Bolt Torque Calculator - SAE Grade 2, 5, 8 and Metric 8.8

Bolt torque from size, grade and thread condition

Pick the thread, the size, the SAE grade or metric property class and the thread condition. The bolt torque calculator works out the clamp load the bolt should carry, 75% of its minimum proof load unless you choose another share, and the torque that produces it from T = K x D x F. You get ft-lb, in-lb and N-m, plus three tables: the same bolt in the other grades, every size in your grade, and your bolt under five thread conditions. Inch coarse (UNC) and fine (UNF) from 1/4 to 1 1/2 in, metric coarse from M5 to M24.

Quick start: a 1/2-13 Grade 5 bolt, plain and dry, comes out at 75.4 ft-lb (102.2 N-m) for 9,046 lb of clamp load. The same bolt with oiled threads needs only 56.5 ft-lb to reach the same clamp load.

Reading a bolt before you choose

Thread - inch coarse (UNC), inch fine (UNF) or metric coarse. Count threads per inch with a thread gauge, or measure the pitch in mm.
Size - the nominal diameter, for example 1/2 in or M10. The threads per inch or the pitch come with it.
Grade or property class - read the head: SAE Grade 2 has no marks, Grade 5 three radial lines, Grade 8 six; metric bolts carry the class number, such as 8.8 or 10.9.
Thread condition - plain and dry, zinc plated and dry, lubricated, hot-dip galvanized, waxed, or your own nut factor K.
Clamp load - optional, as a share of the proof load. Empty means 75%, the basis Fastenal and Portland Bolt use in their charts.

Seven bolt grades side by side

Minimum proof stress from SAE J429 and ISO 898-1, and what it means for one common size with plain, dry threads.

SAE Grade 2

No head marks. Proof stress 55,000 psi up to 3/4 in, 33,000 psi above. 1/2-13: 48.8 ft-lb.

SAE Grade 5

Three radial lines. 85,000 psi up to 1 in, 74,000 psi above. 1/2-13: 75.4 ft-lb.

SAE Grade 8

Six radial lines. 120,000 psi at every size. 1/2-13: 106.4 ft-lb.

Class 4.6

Proof stress 225 MPa. M10: 19.6 N-m (173 in-lb).

Class 8.8

580 MPa up to M16, 600 MPa above. M10: 50.5 N-m (37.2 ft-lb).

Class 10.9

830 MPa. M10: 72.2 N-m (53.3 ft-lb).

Class 12.9

970 MPa. M10: 84.4 N-m (62.2 ft-lb).

Inch bolt torque chart, Grade 5 and Grade 8

Coarse thread (UNC), plain and dry (K = 0.20), clamp load at 75% of proof. These are the values the calculator prints; they match the Fastenal chart to within its rounding.

Size	Grade 5	Grade 8	Grade 8 clamp load
1/4-20	101 in-lb	143 in-lb	2,862 lb
5/16-18	209 in-lb	24.6 ft-lb	4,716 lb
3/8-16	30.9 ft-lb	43.6 ft-lb	6,975 lb
7/16-14	49.4 ft-lb	69.8 ft-lb	9,567 lb
1/2-13	75.4 ft-lb	106.4 ft-lb	12,771 lb
9/16-12	108.7 ft-lb	153.5 ft-lb	16,371 lb
5/8-11	150.1 ft-lb	211.9 ft-lb	20,340 lb
3/4-10	266.6 ft-lb	376.3 ft-lb	30,105 lb
7/8-9	429.2 ft-lb	606.0 ft-lb	41,553 lb
1-8	643.6 ft-lb	908.6 ft-lb	54,513 lb

Fine thread (UNF) torque, Grade 5 and Grade 8

Same conditions as the coarse chart: plain and dry, 75% of proof. A fine thread has a larger tensile stress area at the same diameter, so it carries more clamp load and takes more torque. The last column is the stress area from the Fastenal chart, which is what the calculator multiplies by the proof stress.

