FASTENER ENGINEERING TOOL
Bolt Torque Calculator
Calculate metric and SAE bolt tightening torque using T = K × D × F. Get torque in Nm and ft-lb, with target preload and K-factor included in the result.
SIMPLE AND ACCURATE
How to Use the Bolt Torque Calculator
Enter five inputs and the calculator returns everything else.
Bolt Size and
Thread Series
Metric coarse or fine (M8, M10, M12), or imperial UNC/UNF (1/2-13, 3/4-10).
Property Class
or Grade
ISO 4.6, 8.8, 10.9 or 12.9, or SAE Grade 2, 5 or 8. This sets the proof load.
Surface and
Lubrication Condition
Dry, zinc-plated, oiled, anti-seize or heavily galvanized. This sets the K-factor.
Preload
Target
Set the percentage of proof load you intend to reach. 75% is a common default.
Units
Choose Nm, ft-lb or in-lb. The other units remain visible alongside the result.
BOLT TORQUE CALCULATOR
Calculate Torque or Clamp Load
Use the standard torque-tension relationship T = K × D × F to estimate tightening torque or reverse the calculation to estimate preload.
INPUT PARAMETERS
Joint Conditions
mm
Example: M12 = 12 mm
K
Friction assumption for the joint
kN
Enter the target clamp load for the joint.
Nm
Enter the torque applied to the fastener.
CALCULATED RESULT
Live Result
Tightening Torque
108.0
Nm
Estimated torque required to produce the entered target preload.
Torque
108.0 Nm
Torque
79.7 ft-lb
Torque
956.0 in-lb
Clamp Load
45.0 kN
EQUATION
T = K × D × F
0.20 × 0.012 m × 45,000 N
K
0.20
Nut factor
D
12 mm
Diameter
F
45.0 kN
Clamp load
K is an estimate, not a constant.
Surface finish, plating, thread condition and lubrication can materially change friction and therefore the resulting clamp load.
FRICTION REFERENCE
Typical Nut Factor (K)
Reference values only. Actual friction should be established from the specified fastener, finish, lubricant and assembly process.
Well Lubricated
0.10–0.12
Anti-Seize
0.12
Lightly Oiled
0.15–0.18
Dry / Zinc-Plated
0.20
Black Finish
0.30
Heavy Galvanized
0.35+
SAE TORQUE REFERENCE
Bolt Torque Chart: Metric and SAE
FORMULA
T = K × D × F
SAE Grade 5 and Grade 8 values below use K = 0.20 for dry or zinc-plated fasteners and a target preload of 75% of proof load.
Nut Factor
0.20
Dry / zinc-plated
Target Preload
75%
Of proof load
Grade 5 Proof
85,000
psi
Grade 8 Proof
120,000
psi
| Bolt Size (UNC) | Stress Area (in²) | Grade 5 Torque | Grade 8 Torque |
|---|---|---|---|
| 1/4-20 | 0.0318 | 8.4 ft-lb 11.5 Nm | 11.9 ft-lb 16.2 Nm |
| 5/16-18 | 0.0523 | 17.4 ft-lb 23.5 Nm | 24.5 ft-lb 33.2 Nm |
| 3/8-16 | 0.0775 | 30.9 ft-lb 41.9 Nm | 43.6 ft-lb 59.1 Nm |
| 7/16-14 | 0.1063 | 49.4 ft-lb 67.0 Nm | 69.8 ft-lb 94.6 Nm |
| 1/2-13 | 0.1419 | 75.4 ft-lb 102.2 Nm | 106.4 ft-lb 144.3 Nm |
| 5/8-11 | 0.2260 | 150.1 ft-lb 203.5 Nm | 211.9 ft-lb 287.3 Nm |
| 3/4-10 | 0.3340 | 266.2 ft-lb 360.9 Nm | 375.8 ft-lb 509.5 Nm |
!
Calculated reference values — not a manufacturer specification. These values assume dry threads, clean surfaces and a steel-to-steel joint. For safety-critical or structural applications, use the fastener or equipment manufacturer’s specified value.
FRICTION MATTERS
Why Published Torque Tables Disagree
Different torque tables can give different answers for the same bolt because their friction assumptions differ. An M10 Class 8.8 bolt shows the effect clearly.
EXAMPLE FASTENER
M10 · Class 8.8
Proof strength
640 MPa
Tensile stress area
58.0 mm²
Proof load
37,120 N
Target preload
27,840 N
SAME BOLT · SAME PRELOAD
Dry
K = 0.20
55.7
Nm
VS
Lightly Oiled
K = 0.15
41.8
Nm
25%
difference in tightening torque
from lubrication alone
A torque figure without its K-factor and preload
assumption is incomplete.
Friction can change the result by substantially more than the stated accuracy of a typical calibrated torque wrench.
FASTENER STRENGTH
ISO 898-1 and SAE J429 Strength Values
These values provide the strength basis used when establishing a preload target for manual torque calculations.
METRIC
ISO 898-1
| Class | Tensile Strength | Yield / Proof |
|---|---|---|
| 4.6 | 400 MPa | 240 MPa |
| 8.8 | 800 MPa | 640 MPa |
| 10.9 | 1040 MPa | 940 MPa |
| 12.9 | 1220 MPa | 1100 MPa |
IMPERIAL
SAE J429
| Grade | Minimum Proof Strength |
|---|---|
| Grade 2 | 55,000 psi |
| Grade 5 | 85,000 psi |
| Grade 8 | 120,000 psi |
Confirm the current edition of the applicable standard
before using these values for design or purchase
specifications. Stainless fastener grades are governed
separately.
