How to Calculate Bolt Preload and Tightening Torque (VDI 2230)
VDI 2230
Most bolted joints don't fail because the torque was wrong — they fail because the resulting preload (the clamp force the bolt actually holds) was wrong. Torque is only the means; preload is the goal. This guide shows the VDI 2230 relationship between the two, why most of your applied torque never becomes clamp force, and a worked example you can reproduce.
VDI 2230 — "Systematic calculation of highly stressed bolted joints" — is the method most engineers and reviewers expect to see for a structural bolted connection. The MechanixCalc bolt calculator runs it end-to-end; this guide explains what it's doing.
Preload vs. torque — what you're really controlling
Preload (F_M) is the tensile clamp force created when you tighten a fastener. A joint stays tight, leak-free and fatigue-resistant only while a sufficient preload remains, so preload — not torque — is the design target. You apply torque because it's what a wrench can measure, but the relationship between the two is dominated by friction.
The torque–preload formula
For a metric fastener, VDI 2230 relates tightening torque to preload through three terms: the thread pitch (the useful part that creates clamp force), the thread flank friction, and the friction under the bolt head or nut face.
M_A = F_M · (0.16·P + 0.58·d₂·µ_th + µ_b·D_Km/2)where M_A = tightening torque; F_M = preload; P = thread pitch; d₂ = thread pitch diameter; µ_th = thread friction; µ_b = head/underhead friction; D_Km = effective bearing diameter (≈1.36·d)
Why ~90% of the torque is lost to friction
Only the first term (0.16·P) does the useful work of stretching the bolt; the other two are friction. With typical friction coefficients µ ≈ 0.12–0.14, only about 10–15% of the applied torque becomes preload — the rest is consumed turning against the threads and the bearing face. That's why friction scatter (dry vs. lubricated, plated vs. plain) is the single biggest source of preload uncertainty, and why a torque value is meaningless without stating the friction condition it assumes.
Worked example
Find the tightening torque for an M12 class 10.9 bolt at a target preload of 43 kN, with friction µ_th = µ_b = 0.12.
Given
- BoltM12, class 10.9
- Target preload F_M43 kN
- Thread pitch P1.75 mm
- Pitch diameter d₂10.863 mm
- Friction µ_th = µ_b0.12
Result
- Tightening torque M_A≈ 87 N·m
- Useful (preload) share of torque≈ 14% (0.28 / 2.014)
- Effective bearing diameter: D_Km ≈ 1.36·d = 1.36 × 12 = 16.3 mm.
- Pitch term: 0.16·P = 0.16 × 1.75 = 0.28 mm.
- Thread-friction term: 0.58·d₂·µ_th = 0.58 × 10.863 × 0.12 = 0.756 mm.
- Head-friction term: µ_b·D_Km/2 = 0.12 × 16.3 / 2 = 0.978 mm.
- Sum of terms = 0.28 + 0.756 + 0.978 = 2.014 mm.
- Torque: M_A = F_M × 2.014 = 43 000 N × 2.014 mm = 86 600 N·mm ≈ 87 N·m.
Illustrative — d₂, D_Km and the friction values depend on the exact bolt and joint. The calculator computes the full VDI 2230 chain (preload, torque, and the fatigue safety factor) for your inputs.
Do it on your own numbers
Run the full preload, torque and fatigue check for your bolt. Free 30-minute preview, no sign-up.
Open the Bolted JointFrequently asked questions
What is the difference between bolt torque and preload?
Torque is the rotational effort you apply with a wrench; preload is the tensile clamp force the bolt actually holds as a result. Preload is the real design target — the joint stays tight only while enough preload remains — but only ~10–15% of the applied torque becomes preload; the rest fights friction.
Why does friction matter so much in bolt tightening?
About 85–90% of tightening torque is consumed by thread and underhead friction, so a small change in the friction coefficient (e.g. dry vs. lubricated) produces a large change in the preload you actually achieve. That's why a torque spec is incomplete without the friction condition it assumes.
What preload should I target?
A common target is roughly 70–75% of the bolt's proof load (or yield), leaving margin for external load and embedding/relaxation. The exact value comes from the joint's external load, the required clamp force, and the tightening-method scatter — which is what VDI 2230 sizes.
Which standard governs this calculation?
VDI 2230 ("Systematic calculation of highly stressed bolted joints") is the governing method; bolt property classes and proof loads come from ISO 898-1. The MechanixCalc bolt calculator implements the VDI 2230 torque–preload and fatigue chain.
Is the bolt calculator free?
You can use it during a free 30-minute preview with no sign-up, and a free 14-day account trial unlocks every calculator with no credit card. The branded PDF report and saved calculations are part of a paid plan.