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Bolt torque, grades and vehicle specs, sourced

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Bolt torque calculator (SAE and metric)

Pick the system, size, thread, grade and thread condition to get a tightening torque and the clamp load it targets. The default, an M12 x 1.75 class 10.9 bolt with plain dry threads, comes out at 93 ft-lb (126 N·m) for 52.5 kN of clamp load.

By the Torque Reference editors 9 sources cited Last reviewed Method
Contents6 sections

93 ft-lb 126 N·mM12 x 1.75, Class 10.9, plain dry (K 0.20), clamp load 52.5 kN (75% of proof).

Key takeaways

  • Torque = nut factor K × nominal diameter D × target clamp load F, the short-form equation in the Fastenal guide and NASA RP-1228 ⁠1 ⁠2.
  • F defaults to 75% of the bolt's proof load, the basis Fastenal gives for standard bolts ⁠1.
  • Thread condition moves the answer more than anything else you pick: oiled threads need 25% less torque than plain dry ones for the same clamp load.
  • Use the maker's torque when there is one, and never use a table torque on torque-to-yield bolts or pretensioned structural bolts ⁠3.

Quick facts

FormulaT = K × D × F
Clamp load basis75% of proof load (adjustable)
Nut factors0.20 dry, 0.18 zinc, 0.15 oiled, 0.16 anti-seize, 0.11 moly
Expected scatterAbout ±25% with a torque wrench ⁠2

How it works

The calculator does three steps, the same ones behind every chart on this site.

  1. Clamp load

    F = percent of proof × proof stress × tensile stress area. Proof stress comes from SAE J429 for inch grades (as reproduced by Fastenal) and ISO 898-1 for metric classes ⁠1 ⁠4. Stress area comes from the thread you pick, so coarse and fine threads give different answers.

  2. Torque

    T = K × D × F, where D is the nominal diameter and K is the nut factor for the thread condition ⁠1. In inch units the result is in in-lb, divided by 12 for ft-lb. In metric units it is N·mm, divided by 1,000 for N·m.

  3. Display

    Results under 10 ft-lb show in in-lb, everything else in whole ft-lb, with N·m alongside, converted with the NIST SP 811 factor 1 ft-lb = 1.355818 N·m ⁠5.

The nut factors are fixed values with a source each: 0.20 for plain dry steel, 0.18 zinc-plated and 0.15 lubricated, the values printed under Fastenal’s torque tables ⁠1, 0.16 for copper anti-seize ⁠6 and 0.11 for moly paste ⁠7. K is not a friction coefficient. It lumps together thread geometry, thread friction and underhead friction ⁠1.

Grades follow the strength breaks in the standards. Class 8.8 proof stress is 580 MPa up to M16 and 600 MPa above it ⁠4. SAE Grade 2 drops from 55,000 to 33,000 psi above 3/4 in, and Grade 5 drops from 85,000 to 74,000 psi above 1 in ⁠1. If a grade does not cover the size, the calculator says so instead of giving a number.

Worked example

M12 x 1.75, class 10.9, plain dry threads, 75% of proof:

M12 x 1.75 class 10.9, plain dry
Step Value
Proof stress, class 10.9 830 MPa
Tensile stress area, M12 x 1.75 84.3 mm²
Clamp load, 0.75 × 830 × 84.3 52,477 N (52.5 kN)
Torque, 0.20 × 12 mm × 52,477 N 125,945 N·mm
Result 126 N·m, 93 ft-lb

Change one input at a time and watch what moves:

  • Fine thread. M12 x 1.25 has a 92.1 mm² stress area, so it carries about 9% more clamp load (57.3 kN) and needs 101 ft-lb (138 N·m).
  • Oiled threads. K drops from 0.20 to 0.15, and the same 52.5 kN needs 70 ft-lb (94 N·m).
  • Class 8.8. Proof stress falls to 580 MPa, giving 65 ft-lb (88 N·m) for 36.7 kN.

For an inch check, switch to SAE and pick 1/2-13 Grade 5, plain dry. F = 0.75 × 85,000 psi × 0.142 in² = 9,050 lbf, and T = 0.20 × 0.5 in × 9,050 lbf = 905 in-lb, or 75 ft-lb (102 N·m). That matches the worked example in the Fastenal guide ⁠1.

Reading the result

The torque is the nominal value that should produce the clamp load shown, if the real nut factor matches the one you picked. It rarely matches exactly. With a torque wrench the clamp load scatters by about ±25% ⁠2, and NASA’s design default for torque control without test data is 25% for lubricated and 35% for dry or as-received hardware ⁠8. A result of 52.5 kN really means somewhere between about 39 and 66 kN.

The percent-of-proof box lets you aim lower or higher. Fastenal uses 75% for standard bolts and 90% for special applications such as structural bolts ⁠1. Going above 75% eats into the margin that covers the scatter. Proof load is usually 75% of theoretical yield ⁠2, so a 90% target on a dry bolt leaves very little room.

Common mistakes

  • Picking dry when the threads are oiled. A dry torque on an oiled bolt overshoots the clamp load by a third (0.20 ÷ 0.15). Choose the condition the threads will actually be in, or use the lubricated torque calculator to scale a dry spec.
  • Reading in-lb as ft-lb. Small bolts come out in in-lb: an M6 class 8.8 dry bolt is 93 in-lb (10 N·m). Setting 93 ft-lb is twelve times too much.
  • Using it where a procedure exists. Torque-to-yield bolts are tightened by angle and replaced, not torqued from a table ⁠9. Pretensioned structural bolts must not be installed with table torque ⁠3. Wheel nuts have a vehicle-specific spec in the owner’s manual: see lug nut torque by vehicle.
  • Guessing the grade. If the head marks are unclear, check them with the grade identifier before you pick one. An unmarked bolt should be treated as Grade 2 or lower.
  • Using the nominal size for the wrong thread. The calculator lists the thread series for each size. A fine-thread bolt run with the coarse-thread stress area is about 5 to 15% short on clamp load, depending on the size.

The full method, with every input and its source, is on the methodology page.

Sources

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  9. 9

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