CALCULATION LIMITS
The Error Budget
±4%
Typical click-wrench
deviation above 10 Nm
The friction assumption inside a torque calculation can introduce substantially more uncertainty than the torque wrench used to apply the result.
Torque by Hand
±35%
Torque Wrench
±25%
Turn-of-the-Nut
±15%
Load-Indicating Washer
±10%
Bolt Elongation
±3–5%
Strain Gauge
±1%
K
Friction is often the dominant uncertainty.
A ±4% wrench does not create a ±4% joint when the K-factor itself may be wrong by 20–25%. Confirming lubrication, coating and thread condition can be more valuable than improving wrench accuracy alone.
01
Tighten the Assumption
Confirm finish and lubrication or use torque-tension testing for critical work.
02
Calibrate Critical Joints
Reused fasteners, damaged threads, washers and temperature can all shift friction.
03
Respect Significant Figures
A calculated 55.7 Nm is a target based on assumptions, not an exact physical outcome.
STRUCTURAL FASTENING
Torque Is Not the Preferred Method for Every Joint
Structural bolting may use installation methods that control tension more directly than a calculated torque value.
01
Turn-of-the-Nut
Direct installation method
02
DTI Washers
Direct tension indication
03
Tension-Control Bolts
Twist-off installation
04
Calibrated Wrench
Torque-based installation method
!
Structural Connection
For structural connections, follow the project specification and the applicable structural bolting standard. A general-purpose torque calculator should not replace the specified installation procedure.
TORQUE TOOL SELECTION
From Torque Value to Torque Tool
REFERENCE
ISO 6789
A calculated torque value is useful only when the tool applying it has the appropriate type, working range, accuracy and documentation for the application.
| ISO 6789 Type | Behaviour | Typical Permitted Deviation |
|---|---|---|
| Type I Indicating | Shows achieved torque | Around ±6% |
| Type II Setting · above 10 Nm | Signals at preset torque | Around ±4% |
| Digital Indicating tools | Electronic torque display | Confirm declared accuracy |
01
Type & Class
Type I indicates achieved torque. Type II signals when a preset torque is reached. Required accuracy depends on the specific tool class.
02
Working Range
Accuracy is generally best through the useful middle portion of a wrench’s range. Avoid choosing one oversized wrench for every torque requirement.
03
Calibration Interval
A common baseline is 12 months or approximately 5,000 cycles, with shorter intervals for heavy-use or safety-critical service.
04
Documentation
Determine whether the quality system requires a Declaration of Conformance or a calibration certificate with measurement uncertainty.
PRACTICAL RANGE SELECTION
~20–80%
Keep the Target Inside the Useful Range
0%
20%
80%
100%
For example, using a 200 Nm wrench at only 25 Nm places the fastener close to the bottom of the tool’s range, where relative error becomes more important.
ISO 6789-1
Declaration of Conformance
Suitable where the application requires confirmation that the torque tool conforms to the applicable product requirements.
ISO 6789-2
Calibration Certificate
Used where measurement uncertainty and stronger calibration traceability are required by the customer’s quality system.
VERIFICATION NOTE
Confirm the exact tool, certificate and applicable
standard before publishing a compliance claim.
ISO 6789 applicability is product- and market-specific. Verify the exact SKU, declared accuracy, documentation and current edition required by the destination market.
Frequently Asked Questions (FAQs)
Find answers to common questions about our hand tools, their care, and usage. If you need additional information, our customer support team is ready to assist you.
What is the formula for bolt torque?
Bolt torque is T = K × D × F — the nut factor multiplied by nominal bolt diameter and target preload. Preload is normally set at 70 to 75 percent of the bolt’s proof load. An extended form, T = K × F × d × (1 − l/100), adds an explicit lubrication percentage.
What K factor should I use for bolt torque?
Use K = 0.20 for dry, as-received or zinc-plated steel, 0.15 to 0.18 for lightly oiled threads, 0.12 with anti-seize, and 0.30 or higher for a non-plated black finish or heavily galvanized and rusty conditions. Treat these as starting points and confirm against a torque-tension test for any critical joint, because the K range is wide enough to change the answer by a third.
How do I convert bolt torque to clamping force?
Rearrange the equation to F = T / (K × D). Divide the applied torque by the nut factor and the nominal diameter. Expect the result to be much smaller than the input suggests, because only about 10 percent of applied torque becomes clamp load — the rest is lost to friction under the head and in the threads.
Does lubrication change bolt torque?
Yes, substantially — more than any other single variable. Heavily lubricating a bolt can roughly halve the torque needed for a given preload, or conversely double the preload produced by the same torque. Over-lubricated fasteners torqued to a dry specification is a common and genuine cause of bolt failure.
How accurate is a bolt torque calculator?
A torque calculation typically carries ±25–30% uncertainty in the resulting preload, driven almost entirely by the friction assumption. A torque wrench under ISO 6789 is usually accurate to about ±4% to ±6%. The calculator and the friction model — not the wrench — dominate the error, so verifying the lubrication condition matters more than buying a more accurate tool.
What is the difference between proof load and yield strength?
Proof load is the maximum tensile load a fastener sustains without measurable permanent set, typically 85 to 92 percent of yield load. It is used as the design reference because it keeps the bolt elastic. Yield strength is the stress at which the material begins to deform permanently. Tightening to 70 to 75 percent of proof load leaves margin for the uncertainty in torque-to-preload conversion.