Size	Grade 5	Grade 8	Stress area
1/4-28	116 in-lb	164 in-lb	0.0364 sq in
5/16-24	231 in-lb	27.2 ft-lb	0.0581 sq in
3/8-24	35.0 ft-lb	49.4 ft-lb	0.0878 sq in
7/16-20	55.2 ft-lb	77.9 ft-lb	0.1187 sq in
1/2-20	85.0 ft-lb	120.0 ft-lb	0.1600 sq in
9/16-18	121.3 ft-lb	171.3 ft-lb	0.2030 sq in
5/8-18	170.0 ft-lb	240.0 ft-lb	0.2560 sq in
3/4-16	297.2 ft-lb	419.6 ft-lb	0.3730 sq in
7/8-14	473.7 ft-lb	668.7 ft-lb	0.5095 sq in
1-14	722.4 ft-lb	1,019.9 ft-lb	0.6799 sq in

Metric bolt torque in N-m, classes 8.8 and 10.9

Metric coarse thread, plain and dry, 75% of proof. The jump between M16 and M18 in class 8.8 is partly the higher proof stress above M16.

Size	Class 8.8	Class 10.9
M6 x 1	10.5 N-m	15.0 N-m
M8 x 1.25	25.5 N-m	36.5 N-m
M10 x 1.5	50.5 N-m	72.2 N-m
M12 x 1.75	88.0 N-m	125.9 N-m
M14 x 2	140.1 N-m	200.4 N-m
M16 x 2	218.5 N-m	312.7 N-m
M18 x 2.5	311.0 N-m	430.3 N-m
M20 x 2.5	441.0 N-m	610.1 N-m
M24 x 3	762.5 N-m	1,054.8 N-m

Thread conditions, from slick to grippy

The nut factor K lumps friction under the head and in the threads into one number. Lower K means less torque for the same clamp load. For the quick start bolt (1/2-13 Grade 5, 9,046 lb):

Waxed, K 0.10

37.7 ft-lb. Portland Bolt's figure for pressure wax as supplied on high-strength nuts.

Lubricated, K 0.15

56.5 ft-lb. Fastenal's value for lubricated threads.

Zinc plated, dry, K 0.17

64.1 ft-lb. Fastenal notes thread lockers they tested landed close to this too.

Plain and dry, K 0.20

75.4 ft-lb. The default in both charts.

Hot-dip galvanized, K 0.25

94.2 ft-lb, from Portland Bolt.

Patterns the charts hide

Fine thread takes more torque. A 1/2-20 UNF Grade 5 has a stress area of 0.1600 sq in against 0.1419 for 1/2-13, so 85.0 ft-lb instead of 75.4.

Grade 2 steps down at 7/8 in. Its proof stress drops from 55,000 to 33,000 psi, so a 7/8-9 Grade 2 gets 166.6 ft-lb, less than the 172.5 ft-lb of a 3/4-10.

Torque follows K one to one. Oiling a dry bolt (0.20 to 0.15) without lowering the torque raises the clamp load by a third, which takes a bolt set at 75% right up to its proof load.

Bolt torque questions

How is bolt torque calculated?

T = K x D x F: nut factor times nominal diameter times clamp load. For 1/2-13 Grade 5: 0.20 x 0.5 in x 9,046 lb = 905 in-lb, divided by 12 = 75.4 ft-lb.

Why 75% of the proof load?

Proof load is the force a bolt must carry without permanent stretch. Fastenal and Portland Bolt both base their charts on 75% of it for reusable joints, which leaves room for the scatter in friction. You can choose 40 to 90% in the form.

Should I use this for head bolts or lug nuts?

No. Cylinder head bolts, connecting rod bolts, wheel lug nuts and torque-to-yield bolts have specs and sequences from the vehicle or engine maker. Use this tool for general fasteners without a published spec.

How do I convert ft-lb to N-m and in-lb?

Multiply ft-lb by 1.3558 for N-m and by 12 for in-lb. The result shows all three; small bolts are easier to set with an in-lb wrench.

Does the nut matter?

Yes. Fastenal notes that the chart torques are reachable only if the nut or tapped hole has a proof load at least equal to the bolt's minimum tensile strength. A soft nut or a shallow thread in aluminum strips first.

Why does my chart show a slightly different number?

Charts round, and some list proof loads from a table instead of proof stress x stress area. Portland Bolt prints 75 ft-lb for a plain 1/2-13 Grade 5 and 107 for Grade 8; this calculator gives 75.4 and 106.4.

Is a Grade 8 bolt the same as a class 10.9?

They are close in strength, not identical. Grade 8 has a proof stress of 120,000 psi, class 10.9 has 830 MPa, which is about 120,400 psi. Sizes differ too: a 1/2-13 Grade 8 gets 106.4 ft-lb, the nearest metric size, an M12 class 10.9, gets 92.9 ft-lb, because it is slightly smaller and has a smaller stress area.

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