What is the formula for calculating bolt tightening torque?

T = K × F × d, where K is the torque coefficient (0.10–0.25), F is target preload force (N), and d is nominal bolt diameter (m).

What is K-factor in bolt tightening?

K-factor combines thread and bearing surface friction. Typical values: dry 0.20-0.25, oiled 0.14-0.18, MoS₂ 0.10-0.12, PTFE 0.08-0.10.

What is the recommended preload for bolts?

Typically 65-75% of yield strength for standard applications. F = S × As × η.

How does lubrication affect bolt torque?

Lubrication reduces K-factor significantly. Using wrong K-factor causes over- or under-tightening.

What is the correct bolt tightening sequence?

Tighten in multiple passes using cross/star pattern: snug → 30% → 70% → 100%.

What is the Kellermann-Klein equation?

T = F × [P/(2π) + 1.154·μth·d₂ + μb·(Do+dh)/4]. Separates thread and bearing friction.

Free Engineering Tool

Bolt Tightening Torque Calculator

Calculate the required torque to achieve proper bolt preload force. Supports coarse & fine pitch metric threads, imperial UNC, and multiple lubrication conditions.

ISO 898ISO 16047VDI 2230ASME B1.1

Bolt Size

Property Class / Grade

Strength Basis (strict mode)

Lubrication (affects K-factor)

Target Preload (% of Rp)

Joint Type (informational)

Tightening Method (affects range)

Number of Bolts (for pattern)

Results

Recommended Torque

—

Preload Force

—

K-Factor

—

Stress Area

—

Torque Range (based on tightening method accuracy)

———

Tightening Sequence

1Hand-tighten all bolts until snug
2Tighten to — (30% torque) in pattern
3Tighten to — (70% torque) in pattern
4Tighten to — (100% torque) — smooth motion

Recommended Tightening Pattern

Torque Calculation Formula

The required tightening torque is calculated using the simplified VDI 2230 nut-factor method:

T = K × F × d

T — tightening torque (N·m)
K — torque coefficient / nut factor (0.10–0.25)
F — target preload force (N)
d — nominal bolt diameter (m)

Preload Force

F = S × A_s × η

S — strength basis: Rp (yield) or Sp (proof stress)
A_s — tensile stress area (mm²)
η — utilization factor (0.50–0.90)

Kellermann-Klein Equation

T = F × [ P/(2π) + 1.154·μ_th·d₂ + μ_b·(D_o+d_h)/4 ]

K-Factor Reference

Surface Condition	K-factor	Notes
Dry	0.20–0.25	High scatter
Light oil	0.14–0.18	Most common
MoS₂	0.10–0.12	High loads, stainless
PTFE	0.08–0.10	Caution: over-torque
Zinc plated	0.17–0.20	Depends on coating
Anti-seize	0.11–0.13	High-temperature
Wax patch	0.09–0.11	Automotive OEM

Bolt Property Classes (ISO 898-1)

Class	R_m	R_p	S_p	Application
4.6	400	240	225	Non-critical
5.6	500	300	280	General
8.8	800	640	580/600	Standard structural
10.9	1040	940	830	High-strength
12.9	1220	1100	970	Critical, max load

Practical Examples

Pump Flange — M12 × 1.75, class 8.8, oiled

K=0.16, F=640×84.3×0.75=40,464 N → T=0.16×40464×0.012=77.7 N·m

Gearbox — M20 × 2.5, class 10.9, anti-seize

K=0.12, F=900×245×0.75=165,375 N → T=0.12×165375×0.020=397 N·m

Over-torquing strips threads or fractures bolt
Under-torquing → loosening, leaks, fatigue failure
Calibrate torque wrenches annually
Clean threads before assembly
Do not reuse class 10.9+ bolts
This is an engineering estimate — test per ISO 16047 for critical joints

Bolt Tightening Torque Calculator | Free Online Tool | ISO 16047 & VDI 2230

Published by admin on February 15, 2026 February 15, 2026

Results

Torque Calculation Formula

Preload Force

Kellermann-Klein Equation

K-Factor Reference

Bolt Property Classes (ISO 898-1)

Practical Examples