Bolt torque calculator

Torque = K × D × F: the nut factor K (0.20 dry, 0.15 oiled) times the bolt’s diameter times the clamp load you want, usually 75% of the bolt’s proof load. Pick a size, thread, grade and condition to get the torque in ft-lb and N·m.

Sources checked

Work out the torque

Inch (SAE) and metric

Bolt standard

As received, nothing added. K 0.20 (Portland Bolt, Fastenal).

% of proof load

The published charts use 75%.

3/8-16 Grade 5, dry, plain steel

30.9ft-lb41.9N·m

0102030400204060ft-lbN·m
Clamp load
4,941 lbf (22.0 kN)
Proof load
6,588 lbf

The working

  1. Tensile stress area. A = 0.7854 × (D − 0.9743 ÷ n)² = 0.7854 × (0.375 − 0.9743 ÷ 16)² = 0.0775 in²
  2. Proof load. Grade 5 proof stress is 85,000 psi, so A × stress = 6,588 lbf.
  3. Clamp load. 75% of proof load = 4,941 lbf.
  4. Torque. T = K × D × F = 0.2 × 0.375 in × 4,941 lbf = 371 in-lb ÷ 12 = 30.9 ft-lb (41.9 N·m).

How the calculator works

It uses the short-form torque–tension equation, T = K × D × F, the same method behind the bolt torque charts Fastenal and Portland Bolt publish. With the same inputs it gives their numbers: a 1/2-13 Grade 5 bolt, dry, at 75% of proof load comes to 75.4 ft-lb in both.

  1. Stress area. The cross-section that carries the load, smaller than the full diameter because of the thread: 0.7854 × (D − 0.9743 ÷ n)² for inch threads, π/4 × (d − 0.9382 × P)² for metric.
  2. Proof load. Stress area times the grade’s proof stress, the most the bolt can carry without stretching for good.
  3. Clamp load. The share of proof load you’re aiming for, 75% unless you change it.
  4. Torque. Nut factor times nominal diameter times clamp load. Inch: in-lb, divided by 12 for ft-lb. Metric: millimeters times kilonewtons gives N·m.

Grades and proof stress

GradeHead markingProof stress
Grade 2No marks on the head55,000 psi (33,000 over 3/4")
Grade 53 radial lines on the head85,000 psi (74,000 over 1")
Grade 86 radial lines on the head120,000 psi
Class 4.6"4.6" on the head225 MPa
Class 8.8"8.8" on the head580 MPa (600 over M16)
Class 10.9"10.9" on the head830 MPa
Class 12.9"12.9" on the head970 MPa

SAE J429 values from Portland Bolt; ISO 898-1 values from Fastenal. How to tell them apart: bolt grade markings.

Nut factors for each thread condition

ThreadsKNote
Dry, plain steel0.20As received, nothing added. K 0.20 (Portland Bolt, Fastenal).
Zinc plated, dry0.20Published values run 0.17–0.22 (Fastenal); 0.20 is used.
Oiled0.15Light oil on the threads and under the nut. K 0.15 (Fastenal).
Waxed0.10Pressure wax, as supplied on high-strength nuts. K 0.10 (Portland Bolt).
Hot-dip galvanized0.25Thick zinc coating, dry. K 0.25 (Portland Bolt).

Why the lubricated numbers are lower, and when to use them: dry vs lubricated torque.

When not to use it

The maker’s spec comes first

A vehicle, engine or machine maker’s torque spec allows for the joint, gasket and fastener it was designed with. Use it whenever there is one: wheel nuts, head bolts, caliper brackets and anything torque-to-yield all have their own. These numbers are for general-purpose steel bolts when no spec exists.

For wheel nuts, see the lug nut torque for your vehicle, quoted from its owner’s manual. For a whole range of sizes at once, the SAE chart and metric chart use the same method.

To apply the number, see how to use a click torque wrench; if your wrench reads in other units, torque units explained has the factors. Using a crowfoot or torque adapter? The extension calculator gives the wrench setting.

Questions

What is the nut factor K?

A single number for how much of the wrench’s effort goes into friction under the head and in the threads rather than into stretching the bolt. About 90% of the torque is lost that way, which is why K matters so much. Plain dry steel is about 0.20 and lightly oiled threads about 0.15; wax, plating and thread compounds change it again.

Why 75% of proof load?

It’s the target the published charts use for joints that may be taken apart again. It clamps hard while staying clear of the load that would stretch the bolt for good. You can change the percentage above, but tightening near 100% of proof load leaves no room for the scatter in friction.

Does this work for stainless steel, aluminum or torque-to-yield bolts?

No. The proof loads here are for carbon and alloy steel bolts to SAE J429 and ISO 898-1. Stainless and non-ferrous bolts have their own strength values and gall easily, and torque-to-yield bolts are tightened by angle to a maker’s procedure. Use the maker’s spec for those.

Do I need the coarse or fine thread?

Whichever the bolt has: measure the threads per inch (or the pitch in millimeters) with a thread gauge, or count the threads over an inch. A fine thread has a larger stress area, so it takes a little more torque for the same preload.

Sources

  1. Portland Bolt: SAE J429: proof load, yield and tensile strength for Grades 2, 5 and 8 (checked October 14, 2026)
  2. Fastenal: Mechanical properties per ISO 898-1 (externally threaded fasteners): proof stress by property class (checked October 14, 2026)
  3. Engineering Toolbox: Metric bolts: minimum ultimate tensile and proof loads, with ISO 898-1 stress areas (checked October 14, 2026)
  4. Portland Bolt: Thread pitch chart: UNC and UNF threads per inch and tensile stress areas (checked October 14, 2026)
  5. Engineering Toolbox: ISO 724 metric threads: coarse and fine pitches with tensile stress areas (checked October 14, 2026)
  6. Engineering Toolbox: Threaded bolts: tensile stress area formulas for unified and ISO metric threads (checked October 14, 2026)
  7. Fastenal: Torque–tension relationship for A307 A, Grade 5, 8 and 9 coarse thread bolts (75% of proof load; K 0.15 and 0.20) (checked October 14, 2026)
  8. Fastenal: Torque–tension relationship for metric fasteners, property classes 4.6, 8.8, 10.9 and 12.9 (checked October 14, 2026)
  9. Portland Bolt: Bolt torque chart: T = KDP at 75% of proof load, with K for plain, waxed and galvanized bolts (checked October 14, 2026)
  10. Fastenal: Bolted joint design: nut factor (K) by finish and how much preload varies at a given torque (checked October 14, 2026)

Spotted a number that doesn’t match your manual or a source? Tell us; corrections are logged on the sources page.