Bolt Torque Calculator
Tightening torque and clamp load for M3 to M36, UNC and UNF bolts in class 8.8, 10.9, 12.9 and SAE Grade 5 or 8, from T = K x F x d with nut factor presets.
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About this toolWhat is the Bolt Torque Calculator?
Bolts hold joints together by stretching. The tension in a tightened bolt, called preload or clamp force, is what keeps the parts from separating or sliding, and the torque on the wrench is only an indirect way to get there. The short-form relation T = K x F x d links them: tightening torque equals a nut factor K times the preload F times the nominal diameter d. An M10 class 8.8 bolt loaded to 75% of its proof load (25.2 kN) with K = 0.2 needs about 50 N·m.
This calculator works in both directions. Choose a thread series (metric coarse, UNC or UNF), a size and a strength grade, pick the thread condition, and either set the target preload as a percentage of proof load to get the torque, or enter a torque to see the preload it produces. The result shows the tensile stress area, proof load, preload, bolt stress as a share of yield, and torque in N·m, lb·ft and lb·in.
Below the result is a torque chart for every size in the chosen series at the same grade, nut factor and preload target, the same kind of table printed by fastener suppliers, with the assumptions written on top.
- Metric coarse M3 to M36, UNC and UNF 1/4 to 1-1/2 inch
- ISO 898-1 classes 8.8, 10.9, 12.9 and SAE J429 Grades 5 and 8 with size-dependent proof stress
- Nut factor presets from Shigley Table 8-15, or a custom K
- Torque from target preload, or preload from a torque; percent of proof and yield shown
- Full torque chart for the series, copy and share link
How to use the Bolt Torque Calculator
- Pick the thread series: metric coarse, UNC or UNF. The grade list switches between ISO property classes and SAE grades.
- Choose the size and the property class or grade marked on the bolt head.
- Set the thread condition. Use dry, finish unknown (K = 0.20) if you are unsure, or enter a measured K.
- Choose Torque for a target preload and enter the percentage of proof load, or Preload from a torque and enter the wrench setting.
- Read the torque, preload and bolt stress, and use the chart below for other sizes at the same settings.
Worked examples
M10 class 8.8, dry (page default)
Input
M10 x 1.5, class 8.8, K = 0.20, 75% of proof
Output
Preload 25.2 kN; torque 50.5 N·m (37.2 lb·ft); bolt stress 68% of yield
Stress area 58.0 mm² x 580 MPa = 33.6 kN proof load. 75% of that is 25.2 kN, and 0.2 x 25,226 N x 0.010 m = 50.5 N·m.
1/2-13 Grade 5, dry
Input
1/2-13 UNC, Grade 5, K = 0.20, 75% of proof
Output
Preload 9,046 lbf; torque 75.4 lb·ft
Stress area 0.1419 in² x 85 ksi = 12,061 lbf proof. This matches the 75 lb·ft that appears in most Grade 5 torque charts for dry 1/2-13 bolts.
Same torque, different lubricant
Input
M10 class 8.8 at 50 N·m: K = 0.20 vs anti-seize K = 0.12
Output
25.0 kN (74% of proof) vs 41.7 kN (124% of proof)
A dry-thread torque value applied to a bolt coated in anti-seize stretches it past proof load. Always match the torque to the lubrication actually used.
M12 class 10.9, oiled
Input
M12 x 1.75, class 10.9, custom K = 0.15, 75% of proof
Output
Preload 52.5 kN; torque 94.4 N·m
With K = 0.20 the same preload would need 125.9 N·m. The lubricant lowers the torque, not the clamp force you are aiming for.
Background and insight
Proof load is the tensile force a bolt must carry without permanent set, and it is the usual basis for preload. It is the tensile stress area times the proof stress. ISO 898-1 gives proof stresses of 580 MPa for class 8.8 up to M16 (600 MPa above), 830 MPa for 10.9 and 970 MPa for 12.9. SAE J429 gives 85 ksi for Grade 5 from 1/4 to 1 inch (74 ksi from over 1 to 1-1/2 inch) and 120 ksi for Grade 8. Stress areas come from the thread geometry: As = π/4 (d − 0.9382P)² for ISO metric threads and At = 0.7854 (D − 0.9743/n)² for Unified threads in ASME B1.1, which reproduce the tabulated 58.0 mm² for M10 and 0.1419 in² for 1/2-13.
