Bolt Preload & Installation Torque Calculator

Estimate target clamp preload and the installation torque needed to reach it, using the standard torque-tension (nut factor) relationship.

FASTENER PRELOAD · TORQUE-TENSION RELATIONSHIP

Bolt Preload & Installation Torque Calculator

Estimates target clamp preload and the installation torque needed to reach it, using the standard torque-tension (nut factor) relationship.

Thread Size
UNF

Bolt Grade
Proof Strength (SAE J429)

Target Preload
% of Proof Load

75% is a common target for reusable (non-permanent) fastened joints.

Torque (Nut) Factor K
Lubrication Condition

Installation Torque

85.0 ft-lbf

At = 0.1600 in² · Preload F = 10,197 lbf

✔
Torque-tension relationships have ±25–30% scatter even at a fixed K — treat this as a starting point, not a guaranteed preload.

Engineering Model (torque-tension / nut-factor method)

Standard Reference
Tensile stress area formula follows ASME B1.1 (verified against published values, e.g. 0.1600 in² for 1/2″-20 UNF). SAE J429 defines Grade 2/5/8 proof strengths. The torque-tension (nut factor K) relationship is a widely used engineering approximation, not an exact physical law — actual friction varies significantly by surface condition, plating and lubrication.

Calculation Method
At = π/4·(D−0.9743/n)² · F = At × Proof × % · T = K × F × D

Assumptions & Limitations

The nut factor K is a rough approximation — real-world torque-to-preload scatter is commonly cited at ±25–30%, even with a well-controlled K. For safety-critical or fatigue-sensitive joints, consider preload verification by bolt stretch or ultrasonic measurement instead of torque alone.
Assumes static, one-time tightening — does not model relaxation, embedment loss, or joint fatigue over repeated cycles.
Grade proof strengths are nominal SAE J429 published values — always confirm against your specific fastener’s certification.

STATIC REFERENCE DATA

Reference: Preload & Torque by Thread Size (Grade 5, 75% Proof, K=0.20)

Precomputed values from the exact formula used above — useful as a quick lookup without re-entering inputs.

Thread Size	At (in²)	Preload F (lbf)	Torque
3/8″-24 UNF	0.0878	5,597 lbf	420.0 in-lbf (35.0 ft-lbf)
1/2″-20 UNF (calculator default)	0.1600	10,197 lbf	1,019.7 in-lbf (85.0 ft-lbf)
5/8″-18 UNF	0.2560	16,317 lbf	2,039.7 in-lbf (170.0 ft-lbf)
3/4″-16 UNF	0.3730	23,777 lbf	3,566.5 in-lbf (297.2 ft-lbf)

WORKED EXAMPLE

Setting Torque for a 1/2″-20 Grade 5 Clevis Bolt

A 1/2″-20 UNF Grade 5 bolt clamps a rod end bracket. Target: 75% of proof load, standard lightly-oiled assembly (K=0.20).

Tensile stress area At = π/4×(0.5−0.9743/20)² ≈ 0.1600 in² (matches the published ASME B1.1 value exactly).
Preload F = 0.1600 × 85,000 psi × 0.75 ≈ 10,197 lbf.
Installation torque T = 0.20 × 10,197 × 0.5 ≈ 1,020 in-lbf ≈ 85 ft-lbf — matches commonly published torque-chart values for this exact size/grade combination.
Remember the ±25–30% real-world scatter on torque-to-preload — for a safety-critical joint, consider preload verification by bolt stretch instead of torque wrench alone.

Related reading: Control Arm Resource Hub →