Torque Specs for US Bolt Sizes

8 月 14, 2026
8:33 下午

Every bolt size has a recommended torque range based on its diameter, thread pitch, and strength grade. Proper torque ensures consistent clamp force, prevents joint failure, and avoids thread damage. This chart provides baseline torque guidelines for common US bolt sizes from #1 to 2 inch across seven material grades, including both coarse (UNC) and fine (UNF) thread specifications. All values are in lb-ft for dry or slightly lubricated conditions.

Bolt Torque Lookup Tool

Select a bolt size to view recommended torque values across all material grades and thread types.

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Complete Bolt Torque Spec Chart

All torque values in lb-ft for dry, slightly lubricated bolts. Sizes #1–#10 are machine screw sizes; fractional sizes (1/4"–2") are standard bolt sizes. "Coarse" = UNC thread; "Fine" = UNF thread. A dash (—) indicates torque data not applicable for that size/grade combination.

Size	Grade 2		Grade 5		Grade 8		18-8 S/S		Bronze		Brass		A2/A4 S/S
Size	Coarse	Fine	Coarse	Fine	Coarse	Fine	Coarse	Fine	Coarse	Fine	Coarse	Fine	Coarse	Fine
Machine Screw Sizes (#1 – #10)
#1	—	—	—	—	—	—	0.3	0.2	0.2	0.3	—	—	—	—
#2	—	—	—	—	—	—	0.6	0.5	0.4	0.6	—	—	—	—
#3	—	—	—	—	—	—	1.1	0.9	0.8	1.0	—	—	—	—
#4	—	—	—	—	—	—	5.2	4.8	4.3	5.0	—	—	—	—
#5	—	—	—	—	—	—	9.5	8.9	7.9	9.2	—	—	—	—
#6	—	—	—	—	—	—	14.5	13.5	12.0	14.0	—	—	—	—
#8	—	—	—	—	—	—	19.8	18.4	16.2	19.0	—	—	—	—
#10	—	—	—	—	—	—	22.8	31.7	21.2	29.3	18.6	25.9	22.0	30.5
Standard Bolt Sizes (1/4" – 2")
1/4"	4	4.7	6.3	7.3	9	10	6.3	7.8	5.7	7.3	5.1	6.4	6.5	8.0
5/16"	8	9	13	14	18	20	11	11.8	10.3	10.9	8.9	9.7	11.5	12.2
3/8"	15	17	23	26	33	37	20	22	18	20	16	18	21	23
7/16"	24	27	37	41	52	58	31	33	29	31	26	27	32	34
1/2"	37	41	57	64	80	90	43	45	40	42	35	37	44	46
9/16"	53	59	82	91	115	129	57	63	53	58	47	51	59	65
5/8"	73	83	112	128	159	180	93	104	86	96	76	85	96	107
3/4"	125	138	200	223	282	315	128	124	104	102	118	115	132	128
7/8"	129	144	322	355	454	501	194	193	178	178	159	158	200	199
1"	188	210	483	541	682	764	287	289	265	240	235	212	296	298
1-1/8"	258	288	660	738	932	1043	394	396	363	329	322	290	406	408
1-1/4"	340	380	870	974	1229	1376	520	523	479	434	425	383	536	539
1-3/8"	435	486	1113	1246	1572	1760	665	669	613	555	543	490	686	690
1-1/2"	543	607	1389	1555	1962	2196	830	835	765	693	678	612	856	861
1-3/4"	840	939	2149	2406	3036	3399	1285	1292	1184	1073	1050	948	1325	1332
2"	1210	1353	3095	3465	4371	4894	1850	1860	1705	1545	1512	1364	1907	1918

Bolt Grade Reference

Understanding bolt grades is essential for selecting the correct torque specification. Each grade has distinct mechanical properties that determine how much preload the fastener can safely handle.

