Bolt torque and tension

IQ-CAL-008REV 0NOT A CERTIFICATE

IQEG CALCULATION LAB

Bolt torque to pretension

IQ-CAL-008 · Rev 0 Indicative · 11/11/2026 · Not a certificate

INPUTS + RESULTSUNITS AS TABULATED

IQ Engineering Group · Sydney · IQ-CAL-008 · Bolt torque to pretension · millimetres, kN, kNm, MPa unless noted

Select standard, grade, diameter and surface. The table lists ISO 898-1 minimum proof load Fp and minimum tensile Fm, torque to 65% and 90% of proof (T = k d F), AS 4100 Nti where it applies, and Ajax catalogue typical assembly torque (oiled). AS 4100 does not use torque as the compliance method for pretension — part-turn, DTI or a qualified wrench. k varies with surface: do not treat these N·m as a certified specification.

Bolt

STANDARD

GRADE

DIAMETER

SURFACE / LUBE

TARGET TENSION

k typical 0.16–0.22. common AU structural / Ajax as-oiled. ISO Sp 600 MPa · Rm min 830 MPa · As 245 mm².

Glossary

As: Tensile stress area of the thread, mm². ISO 898-1. Proof and tensile loads use As.
Fp: Minimum proof load = Sp × As. The bolt must take this without permanent set. kN.
Fm: Minimum tensile load = Rm × As. Ultimate. kN.
F: Target installed tension. 65% or 80% of Fp, or AS 4100 Nti, or a custom %.
k: Nut factor in T = k d F. As-received black ≈ 0.18. Oiled lower. Do not specify torque from a guessed k.
T: Indicative wrench torque, N·m. Not the AS 4100 pretension method unless a qualified k procedure is used.
Nti: AS 4100 Table 15.2.5.1 installation tension for TF/TB. M20 8.8 = 145 kN. Not a snug 8.8/S value.

Selected bolt

65% of ISO 898-1 min proof load Fp = 147.0 kN.

Outputs

ISO min proof Fp — kN147.0

ISO min tensile Fm — kN203.4

Target F — kN95.6

k used0.18

Torque T — N·m344

T range from k min–max — N·m306–420

Ajax typical (oiled) — N·m431

AS 4100 Nti — kN145

T at Nti — N·m522

Size	As mm²	Fp min kN	Fm min kN	F 65% kN	T 65% N·m	F 90% kN	T 90% N·m	Nti kN	T Nti N·m	Ajax typ. N·m
M12	84.3	48.9	67.4	31.8	69	44.0	95	—	—	89
M16	157	91.1	125.6	59.2	170	82.0	236	95	274	221
M20	245	147.0	203.4	95.6	344	132.3	476	145	522	431
M24	353	211.8	293.0	137.7	595	190.6	823	210	907	745
M27	459	275.4	381.0	179.0	870	247.9	1,205	—	—	1100
M30	561	336.6	465.6	218.8	1,181	302.9	1,636	335	1,809	1488
M36	817	490.2	678.1	318.6	2,065	441.2	2,859	490	3,175	2595

Ajax typical is a catalogue value, not a code torque.

Fp / Fm are ISO 898-1 minima (Sp × As, Rm × As). T = k × d(mm) × F(kN). Ajax column is catalogue typical for oiled coarse-pitch fasteners, not a code value. Worked: M20 8.8, Fp = 600 × 245 / 1000 = 147 kN. 65% = 95.6 kN. k = 0.18 → T = 0.18 × 20 × 95.6 = 344 N·m. Nti = 145 kN → T = 522 N·m.