Bolt Shear Calculator
A bolt shear calculator multiplies bolt area (π × d² ÷ 4) by allowable stress and shear planes: a ¾ in bolt at 21,000 psi carries 9,278 lb in single shear.
How much load can a bolt carry in shear?
This bolt shear calculator finds a bolt’s shear and tension capacity from its diameter and the allowable stresses you enter. The formula is area = π × (diameter ÷ 2)², shear capacity = area × allowable shear stress × number of shear planes, and tension capacity = area × allowable tension stress. A ¾ in bolt at 21,000 psi in single shear carries 9,278 lb.
Enter the bolt diameter in inches, the allowable shear and tension stresses in psi, and 1 for single shear or 2 for double shear.
How the formula works
Picture a bolt joining two steel plates that are being pulled in opposite directions. The plates try to slice the bolt along the joint line. The bolt resists with its whole circular cross-section, so:
- Area = 3.1416 × (diameter ÷ 2)².
- Shear capacity = area × allowable shear stress × shear planes.
- Tension capacity = area × allowable tension stress.
The allowable stress already includes a safety margin under allowable stress design, so the result is a working load, not the load at which the bolt breaks.
The calculator uses the bolt’s full nominal area. Threads have less area than the shank, which is why design standards give a lower shear value when threads cross the shear plane.
Worked examples
- Default: ¾ in bolt, 21,000 psi shear, 29,000 psi tension, single shear
- Area: 3.1416 × 0.375² = 0.442 in²
- Shear: 0.442 × 21,000 × 1 = 9,278 lb
- Tension: 0.442 × 29,000 = 12,812 lb
- Same bolt in double shear
- Shear: 0.442 × 21,000 × 2 = 18,555 lb
- Tension stays 12,812 lb, because shear planes do not affect tension
Capacity by bolt diameter
At 21,000 psi allowable shear, the calculator’s default stress.
| Diameter | Area (in²) | Single shear (lb) | Double shear (lb) |
|---|---|---|---|
| ½ in | 0.196 | 4,123 | 8,247 |
| ⅝ in | 0.307 | 6,443 | 12,885 |
| ¾ in | 0.442 | 9,278 | 18,555 |
| ⅞ in | 0.601 | 12,628 | 25,255 |
| 1 in | 0.785 | 16,493 | 32,987 |
Change the stress and every number in the table scales with it. At 10,000 psi, each figure is a little under half.
Choosing the stress values
The right allowable stress depends on three things:
- Bolt grade. Common A307 bolts are much weaker than A325 or A490 structural bolts.
- Threads in the shear plane. Values are lower when threads cross the joint line.
- Design method. Allowable stress design and load and resistance factor design use different numbers.
Take the values from the design standard your project follows, such as the AISC specification for steel. For timber connections, bolt values come from wood design tables and depend on the wood, not just the bolt. Check local code and your engineer’s requirements.
How to measure
- Diameter: measure the unthreaded shank with calipers, or read the size from the bolt head marking and packaging.
- Shear planes: count how many joint lines the bolt crosses between pieces that slide against each other.
- Grade: read the head marking. Structural bolts are stamped with their grade.
Common mistakes
- Using a hardware-store bolt in a structural joint. An unmarked bolt has unknown strength.
- Forgetting the plates. A strong bolt in a thin plate can tear out of the edge first.
- Counting double shear wrongly. Two plates lapped together is single shear, even with a washer on each side.
- Ignoring combined load. A bolt in shear and tension at once has less capacity in each.
Related tools
Tighten the bolt correctly with the bolt torque calculator. If you are choosing between bolting and welding, compare with the weld strength calculator. For shear stress in other shapes, use the shear stress calculator.
This is a planning estimate. Connection design is safety-critical. Have a licensed structural engineer size and detail any bolted connection that carries real loads.
Frequently asked questions
How much shear can a 3/4 inch bolt take?
With this calculator's default allowable shear stress of 21,000 psi, a ¾ in bolt has 0.442 in² of area and carries 0.442 × 21,000 = 9,278 lb in single shear, or 18,555 lb in double shear. The real allowable value depends on bolt grade, whether threads cross the shear plane, and the design standard you are using.
What's the difference between single shear and double shear?
In single shear the bolt passes through one joint line, like two plates lapped together. In double shear a middle plate is sandwiched between two outer plates, so the bolt must be cut in two places before it fails. That is why this calculator multiplies the capacity by 2 for double shear.
What's the difference between A325 and A490 bolts?
Both are high-strength structural bolts; A490 is the stronger grade and carries higher allowable stresses. Since 2015 both grades are covered by ASTM F3125 as Grade A325 and Grade A490. Always enter the allowable stresses published for the exact grade and design method you are using.
Why does the formula use the bolt's cross-sectional area?
A bolt resists shear across its circular section, so capacity follows area, not diameter. Area is π × (d ÷ 2)², which means doubling the diameter quadruples the area: a 1 in bolt has 0.785 in², four times the 0.196 in² of a ½ in bolt.
Should I check shear capacity, tension capacity, or both?
Check whichever way the load pulls. A lap joint loads the bolt in shear; a hanger pulling straight out loads it in tension. Many real connections do both at once and need a combined shear-and-tension check, plus checks on the plates themselves for bearing and tear-out.
Estimates only. Verify quantities with your supplier before purchasing.