Grade 2

Low/medium carbon steel. Tensile strength: 57,000 psi (up to 3/8"), 55,000 psi (over 3/8"). General-purpose, non-critical applications. Identified by no head markings.

Grade 5

Medium carbon steel, quenched & tempered. Tensile strength: 120,000 psi (up to 3/4"), 105,000 psi (over 3/4"). Most common for automotive and machinery. Identified by 3 radial lines on head.

Grade 8

Alloy steel, quenched & tempered. Tensile strength: 150,000 psi (up to 1-1/2"), 130,000 psi (over 1-1/2"). High-strength structural applications. Identified by 6 radial lines on head.

18-8 / 304 Stainless

Austenitic stainless steel (AISI 304). Tensile strength: 85,000 psi. Good corrosion resistance for indoor and mild outdoor environments. Prone to galling without lubrication. No grade lines on head.

Lubrication Torque Adjustment Factors

How Lubrication Changes Torque

The torque values in the chart above are for dry or slightly lubricated conditions (friction coefficient approximately 0.14). When you apply lubricant, friction decreases and more of the applied torque converts to clamp force. Using the dry torque value on a lubricated bolt can cause over-tightening, yield, or thread failure.

Condition	Friction Coefficient	Torque Adjustment
Dry (as-charted)	approximately 0.14	No adjustment — use chart values directly
Light oil / WD-40	approximately 0.10	Reduce torque by 15%
Motor oil / gear oil	approximately 0.08	Reduce torque by 25%
Anti-seize compound	approximately 0.06	Reduce torque by 30%
Molybdenum disulfide (MoS2)	approximately 0.05	Reduce torque by 35%

For critical joints, always measure preload directly with a load-inducing washer or strain gauge rather than relying on torque alone.

Tightening Best Practices

Multi-Bolt Joint Tightening Procedure

Clean all threads — Remove dirt, rust, and old threadlocker. Damaged threads should be repaired with a tap or die before assembly.
Lubricate consistently — Apply the same lubricant to all bolts in the joint. Mixed lubrication conditions cause uneven clamp load distribution.
First pass: 30% of final torque — Tighten in a star or cross pattern to seat all components evenly and prevent warping.
Second pass: 60% of final torque — Continue the star pattern. This further equalizes clamp load across the joint.
Third pass: 100% of final torque — Complete the star pattern at full specification torque.
Final circular pass — Go around the joint sequentially (not in star pattern) at 100% torque to verify uniformity. Mark each bolt with a paint pen after verification.

Frequently Asked Questions

What is the recommended torque for a Grade 5 3/8 inch bolt?

The recommended torque for a Grade 5, 3/8 inch bolt is 23 lb-ft for coarse thread (UNC) and 26 lb-ft for fine thread (UNF), based on dry or slightly lubricated conditions. These values are baseline guidelines. Always verify with the fastener manufacturer's specifications for critical applications, as surface finish, plating, and lubrication can significantly affect the actual torque-tension relationship.

What is the difference between Grade 2, Grade 5, and Grade 8 bolts?

Grade 2 bolts are made from low or medium carbon steel with minimum tensile strength of 57,000 psi (for sizes up to 3/8 inch). Grade 5 bolts are medium carbon steel, quenched and tempered, with minimum tensile strength of 120,000 psi. Grade 8 bolts are alloy steel, quenched and tempered, with minimum tensile strength of 150,000 psi. As grade increases, the bolt can withstand higher preload, so the recommended torque values increase accordingly.

Should I use the coarse or fine thread torque value?

Use the torque value that matches your bolt's thread series. Coarse thread (UNC) is the default for most general-purpose applications. Fine thread (UNF) is used when higher vibration resistance or finer adjustment is needed. If you are unsure, check the thread pitch: coarse threads have fewer threads per inch (e.g., 3/8-16 has 16 TPI) while fine threads have more (e.g., 3/8-24 has 24 TPI). Using the wrong torque value can result in under-tightening or over-tightening.