The nut factor K is not a friction coefficient; it lumps thread friction, underhead friction and thread geometry into one empirical number. Shigley's Mechanical Engineering Design (Table 8-15, from Bowman's Fastener Facts) lists 0.30 for non-plated black finish, 0.20 for zinc-plated, 0.18 for lubricated, 0.16 for cadmium-plated and 0.12 with anti-seize compound, and suggests K = 0.20 when the condition is unknown. Shigley recommends preload of 75% of proof load for connections that will be reused and 90% for permanent ones.
Because friction varies, torque is an imprecise way to set preload. Two bolts tightened to the same torque can differ in clamp force by 25% or more. Where that matters, engineers use turn-of-nut, direct tension indicators, ultrasonic measurement or torque-angle methods, and VDI 2230 for a full joint analysis.
Common use cases
Machine assembly
Set wrench values for flanges, brackets and covers when the drawing gives a bolt grade but no torque.
Automotive and equipment repair
Check whether a torque from a manual is in a sensible range for the bolt size and grade fitted.
Writing a torque table
Generate a chart for a whole thread series at the lubrication and preload your shop actually uses.
Diagnosing loose or broken bolts
Back out the preload a given torque produced to see whether a joint was under- or over-tightened.
Tips and limitations
- K changes more than any other input. Plating, oil, thread locker and even the washer face shift it, so use the condition you actually assemble with.
- Thread-locking compounds often act as a lubricant while wet. Check the maker's K or torque adjustment before using dry-thread values.
- Do not reuse torque-to-yield bolts such as many cylinder head bolts. Their procedure deliberately takes them past yield.
- Soft joints with gaskets relax after tightening. Retorque per the gasket maker's instructions instead of raising the initial torque.
- Stainless and aluminium fasteners have different strengths and gall easily; this calculator covers carbon and alloy steel grades only.
Frequently asked questions
What is the formula for bolt tightening torque?
T = K x F x d, where K is the nut factor (about 0.2 for dry steel), F the target preload and d the nominal bolt diameter. With F in newtons and d in metres, T is in N·m.
What K factor should I use for bolt torque?
Shigley lists 0.30 for plain black-oxide steel, 0.20 for zinc-plated, 0.18 for lubricated, 0.16 for cadmium-plated and 0.12 with anti-seize. Use 0.20 if the condition is unknown, and a measured value for critical joints.
What preload should a bolt have?
A common target is 75% of proof load for joints that will be taken apart and 90% for permanent ones (Shigley). Proof load is the stress area times the proof stress of the grade, for example 580 MPa for class 8.8 up to M16.
What is the torque for an M12 8.8 bolt?
About 88 N·m (65 lb·ft) dry, with K = 0.20 at 75% of proof load: 84.3 mm² x 580 MPa x 0.75 = 36.7 kN, and 0.2 x 36,660 x 0.012 = 88.0 N·m. Lubricated threads need less.
Should bolts be torqued dry or lubricated?
Use whatever the torque value was written for. Lubricated threads reach the same preload at roughly 10 to 40% less torque, so applying a dry value to oiled bolts can overload them, and a lubricated value on dry threads leaves them loose.
What is the difference between proof load and yield strength?
Proof load is the force a bolt must survive without measurable permanent stretch in the standard test. It sits a little below the 0.2% yield strength: for class 8.8 the proof stress is 580 MPa and the minimum yield 640 MPa.
How much does torque vary the actual clamp force?
With an accurate wrench and controlled lubrication, preload still typically scatters by 25% or more because thread and underhead friction vary. That is why critical joints use turn-of-nut, tension indicators or ultrasonic measurement.
Is there an in-depth guide to the Bolt Torque Calculator?
Yes. “Bolt Tightening Torque and Preload: T = KFd in Practice” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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