liminfoBolt Torque Calculator
Free web tool: Bolt Torque Calculator
Typical: 0.12-0.20
Typically 70-80% of proof
Details
Formula
About Bolt Torque Calculator
The Bolt Torque Calculator computes the recommended tightening torque for metric bolts using the nut-factor (K-factor) method: T = K × D × F. Here, T is the tightening torque in Newton-meters, K is the nut factor (friction coefficient, typically 0.12–0.20 for unlubricated steel), D is the nominal bolt diameter in meters, and F is the bolt preload in Newtons. This formula is widely used in mechanical engineering, structural fastening, and assembly specification documentation.
Mechanics, structural engineers, equipment assemblers, and maintenance technicians use bolt torque calculations to ensure proper joint clamping force without over-stressing the fastener. Under-torquing leads to joint loosening, vibration failure, and leaks. Over-torquing can strip threads, plastically deform the bolt shank, or crack flanges. This calculator covers 14 standard metric bolt sizes from M6 to M36 and five common ISO strength grades: 4.6, 5.8, 8.8, 10.9, and 12.9. Each grade has a defined proof stress, tensile strength, and yield strength. Preload is computed as proof load × preload percentage, where 70–80% of proof load is the industry-standard target.
The tool calculates proof load (proof stress × stress area), target preload (proof load × preload%), tightening torque using T = K × D × F, and clamping force. Results are shown in both N·m and ft·lb for compatibility with metric and imperial torque wrenches. A detailed breakdown panel shows stress area (mm²), proof stress (MPa), proof load (kN), preload (kN), friction factor, and the full formula substitution. All computation is done client-side with no data leaving your browser.
Key Features
- Covers M6 through M36 metric bolt sizes with accurate stress area data
- Supports ISO strength grades 4.6, 5.8, 8.8, 10.9, and 12.9
- Calculates tightening torque using the industry-standard T = K × D × F formula
- Adjustable friction coefficient (K) for dry, lubricated, or coated conditions
- Adjustable preload percentage (typically 70–80% of proof load)
- Dual-unit output: torque in N·m and ft·lb simultaneously
- Detailed breakdown: stress area, proof stress, proof load, preload, clamping force
- Full formula substitution displayed for verification and documentation
Frequently Asked Questions
What is bolt torque and why does it matter?
Bolt torque is the rotational force applied to tighten a fastener. Proper torque creates a specific clamping force (preload) that holds joint members together. Too little torque causes loosening and fatigue failure; too much can yield the bolt, strip threads, or damage the joint. Correct torque is critical in automotive, structural, pressure vessel, and machinery applications.
What is the T = K × D × F formula?
This is the simplified torque-tension relationship: T (torque in N·m) = K (nut factor) × D (nominal diameter in meters) × F (preload in Newtons). The nut factor K combines friction effects at the bolt-head bearing face and the thread helix. It is approximately equal to the friction coefficient for most practical cases.
What is the nut factor (K) and what values should I use?
The nut factor K accounts for friction in both the thread and under the bolt head/nut face. Typical values: 0.10–0.12 for lightly lubricated or waxed bolts, 0.12–0.15 for zinc-plated bolts, 0.15–0.18 for plain steel (as-received), and 0.18–0.22 for dry or oxidized surfaces. The default 0.15 is a conservative value for clean, unlubricated steel.
What are ISO bolt strength grades 8.8, 10.9, and 12.9?
ISO metric bolt strength grades are denoted by two numbers separated by a decimal. The first number × 100 gives the minimum tensile strength in MPa; the first number × the second number × 10 gives the yield strength. Grade 8.8 has 800 MPa tensile and 640 MPa yield. Grade 10.9 has 1040 MPa tensile and 940 MPa yield. Grade 12.9 has 1220 MPa tensile and 1100 MPa yield — the highest common structural grade.
What is bolt preload and why is 75% of proof load used?
Preload is the tension force induced in the bolt shank when it is tightened. It is what creates clamping force. Targeting 75% of proof load is a common engineering practice that ensures adequate clamping force while leaving a safety margin below the proof strength. The proof load is the maximum load the bolt can sustain without permanent deformation.
What is stress area and how is it different from nominal diameter?
The stress area (At) is the effective cross-sectional area of the bolt shank that carries tensile load. It accounts for the thread root geometry and is smaller than the nominal cross-sectional area. For example, M12 has a nominal area of about 113 mm² but a stress area of 84.3 mm². Stress area is used in all fastener strength calculations.
What torque should I use for M12 grade 8.8 bolts?
With the default settings (K = 0.15, 75% preload), an M12 grade 8.8 bolt has a proof load of approximately 48.9 kN, a preload target of about 36.7 kN, and a recommended torque of approximately 66 N·m (48.6 ft·lb). Actual specifications may vary — always verify with your engineering documentation.
Does lubrication affect the required torque?
Yes, significantly. Lubricating a bolt reduces the friction coefficient (K), which means less torque is needed to achieve the same preload. If you apply anti-seize or molybdenum disulfide grease (K ≈ 0.10–0.12), you must reduce the applied torque accordingly — applying the dry torque value to a lubricated bolt can easily over-stress the fastener by 25–40%.