Weld Strength Calculator
This weld strength calculator finds fillet weld capacity from leg size and length with the AISC formula: a ¼ in E70 weld, 10 in long, carries about 37,117 lb.
A weld strength calculator finds how much load a fillet weld can carry. It multiplies the throat thickness (0.707 × the leg size) by the allowable shear stress (0.3 × 70,000 psi for an E70 electrode) and by the weld length. A 1/4 inch fillet weld that is 10 inches long carries 0.707 × 0.25 × 0.3 × 70,000 × 10, about 37,117 lb.
The formula in plain words
Picture a fillet weld as a small triangle of steel filling the corner between two plates. You measure its leg, the flat side of the triangle along each plate.
The weld does not break along the leg. It breaks along the shortest line through the triangle, the throat, which runs from the inner corner out to the face. For equal legs that line is 0.707 times the leg size.
The strength per inch of weld is the throat times the allowable stress. Multiply by the length and you have the total.
- Throat = 0.707 × leg
- Allowable shear stress = 0.3 × 70,000 psi = 21,000 psi for E70
- Capacity = throat × 21,000 × length
Worked examples
Example 1, the default: 1/4 inch leg, 10 inches long, E70.
- Throat: 0.707 × 0.25 = 0.17675 in
- Capacity: 0.17675 × 21,000 × 10 = 37,117.5 lb, which the calculator shows as 37,117
Example 2, a joint welded on two sides: a 1/4 inch leg weld, 12 inches on each side, 24 inches in total.
- Capacity: 0.17675 × 21,000 × 24 = 89,082 lb
Example 3, a smaller weld: 3/16 inch leg, 12 inches long.
- Throat: 0.707 × 0.1875 = 0.13256 in
- Capacity: 0.13256 × 21,000 × 12 = 33,406 lb
Capacity of a 10 inch fillet weld, E70
| Leg size | Capacity | Per inch of weld |
|---|---|---|
| 3/16 in | 27,838 lb | about 2,784 lb |
| 1/4 in | 37,117 lb | about 3,712 lb |
| 5/16 in | 46,397 lb | about 4,640 lb |
| 3/8 in | 55,676 lb | about 5,568 lb |
| 1/2 in | 74,235 lb | about 7,424 lb |
How to use the number
The result is the load the weld can carry in shear, under the allowable stress assumption. A designer compares it with the force on the joint, with a margin chosen under the applicable code. The weld is only one part of a connection. The base metal, the plates, and the supports need checks too.
Enter the effective length. Count only the length that is fully formed, and take off a bit at each end if the weld starts and stops.
Common mistakes
- Entering the throat as the leg, or the leg as the throat. The calculator wants the leg.
- Counting a weld that is not continuous as if it were. Intermittent welds carry load only along the welded segments.
- Mixing up allowable and ultimate strength. This is an allowable value, which already includes a safety margin.
- Assuming the base metal is stronger than the weld. Thin plate can tear before the weld does.
- Using the result for lifting, towing, or pressure parts. These need an engineer and a qualified welder.
Bigger weld or longer weld?
Capacity rises with leg size, but the metal in the weld rises with the square of it. A fillet’s cross-section is half the leg squared. A 1/4 inch leg is 0.03125 in² and a 1/2 inch leg is 0.125 in², four times the metal for twice the strength.
So a 20 inch weld at 1/4 inch also carries 74,235 lb, the same as a 10 inch weld at 1/2 inch, and uses half the metal: 20 × 0.03125 = 0.625 against 10 × 0.125 = 1.25 cubic inches. Length is often the cheaper way to add capacity when the joint has room. Welder qualification and inspection decide whether the weld reaches its rated strength.
Reading the answer
The calculator gives an allowable load in pounds for the length you entered. If the force on the joint is higher, you need a larger leg, a longer weld, or a weld on more sides. A designer also checks that the plates themselves can carry the force and that the weld size is not too large for the thinnest plate. Take the result to an engineer for any structural use.
Related calculators
For bolted joints, use the bolt shear calculator and the bolt torque calculator. The shear stress calculator checks stress in a plate or pin, and the steel weight calculator estimates the weight of the members. Structural connections should be designed and reviewed by a licensed engineer.
Frequently asked questions
Why is there a 0.707 factor in the weld strength formula?
A fillet weld has a triangular cross-section with two equal legs, and it fails through the shortest distance from the corner to the face, the throat. For equal legs the throat is 0.707 times the leg size. A 1/4 inch leg has a throat of 0.177 inch.
What does E70 electrode mean?
E70 means the filler metal has a minimum tensile strength of 70,000 psi. It is the common choice for mild steel, covering E70XX stick electrodes and ER70S wire. This calculator uses Fexx = 70,000 psi and an allowable shear of 0.3 times that, which is 21,000 psi.
How do I find the total capacity of a weld with multiple segments?
Add the lengths of all the segments that carry the load, then enter that total. Two 12 inch welds of 1/4 inch leg carry the same as one 24 inch weld: 89,082 lb. Skip the short unwelded gaps and the ends of each weld where the weld may be incomplete.
Is this the same as the AWS D1.1 allowable stress?
It is the AISC allowable stress approach for a fillet weld loaded in shear, 0.3 times the electrode strength on the effective throat. Codes and design methods differ in details, such as load direction and base metal checks, so have a qualified engineer confirm the design.
How much does the weld capacity change with leg size?
It rises in direct proportion. At 10 inches long, a 3/16 inch weld carries 27,838 lb, a 1/4 inch weld 37,117 lb, a 5/16 inch weld 46,397 lb, a 3/8 inch weld 55,676 lb, and a 1/2 inch weld 74,235 lb.
Estimates only. Verify quantities with your supplier before purchasing.