Quick Answer: How Strong Is a Bolt?
A bolt fails two ways: it breaks in tension when pulled, or cuts in shear when loaded sideways. Tensile capacity equals the material ultimate strength times the tensile stress area. Single shear is about 60% of that, and double shear is twice the single-shear value. A 1/2″-13 Grade 8 bolt (Aₜ = 0.1419 in², 150 ksi) carries roughly 21,000 lbf in tension and 12,800 lbf in single shear.
This calculator computes tensile capacity, single shear, double shear, proof load and safety factor for SAE Grade and metric class bolts per SAE J429 and ISO 898. Inputs include bolt system (inch or metric), bolt size and tensile stress area, grade or property class, load type and applied load. Proof load is about 90% of yield, the limit a bolt can carry without permanent stretch.
Common bolt grades: Grade 2 → 60 ksi | Grade 5 → 120 ksi | Grade 8 → 150 ksi | Class 8.8 → 800 MPa (116 ksi) | Class 10.9 → 1040 MPa (151 ksi) | Class 12.9 → 1220 MPa (177 ksi) | safety factor 2.0–5.0.
Bolt Shear Strength Chart
These tables use the same formula as the calculator above: single shear = 0.60 × tensile strength × tensile stress area, with the threads in the shear plane and no safety factor applied. Double shear is twice the single-shear value. Tensile capacity is the single-shear value divided by 0.6. Pick the same size and grade in the calculator and you get the same number.
Metric bolts, single shear in kN
| Size | Stress area At (mm²) | Class 8.8 (800 MPa) | Class 10.9 (1040 MPa) | Class 12.9 (1220 MPa) |
|---|---|---|---|---|
| M3 x 0.5 | 5.03 | 2.41 | 3.14 | 3.68 |
| M4 x 0.7 | 8.78 | 4.21 | 5.48 | 6.43 |
| M5 x 0.8 | 14.2 | 6.82 | 8.86 | 10.4 |
| M6 x 1.0 | 20.1 | 9.65 | 12.5 | 14.7 |
| M8 x 1.25 | 36.6 | 17.6 | 22.8 | 26.8 |
| M10 x 1.5 | 58.0 | 27.8 | 36.2 | 42.5 |
| M12 x 1.75 | 84.3 | 40.5 | 52.6 | 61.7 |
| M14 x 2.0 | 115 | 55.2 | 71.8 | 84.2 |
| M16 x 2.0 | 157 | 75.4 | 98.0 | 115 |
| M20 x 2.5 | 245 | 118 | 153 | 179 |
| M24 x 3.0 | 353 | 169 | 220 | 258 |
Inch bolts, single shear in lbf
| Size | Stress area At (in²) | Grade 2 (60 ksi) | Grade 5 (120 ksi) | Grade 8 (150 ksi) |
|---|---|---|---|---|
| #4-40 | 0.00604 | 217 | 435 | 544 |
| #6-32 | 0.00909 | 327 | 654 | 818 |
| #8-32 | 0.014 | 504 | 1,008 | 1,260 |
| #10-24 | 0.0175 | 630 | 1,260 | 1,575 |
| #10-32 | 0.02 | 720 | 1,440 | 1,800 |
| 1/4-20 | 0.0318 | 1,145 | 2,290 | 2,862 |
| 5/16-18 | 0.0524 | 1,886 | 3,773 | 4,716 |
| 3/8-16 | 0.0775 | 2,790 | 5,580 | 6,975 |
| 7/16-14 | 0.1063 | 3,827 | 7,654 | 9,567 |
| 1/2-13 | 0.1419 | 5,108 | 10,217 | 12,771 |
| 5/8-11 | 0.226 | 8,136 | 16,272 | 20,340 |
| 3/4-10 | 0.3345 | 12,042 | 24,084 | 30,105 |
| 7/8-9 | 0.4617 | 16,621 | 33,242 | 41,553 |
| 1-8 | 0.6057 | 21,805 | 43,610 | 54,513 |
Grade 2 is listed at 60 ksi, the SAE J429 value for sizes over 3/4 in. Sizes 1/4 to 3/4 in are rated 74 ksi, so the Grade 2 column is conservative for small bolts. Tensile stress areas follow ASME B1.1 and ISO 898-1.
Most-asked bolt shear strengths
- M12 shear strength: 40.5 kN (9,097 lbf) single shear for class 8.8, 52.6 kN for 10.9 and 61.7 kN for 12.9. Double shear is twice that.
- M10 class 8.8: 27.8 kN single shear, 55.7 kN double shear.
- M8 class 8.8: 17.6 kN single shear. M6 class 8.8: 9.65 kN. M16 class 8.8: 75.4 kN.
- 3/8-16 Grade 8: 6,975 lbf single shear; Grade 5 is 5,580 lbf.
- 1/2-13 Grade 8: 12,771 lbf single shear; Grade 5 is 10,217 lbf.
- 1/4-20 Grade 8: 2,862 lbf single shear; Grade 5 is 2,290 lbf. 5/16-18 Grade 8: 4,716 lbf.
Read these as ultimate values. They are the load at which the bolt shears through, not a working load. Divide by a safety factor (2 for static loads, 3 for cyclic, 4 or more for shock) and confirm the grade marking on the actual fastener. In a tension joint, proof load governs, not tensile strength, and a properly preloaded joint carries shear by friction between the plates rather than on the bolt shank. The thread engagement calculator checks whether the threads strip before the bolt breaks, and the bolt torque calculator gives the tightening torque for that preload.
How Bolt Strength Works
A bolt has two ways to fail. It can break in tension when pulled. It can cut in shear when loaded sideways. This calculator checks both.
Bolt Grades
Bolt grade tells you the material strength. Grade 2 is soft. Grade 5 is medium. Grade 8 is strong. The head has marks to show the grade. Count the radial lines on the bolt head. Three lines means Grade 5. Six lines means Grade 8. No lines means Grade 2.
Metric Classes
Metric bolts use numbers like 8.8 and 10.9. The first number times 100 is the tensile strength in MPa. Class 8.8 has 800 MPa. The second number is the yield-to-tensile ratio. Higher numbers mean stronger bolts.
Shear vs Tensile
Tensile load pulls the bolt lengthwise. Think of hanging a weight from a bolt. Shear load pushes the bolt sideways. Think of two plates sliding past each other with a bolt holding them together. A bolt is stronger in tension than in shear.
Pro tip: When possible design joints in shear, not tension. Shear joints are more predictable and less sensitive to preload. Save tension for cases where shear is not possible.
Single vs Double Shear
Single shear has one cutting plane. Double shear has two. Double shear doubles the capacity because the load splits across two planes. A double-shear joint is the strongest way to use a bolt in shear.
Safety Factor
Safety factor is the bolt capacity divided by the applied load. A safety factor of 2.0 means the bolt can handle twice the load. Use 2.0 for static loads, 3.0 for cyclic, and 4.0 or more for critical or shock loads.
