Bolt Torque & Preload Calculator
Work out the tightening torque for a bolt, or the clamping force a torque gives. Metric ISO and UNC/UNF threads, property classes 4.6 to 12.9 and SAE Grades 5 and 8, the tensile stress area, the proof load and a torque table for every common size.
How to Use
- Choose what to work out: the Torque to tighten a bolt to a preload, or the Preload (clamping force) a known torque gives.
- Pick the thread system (metric, UNC coarse or UNF fine) and the size; for a metric fine thread type its pitch, otherwise the ISO coarse pitch is used.
- Pick the strength: a metric property class (4.6, 8.8, 10.9, 12.9) or SAE Grade 5 or 8.
- Pick the nut factor K for the finish and lubrication (0.20 for a dry or zinc-plated bolt), then the preload as a percentage of the proof load (75% is usual) or the torque.
- Read the torque in N·m and lbf·ft, the preload and the proof load; the table under the drawing gives the torque for every common size at the same settings.
- Press a preset to load an M10 or M12 bolt, a ½ in Grade 5 or ⅜ in Grade 8 bolt, or a torque to check.
Worked Example
An M10 class 8.8 bolt. The ISO coarse pitch of M10 is 1.5 mm, so the tensile stress area is π/4 × (10 − 0.9382 × 1.5)² = 57.99 mm². Class 8.8 has a proof stress of 580 MPa up to M16, so the proof load is 580 × 57.99 = 33.63 kN, and 75% of that is a preload of 25.23 kN. Dry, with K = 0.20, the torque is 0.20 × 0.010 m × 25,225 N = 50.45 N·m (37.21 lbf·ft).
A ½ in Grade 5 bolt. ½-13 UNC has A_t = π/4 × (0.5 − 0.9743 ÷ 13)² = 0.1419 in². At Grade 5’s 85 ksi proof stress the proof load is 12,061 lbf; 75% is 9,046 lbf, and T = 0.20 × 0.5 in × 9,046 lbf = 904.6 lbf·in = 75.38 lbf·ft.
The common mistake: a dry torque on a lubricated bolt. Most torque charts assume a dry bolt. Put 50.45 N·m on the same M10 with anti-seize on its threads (K about 0.12) and the preload is 50.45 ÷ (0.12 × 0.010) = 42.04 kN instead of 25.23 kN: 125% of the proof load and past the bolt’s 37.11 kN minimum yield load, so it stretches permanently or snaps. Lubricant means less torque, not the same.
Show Work
Formulas
From Whitworth’s Threads to Property Classes
Until the 1840s every workshop cut its own screw threads, and a nut from one maker rarely fitted a bolt from another. In 1841 Joseph Whitworth proposed a standard thread with a 55° flank angle and a fixed number of threads per inch for each diameter, and British industry adopted it. In 1864 William Sellers proposed a simpler 60° thread in the United States, and in 1948 the United States, Britain and Canada agreed the Unified thread (UNC and UNF) so that wartime equipment could share fasteners. The ISO metric thread, also 60°, is now the standard almost everywhere else.
Strength grades followed: SAE J429 marks inch bolts with radial lines on the head (three for Grade 5, six for Grade 8), and ISO 898-1 stamps metric bolts with a class such as 8.8, where the first number is a hundredth of the tensile strength in MPa and the second is ten times the ratio of yield to tensile strength.
Torque is only an indirect way to reach a preload. Most of the turning effort is lost to friction under the head and in the threads, which is why the nut factor matters so much and why critical joints use load-indicating washers, ultrasonic measurement or tightening by angle instead.
About This Tool
This calculator finds the tightening torque for a target preload, or the preload a torque produces, from the short-form equation T = K·D·F. The tensile stress area is calculated from the nominal diameter and pitch (ISO 261 coarse pitches, or ASME B1.1 threads per inch), and the proof, yield and tensile strengths are the minimums in ISO 898-1 (class 8.8: proof stress 580 MPa up to 16 mm and 600 MPa above) and SAE J429 (Grade 5: 85 ksi proof, ¼ to 1 in; Grade 8: 120 ksi). The nut factors are typical values after Shigley’s Mechanical Engineering Design.
The nut factor is the weak point: two bolts tightened to the same torque can differ in preload by a quarter or more. Where a manufacturer gives a torque, use theirs. Everything runs in your browser; nothing you enter is sent anywhere.
Related tools: Torque Calculator, Stress, Strain & Young’s Modulus Calculator, and Pressure Vessel Hoop Stress Calculator.
Frequently Asked Questions
How do you calculate bolt torque?
Use T = K × D × F: nut factor times nominal diameter times preload. An M10 class 8.8 bolt has a proof load of 580 MPa × 57.99 mm² = 33.63 kN; at 75% that is 25.23 kN, so with K = 0.20 the torque is 0.20 × 0.010 m × 25,225 N = 50.45 N·m, or 37.21 lbf·ft.
What is the tensile stress area of a bolt?
It is the effective area of the threaded part, between the root and pitch diameters. For metric threads A_t = π/4 × (d − 0.9382p)², so M10 × 1.5 gives 57.99 mm² and M12 × 1.75 gives 84.27 mm². For inch threads A_t = π/4 × (d − 0.9743/n)², so ½-13 UNC gives 0.1419 in².
What nut factor K should I use?
Around 0.20 for a plain dry or zinc-plated steel bolt, about 0.18 lubricated with oil, 0.16 cadmium-plated, 0.12 with anti-seize compound and 0.30 for black-oxide finish (typical values from Shigley’s). The same 50.45 N·m on an M10 gives 25.23 kN at K = 0.20 but 42.04 kN at K = 0.12.
How much clamping force does a torque give?
Turn the formula round: F = T ÷ (K × D). 200 N·m on a dry M16 class 8.8 bolt gives 200 ÷ (0.20 × 0.016) = 62.5 kN, which is 68.8% of its 90.87 kN proof load and a bolt stress of 398.9 MPa.
Why tighten to 75% of the proof load?
A high preload keeps the joint clamped under varying load, which protects the bolt from fatigue and stops it working loose, while staying clear of the proof load leaves room for scatter in the nut factor. Shigley’s recommends 75% for reusable joints and 90% for permanent ones. A ½-13 UNC Grade 5 bolt at 75% carries 9,046 lbf and needs 75.38 lbf·ft dry.
How do I use the Bolt Torque & Preload Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
Common Use Cases
Machinery assembly
An M12 class 10.9 bolt, oiled (K = 0.18), at 75% of its 69.94 kN proof load needs 113.3 N·m for 52.46 kN of clamp.
Automotive and equipment
A ⅜-24 UNF Grade 8 bolt, lubricated, at 75% of proof needs 44.46 lbf·ft (60.28 N·m) for 7,905 lbf of preload.
Structural and agricultural
A ½-13 UNC Grade 5 bolt, dry, at 75% of its 12,061 lbf proof load needs 75.38 lbf·ft.
Checking a manual figure
120 N·m on an M14 × 1.5 fine-thread class 10.9 stud, dry, gives 42.86 kN, which is only 41.5% of its 103.4 kN proof load.
Torque charts
The size table gives every metric size from M6 (10.5 N·m) to M24 (761.4 N·m) for class 8.8 at 75% of proof and K = 0.20.
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