Bolt Torque & Preload Calculator
Torque for a target preload — or preload from a torque — with every formula shown.
Where the torque goes (VDI 2230)
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How it was calculated
Torque wrench extension
For a crowfoot or adapter that lengthens the wrench. Measure both lengths in the same unit.
Every size in this series
| Size | Torque | Preload, kN | Proof load, kN | As, mm² |
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Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Bolt Torque & Preload Calculator
Choose a metric (coarse or fine) or UNC/UNF thread and its property class or SAE grade, and the calculator works out the tensile stress area, the proof load and the preload you are aiming for — 75% of the proof load for reusable joints, 90% for permanent ones — and the torque that produces it. Or enter a torque to see the preload it gives.
Use the simple nut-factor method (T = K × F × d, with Shigley’s K values for the bolt finish) or the VDI 2230 formula with separate thread and head friction, which also shows where the torque goes and whether the bolt stays below yield while it is tightened. A third section corrects the torque-wrench setting for a crowfoot or other extension.
How to use it
- Pick the thread series and size, then the property class (ISO 898-1, such as 8.8 or 10.9), a stainless class (A2-70 …) or the SAE grade for inch bolts.
- Choose to solve for the torque (enter the preload as a % of the proof load) or for the preload (enter a torque).
- Pick the method. Nut factor: choose the finish or type K (0.2 when it is unknown). VDI 2230: enter the thread and head friction coefficients (0.12 is typical for lightly oiled steel) and check the head and hole diameters.
- Read the torque in N·m, lbf·ft and lbf·in, with the working. The table lists the same calculation for every size in the series.
- If you use a crowfoot or adapter, enter the wrench length and extension under Torque wrench extension to get the setting to dial in.
Examples
A_s = 58.0 mm², proof stress 580 MPa, 75% preload, K = 0.2
Proof load 33.6 kN → preload 25.2 kN → T = 0.2 × 25.2 kN × 10 mm = 50.5 N·m
A_s = 0.1419 in², proof 85 ksi, 75%, K = 0.2
Preload 9,046 lbf → T = 0.2 × 9,046 × 0.5 = 905 lbf·in (75.4 lbf·ft)
25.2 kN preload, μ_G = μ_K = 0.12, hex head 16 mm, 11 mm hole
M_A = 42.3 N·m: 14% stretches the bolt, 37% is thread friction, 48% head friction; equivalent stress 83% of yield
300 mm wrench, 50 mm extension in line, 100 N·m needed
Set 100 × 300 ÷ 350 = 85.7 N·m
The formulas
- Tensile stress area: A_s = π/4 × ((d2 + d3) ÷ 2)² for ISO threads (ISO 898-1), A_s = 0.7854 × (D − 0.9743 ÷ n)² for UNC/UNF
- Proof load: F_p = A_s × S_p; target preload F_i = 0.75 F_p (reusable) or 0.90 F_p (permanent)
- Nut factor: T = K × F_i × d
- VDI 2230: M_A = F_M × (0.16 P + 0.58 d2 μ_G + μ_K × D_Km ÷ 2), with D_Km = (d_w + d_h) ÷ 2 the mean diameter of the head’s bearing face
- Stress while tightening: σ_red = √(σ² + 3τ²), from the tension and the thread torque
- Wrench extension: setting = T × L ÷ (L + E × cos θ)
Property classes and grades
An ISO class such as 8.8 is read as 8 × 100 = 800 MPa nominal tensile strength and 0.8 × 800 = 640 MPa yield. ISO 898-1 proof stresses: 4.6 = 225 MPa, 5.6 = 280, 5.8 = 380, 8.8 = 580 (600 above M16), 9.8 = 650 (M16 and below), 10.9 = 830, 12.9 = 970 MPa. SAE J429 proof strengths: Grade 2 = 55 ksi (33 ksi above 3/4″), Grade 5 = 85 ksi (74 ksi above 1″), Grade 8 = 120 ksi, all from 1/4″ to 1 1/2″. Stainless classes (ISO 3506-1) have no proof stress, so their 0.2% proof stress is used: 210 MPa for A2-50, 450 for A2-70, 600 for A2-80.