How does lubrication affect bolt torque values?

Lubrication significantly reduces friction between the threads and under the bolt head, which means more of the applied torque converts to clamp force. The torque values in this chart are for dry or slightly lubricated bolts. If using oil, reduce the torque by approximately 15 percent. If using molybdenum disulfide (MoS2) or anti-seize compound, reduce torque by 20 to 30 percent. Over-torquing a lubricated bolt to the dry specification can cause yield or thread stripping.

What torque should I use for stainless steel bolts?

Stainless steel bolts (18-8 / 304 and A2/A4 / 316) have lower strength than Grade 5 steel bolts, so they require lower torque values. For example, a 3/8 inch 18-8 stainless bolt has a recommended torque of 20 lb-ft (coarse), compared to 23 lb-ft for Grade 5. Stainless steel is also prone to galling, especially when dry. Use a lubricant or anti-seize compound on stainless threads and reduce torque by an additional 10 to 15 percent to prevent galling during tightening.

Can I use a torque wrench to tighten small machine screws like #4 or #6?

Yes, but standard click-type torque wrenches are not accurate at very low torque values (below 5 lb-ft). For machine screws #0 through #10, use a precision inch-pound torque wrench or a torque screwdriver calibrated for the 1 to 50 in-lb range. The torque values in this chart are in lb-ft, so you will need to convert: multiply by 12 to get inch-pounds. For example, a #6 Grade 5 bolt at 14.5 lb-ft equals 174 inch-pounds.

What is the torque specification for a 1/2 inch Grade 8 bolt?

A 1/2 inch Grade 8 bolt has a recommended torque of 80 lb-ft for coarse thread (UNC, 1/2-13) and 90 lb-ft for fine thread (UNF, 1/2-20). These values assume dry or slightly lubricated conditions with a friction coefficient of approximately 0.14. For structural applications, always follow the applicable code requirements (such as RCSC specifications for A490 bolts, which is the structural equivalent of Grade 8).

How do I convert bolt torque from lb-ft to N-m?

To convert from lb-ft to Newton-meters (N-m), multiply the lb-ft value by 1.3558. For example, a 3/8 inch Grade 5 bolt at 23 lb-ft equals 23 x 1.3558 = 31.2 N-m. Common conversion factors: 1 lb-ft = 1.3558 N-m, 1 N-m = 0.7376 lb-ft, 1 lb-ft = 12 lb-in. Many torque wrenches are dual-calibrated in both units. For quick estimation, multiply lb-ft by 1.36 to get approximate N-m.

Why do fine thread bolts sometimes have different torque values than coarse?

Fine thread (UNF) and coarse thread (UNC) bolts of the same size and grade can have different torque specifications because fine threads have a larger tensile stress area (the minor diameter is larger) and a smaller thread pitch angle. The larger stress area allows slightly higher preload before yielding, while the smaller pitch angle changes the torque-tension relationship. For high-strength grades (5 and 8), fine thread torque values are typically slightly higher.

What tightening pattern should I use for multi-bolt flanges and joints?

For multi-bolt joints, always use a star or cross pattern to ensure even clamping and prevent distortion. The recommended procedure is: first pass at 30 percent of final torque, second pass at 60 percent, third pass at 100 percent, and a final circular pass at 100 percent to verify uniformity. Never tighten bolts sequentially in a circle, as this traps stress on one side and can warp the joint. For critical flanged connections, refer to ASME PCC-1 guidelines for bolted joint assembly.

Disclaimer: The torque data provided in this chart is for general reference purposes only. Actual torque requirements may vary based on specific application conditions, surface finish, plating, lubrication, ambient temperature, and fastener manufacturer specifications. Always consult the original equipment manufacturer's recommendations, SAE J429, or applicable engineering standards before assembly. GUXIANG does not guarantee the accuracy or completeness of this data and assumes no liability for errors, omissions, or consequences arising from the use of this information.