What does this bolt torque calculator calculate?

It calculates tightening torque, bolt preload, tensile stress area, external and internal thread shear areas, and minimum length of engagement for Unified inch screw threads.

Which thread series are included?

The calculator includes Unified inch screw thread sizes in UNC, UNF and UNEF series from 1/4 inch to 1 1/2 inch.

What is the torque equation used?

The simplified tightening torque equation is T = K d Fi, where K is the torque factor, d is nominal bolt diameter and Fi is the calculated preload.

AmesWeb

Unified Thread Bolt Torque Calculator

Calculate bolt tightening torque, preload and thread strength for Unified inch screw threads. The calculator covers common UNC, UNF and UNEF bolt sizes and steel bolt grades, and reports the dimensional and strength parameters used in the calculation.

This tool is intended for preliminary engineering calculations for bolted joints. It estimates tightening torque from preload by using a torque factor and also calculates tensile stress area, thread shear areas and minimum engagement length against thread stripping.

UNCUNFUNEFASME B1.1Bolt preloadThread engagement

Bolt Torque Calculator

Nominal size and threads per inch Unified thread size, such as 1/2-20 UNF.

Bolt / screw material grade Material data is selected according to the selected bolt diameter.

Use material database

Yes No, enter strengths manually

Internal thread strength greater than external thread strength

Yes No

Min. proof strength of bolt, Sp 10³ psi

Min. tensile strength of bolt, Sut.ext 10³ psi

Min. tensile strength of internal thread, Sut.int 10³ psi; required only if internal thread is weaker.

Max. allowed preload, PPS % of proof strength. Common values: 75% nonpermanent, 90% permanent.

Torque factor, K Use supplier values when available.

Design factor against stripping, ns Must be at least 1.0.

Torque output unit

Note: The torque factor K strongly depends on lubrication, surface finish, coating, washer condition and tightening method. Manufacturer or supplier test data should be used for final design whenever possible.

Results

Selected thread
Nominal size	---	---
Threads per inch, n	---	---
Series designation	---	---
Material properties
Proof strength, Sp	---	10³ psi
Bolt tensile strength, Sut.ext	---	10³ psi
Internal thread tensile strength, Sut.int	---	10³ psi
Required dimensions for calculations
Basic major diameter, d	---	in
Min. major diameter of external thread, d_min	---	in
Min. pitch diameter of external thread, d_2min	---	in
Max. minor diameter of internal thread, D_1max	---	in
Max. pitch diameter of internal thread, D_2max	---	in
Thread tensile stress area, A_t	---	in²
Shear area of external thread, A_s	---	in²
Shear area of internal thread, A_n	---	in²
Torque and preload results
Calculated preload, F_i	---	lbf
Estimated required torque, T	---	lbf·in
Minimum length of engagement to prevent stripping, L_e	---	in

Bolt torque calculation method

The calculator estimates the required tightening torque by using a common torque factor relationship. The preload is calculated as a percentage of the bolt proof load, and torque is then estimated from the nominal bolt diameter and torque factor.

F_i = A_t × S_p × PPS / 100
T = K × d × F_i

Where F_i is preload, A_t is tensile stress area, S_p is proof strength, PPS is the selected preload percentage, K is the torque factor and d is the basic major diameter of the bolt.

Thread strength and engagement length

The page also estimates the minimum length of engagement required to reduce the risk of thread stripping. The calculation uses Unified screw thread geometry, external and internal thread tolerance values and the selected design factor against stripping.

For tolerance calculations, class 2A is used for external threads and class 2B is used for internal threads. If the internal thread material is weaker than the bolt material, the engagement length is adjusted using the ratio of external and internal thread stripping capacities.

Definitions

Preload: Axial tension force created in a bolt during tightening.

Proof load: Maximum load a bolt can withstand without permanent set.

Torque factor K: Empirical factor that represents friction and tightening conditions.

Major diameter: Diameter of the crest of an external thread or root of an internal thread.

Minor diameter: Diameter of the root of an external thread or crest of an internal thread.

Threads per inch: Number of full thread turns per inch of thread length.

UNC: Unified National Coarse thread series for general engineering applications.

UNF: Unified National Fine thread series, often used when a finer pitch or greater tensile stress area is desired.

UNEF: Unified National Extra Fine thread series used when finer pitches are required.

Practical notes for bolt tightening

Torque tightening is practical and widely used, but it is less accurate than direct bolt tension or elongation measurement because friction in the threads and under the nut or bolt head can vary significantly. When preload accuracy is important, elongation measurement, load indicating washers, hydraulic tensioning or ultrasonic bolt measurement may be considered.

For reused fasteners or nonpermanent joints, a preload value around 75% of proof strength is commonly used as a starting point. Permanent joints may use higher values, but the final selection should consider joint stiffness, fatigue, embedment, material limits, temperature, lubrication and applicable design standards.

Related tools and resources

References

ASME B1.1 — Unified Inch Screw Threads (UN and UNR Thread Form).
Oberg, E.; Jones, F. D.; Horton, H. L.; Ryffel, H. H. Machinery's Handbook, Industrial Press.
Budynas, R.; Nisbett, K. Shigley's Mechanical Engineering Design, McGraw-Hill.
Barrett, T. R. NASA RP-1228, Fastener Design Manual.
Bickford, J.; Nassar, S. Handbook of Bolts and Bolted Joints, Marcel Dekker.