Choosing K or the friction coefficients
Most of the torque is lost to friction — typically only about 10–15% stretches the bolt — so the result is only as good as the friction value. Shigley’s nut factors: non-plated black finish 0.30, zinc-plated 0.20, lubricated 0.18, cadmium-plated 0.16, with anti-seize 0.12; 0.2 is the usual default. For VDI 2230, μ around 0.12 suits lightly oiled steel; dry, plated or stainless parts can be much higher. A lubricated bolt torqued to a “dry” figure is over-tightened.
Torque wrench extensions
A crowfoot or adapter that sticks out in line with the wrench makes the lever longer, so the bolt gets more torque than the wrench shows. Set the wrench to T × L ÷ (L + E), where L is the wrench length from the hand-hold mark to the square drive and E the extension from the square drive to the bolt centre. At 90° to the wrench the extension adds nothing; folded back towards the handle the setting goes up.
Sources
- ISO 898-1:2013, Mechanical properties of fasteners made of carbon steel and alloy steel — Part 1: Bolts, screws and studs
- ISO 3506-1:2020, Fasteners — Mechanical properties of corrosion-resistant stainless steel fasteners — Part 1
- SAE J429, Mechanical and Material Requirements for Externally Threaded Fasteners
- VDI 2230 Part 1:2015, Systematic calculation of highly stressed bolted joints — tightening torque and assembly preload
- Budynas & Nisbett, Shigley’s Mechanical Engineering Design, 11th ed., §8-8 and Table 8-15 (nut factors and preload)
- ISO 272 (hexagon widths across flats), ISO 4762 (socket head cap screws), ISO 273 (clearance holes), ASME B18.2.1 and B18.3 (inch heads)
Limitations
- Friction is the big unknown: the same torque can give very different preloads with a different finish, lubricant or surface. Use the fastener or equipment maker’s torque specification where there is one.
- It does not design the joint: clamp load needed, embedding, thermal expansion, fatigue, gaskets and the strength of the threads in soft materials (aluminium, cast iron) are not checked.
- The VDI 2230 bearing diameter defaults to the hex width across flats; a real hex head bears on a slightly smaller circle, which lowers the torque by a few percent.
- SAE J429 covers 1/4″ to 1 1/2″ and ISO 898-1 covers M1.6 to M39; outside those sizes the class values are applied as an assumption and the result says so.
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Frequently asked questions
How much torque does an M10 8.8 bolt need?
With the nut-factor method, K = 0.2 and a preload of 75% of the 33.6 kN proof load, about 50 N·m. Lubricated (K ≈ 0.18) it is about 45 N·m; with VDI 2230 and μ = 0.12, about 42 N·m for the same preload.
What is the nut factor K?
A single number that lumps together thread and head friction and thread geometry, so that T = K × F × d. Shigley gives 0.30 for non-plated black bolts, 0.20 for zinc-plated, 0.18 lubricated, 0.16 cadmium-plated and 0.12 with anti-seize.
Why 75% or 90% of the proof load?
Shigley recommends a preload of 75% of the proof load for joints that will be taken apart and reused, and 90% for permanent joints. The proof load is the force the bolt must carry without permanent stretch, so both stay below yield.
Do lubricated bolts need less torque?
Yes. Less friction means more of the torque becomes preload, so a lubricated bolt torqued to a dry value is over-tightened and can yield. Always use the torque that matches the lubrication.
What do 8.8, 10.9 and 12.9 mean?
The first number × 100 is the nominal tensile strength in MPa; the second × 10 is the yield-to-tensile ratio in percent. So 10.9 is about 1,000 MPa tensile with a yield of about 90% of that. Higher classes allow more preload and torque.
How do I correct a torque wrench for a crowfoot?
If the crowfoot points straight out from the wrench, set the wrench to T × L ÷ (L + E). A 300 mm wrench with a 50 mm crowfoot needs a setting of 85.7 N·m for 100 N·m at the bolt. Turned 90° to the wrench, no correction is needed.