolting applications where they have been validated against the specified procedure.
ASME PCC-1 is the Guidelines for Pressure Boundary Bolted Flange Joint Assembly. It provides best practices for achieving leak-free flanged joints including bolt-up sequences, multi-pass torque procedures, gasket selection, and training requirements for bolting technicians. Many refineries and chemical plants now require PCC-1 compliance for all flanged connections. Following PCC-1 procedures reduces flange leaks by 80-90% compared to uncontrolled bolting practices.
Disclaimer: This calculator provides estimates based on the short-form K-factor equation. Actual bolt preload depends on friction conditions, thread quality, bolt condition, and operator technique. Always verify critical bolted joints against applicable codes (ASME PCC-1, AISC, API, etc.). ToolGrit is not responsible for joint integrity, safety, or compliance outcomes.

Learn More

Shops & Outbuildings

Bolt Torque vs Tension: K-Factor, Preload & Lubrication Effects

Understanding the relationship between applied torque and bolt tension. K-factor (nut factor), lubrication effects, preload targets, and why 90% of your torque is friction.

Read Guide →
Shops & Outbuildings

Flange Bolting: ASME B16.5 Patterns, Gaskets & Torque Sequences

ASME B16.5 flange bolt patterns by size and class. Raised face vs RTJ, gasket selection, bolt torque sequences (star pattern), and common flange leak causes.

Read Guide →

Related Tools

Shops & Outbuildings Live

Shop Heater BTU Sizing Calculator

Calculate the exact BTU output your shop or garage heater needs. Factors in wall R-values, ceiling insulation, slab edge loss, overhead door infiltration, and air changes per hour to size propane, natural gas, and electric heaters correctly.

Launch Tool →
Shops & Outbuildings Live

Overhead Door Infiltration Loss Calculator

Calculate heat loss through overhead doors in shops, garages, and warehouses. Compares open-door vs closed-door losses, seal condition impact, and annual cost of infiltration with payback on door seals and high-speed doors.

Launch Tool →
Shops & Outbuildings Live

Long-Run Voltage Drop Calculator

Calculate voltage drop for long wire runs to detached shops, barns, garages, and outbuildings. Compares copper vs aluminum, shows motor starting voltage impact, and recommends the right wire size for your distance and load.

Launch Tool →