Bolt Torque Calculator
Enter your bolt diameter, bolt grade, desired clamping load, and surface condition to get the correct tightening torque instantly. Switch to reverse mode to find the clamping force produced by a known torque. Supports metric (ISO grades) and imperial (SAE grades) bolts, plus a custom K-factor option for any lubricant or plating.
What is bolt torque and why does it matter?
When you tighten a bolt, you are not really holding parts together with the bolt itself: you are stretching the bolt shaft slightly so that it acts like a compressed spring, clamping the joint with a large axial force called the preload or clamping force. Torque is simply the rotational force applied to the wrench; the torque-to-preload relationship is governed by the friction in the thread and under the bolt head. Under-torquing leaves a joint that can loosen under vibration. Over-torquing can stretch the bolt beyond its proof load, strip threads, or crack the clamped material. Getting the torque right is one of the most common and most consequential tasks in mechanical assembly.
The bolt torque formula explained
The standard short-form formula is T = K x F x d. T is the tightening torque (N-m or ft-lbf). F is the target clamping force (N or lbf). d is the nominal bolt diameter (m or in). K is the nut factor or torque coefficient, a dimensionless number that captures how much of the applied torque is consumed by friction rather than converted to clamping force. Roughly 40-50% of torque goes to thread friction, 40-50% to under-head friction, and only 10% actually stretches the bolt to create the preload. Because K is so dominant, the same bolt tightened with dry steel threads and then lubricated threads can differ in clamping force by 25-40% even if the torque reading on the wrench is identical.
Choosing the right K-factor
K-factor values are empirically measured and vary with bolt material, plating, thread form, lubricant, and surface roughness. Common values: dry clean steel 0.20, zinc-plated 0.20, black-oxide finish 0.17, cadmium plating 0.16, light oil or SAE 30-40 0.15, dry-film wax lubricant 0.13, copper-based anti-seize 0.12, molybdenum disulfide (MoS2) 0.10. For critical joints (engine heads, flanges, pressure vessels) you should obtain the K-factor from your fastener supplier or measure it on a torque-preload tester rather than using a tabulated estimate. The custom K option in this calculator lets you enter a lab-measured value.
Metric vs. SAE bolt grades and proof load
Bolt grades define the tensile and proof strength of the bolt material. For metric bolts, ISO 898-1 property classes (4.6, 8.8, 10.9, 12.9) are the standard: the first digit is 1/10 of the nominal tensile strength in MPa, and the second digit times the first gives the nominal yield ratio. For inch bolts, SAE grades (2, 5, 8) and ASTM grades (A307, A325, A490) are used. The proof load is the maximum tensile force a bolt can sustain without permanent deformation; it equals the proof strength times the tensile stress area. Best practice is to target 75-80% of the proof load for general-purpose joints, giving a safety margin while maintaining adequate preload.
Metric bolt recommended torque - dry conditions (K = 0.20, 75% proof load)
| Size | Grade | Condition | Torque (N-m) | Torque (ft-lbf) |
|---|---|---|---|---|
| M4 | 4.8 | Dry | 4.2 | 3.1 |
| M5 | 4.8 | Dry | 8.5 | 6.3 |
| M6 | 4.8 | Dry | 14.2 | 10.5 |
| M8 | 4.8 | Dry | 34.5 | 25.5 |
| M10 | 8.8 | Dry | 76 | 56 |
| M12 | 8.8 | Dry | 131 | 97 |
| M16 | 8.8 | Dry | 317 | 234 |
| M20 | 8.8 | Dry | 619 | 456 |
| M24 | 8.8 | Dry | 1063 | 784 |
| M30 | 10.9 | Dry | 2665 | 1966 |
Approximate tightening torques for ISO metric bolts assembled dry, targeting 75% of the bolt proof load. Values are for guidance only. Consult engineering specifications for structural or safety-critical applications.
Frequently asked questions
What is a K-factor (nut factor) and how do I pick one?
The K-factor is a dimensionless coefficient that relates the applied torque to the resulting clamping force via T = K x F x d. It lumps together thread friction, under-head friction, and thread pitch lead. A typical value is 0.20 for dry clean steel. Use 0.15 for oiled threads, 0.12 for anti-seize, and 0.10 for molybdenum disulfide grease. For safety-critical joints, measure K on a torque-preload test rig rather than relying on a table.
How does lubrication affect the torque I should use?
Lubrication lowers the K-factor, which means that for the same clamping force you need less torque - but also that if you apply the same torque you used for dry bolts to lubricated bolts, you will over-stretch or even break the bolt. Always recalculate torque when you change lubricants or surface treatments. Switching from dry (K=0.20) to anti-seize (K=0.12) reduces the required torque by about 40%.
What percentage of bolt proof load should I target?
For most structural and general-purpose bolted joints, 70-80% of the bolt proof load is recommended, with 75% being the most common standard. A lower value leaves the joint susceptible to self-loosening under vibration. A higher value (up to 90%) is used for torque-to-yield fasteners (TTY) designed to be tightened into the plastic range, which provides very consistent preload but means the bolt should not be reused.
What is the difference between metric grades and SAE grades?
ISO (metric) grades like 8.8 and 10.9 encode the mechanical properties: the first number (times 100) gives the nominal tensile strength in MPa, so grade 10.9 has 1000 MPa tensile strength. SAE grades like Grade 5 and Grade 8 are defined by SAE J429 and are used for inch-series bolts; Grade 8 has about 150,000 psi (1034 MPa) tensile strength. There is no exact cross-mapping: Grade 5 is roughly equivalent to 8.8, and Grade 8 is roughly equivalent to 10.9, but specification details differ.
Can I use these torque values for engine head bolts or structural steel?
No. Engine head bolts, exhaust manifold studs, connecting rod bolts, structural steel connections (AISC), and pressure-vessel flanges all have manufacturer-specific torque procedures - often involving torque-to-yield, angle-torque sequences, or special lubricants. Using generic K-factor calculations for these applications can cause joint failure. Always follow OEM or engineering specifications for safety-critical fasteners.
What is the reverse mode for?
Reverse mode answers "I applied this torque value - what clamping force did I actually achieve?" This is useful when you are inspecting an existing joint, auditing assembly records, or working backwards from a torque wrench reading. Enter the measured torque and the surface condition, and the calculator returns the resulting clamping force and bolt stress.
How accurate is the T = K x F x d formula?
The short-form formula is accurate to about plus or minus 25% for general-purpose joints, which is sufficient for most assembly work. The main uncertainty comes from K-factor variation due to surface condition, plating consistency, thread quality, and operator speed. For precision joints, use a torque-preload tester to measure K on actual production fasteners, or switch to direct preload methods such as bolt elongation measurement or ultrasonic bolt stress.