Welding Calculator
Size and check a fillet weld. Enter the weld configuration, leg size and length, and the electrode — get the effective throat, the unit strength per inch and the allowable weld capacity, plus (with an applied load) the utilization, a pass/fail result and the minimum leg size needed, with by-leg-size and by-electrode comparison tables and a downloadable PDF.
| Leg | Throat | Unit strength | Capacity |
|---|---|---|---|
| Enter a weld to compare | |||
| Electrode | Allowable stress | Unit strength | Capacity |
|---|---|---|---|
| Enter a weld to compare | |||
How to use the welding calculator
Four quick steps turn a fillet weld’s size and electrode into its allowable capacity — the effective throat, unit strength per inch and total capacity, plus a pass/fail check and the minimum leg needed when you add a load.
Pick the configuration
A fillet on one side of the joint, or on both sides.
Enter leg & length
The fillet leg size and the total effective weld length, in inches.
Choose the electrode
E60, E70, E80 or E110 filler metal (FEXX).
Add a load & save a PDF
Enter an applied load for utilization and min leg — then download a report.
The formulas behind the weld capacity
A fillet weld fails in shear across its throat; the capacity is the allowable stress on that throat along the weld length.
▶ Worked example
A single-side 1/4" (0.25 in) fillet, 6 in long, with an E70 electrode, carrying 12,000 lb.
Throat = 0.707 × 0.25 = 0.177 in; allowable stress = 0.30 × 70 = 21,000 psi.
Unit strength = 21,000 × 0.177 = 3,712 lb/in; capacity = 3,712 × 6 = 22,274 lb.
Capacity 22,274 lb · load 12,000 lb · 53.9% used · PASS (min leg 0.1875″)
Welding both sides would double the capacity to about 44,548 lb.
Fillet weld quick reference
The capacity formulas, the strength of each leg size, and how the electrode changes the capacity.
| Quantity | Formula |
|---|---|
| Effective throat | 0.707 × leg |
| Allowable stress | 0.30 × FEXX |
| Unit strength | 0.30 × FEXX × throat × sides |
| Weld capacity | unit strength × length |
| Utilization | load ÷ capacity |
| Min leg for load | load ÷ (0.30·FEXX × 0.707 × length × sides) |
| Leg | Throat | Unit strength | Capacity (6 in) |
|---|---|---|---|
| 1/8″ (0.125) | 0.088″ | 1,856 lb/in | 11,137 lb |
| 3/16″ (0.1875) | 0.133″ | 2,784 lb/in | 16,705 lb |
| 1/4″ (0.25) | 0.177″ | 3,712 lb/in | 22,274 lb |
| 5/16″ (0.3125) | 0.221″ | 4,640 lb/in | 27,842 lb |
| 3/8″ (0.375) | 0.265″ | 5,568 lb/in | 33,411 lb |
| 1/2″ (0.5) | 0.354″ | 7,425 lb/in | 44,548 lb |
Unit strength = 0.30 × 70 ksi × (0.707 × leg). Capacity shown for a 6 inch single-side weld; two sides double it, and capacity scales directly with length.
| Electrode | Strength | Allowable stress | Unit strength | Capacity (6 in) |
|---|---|---|---|---|
| E60 | 60 ksi | 18,000 psi | 3,182 lb/in | 19,092 lb |
| E70 | 70 ksi | 21,000 psi | 3,712 lb/in | 22,274 lb |
| E80 | 80 ksi | 24,000 psi | 4,243 lb/in | 25,456 lb |
| E110 | 110 ksi | 33,000 psi | 5,834 lb/in | 35,002 lb |
E70 is standard for mild structural steel. Allowable stress = 0.30 × the electrode strength; the base metal must also be checked separately.
Everything the calculator returns
One calculation gives the full weld picture — the throat, the capacity, and a load check with the minimum leg.
Weld capacity at a glance
The constants in the check, and the default result.
Built for the way you work
Whether you are laying the bead, designing the connection, or building in the shop, the weld check starts from the same inputs.
Need to know what size and length of fillet carries the load, and whether one side is enough or you should weld both.
- Size the leg for the load
- Weld both sides to double it
- Pick E70 for mild steel
Checking a fillet weld against an ASD service load and want the capacity, utilization and minimum leg in one place.
- Confirm throat and unit strength
- Compare electrodes quickly
- Still check base metal and group
Building a rack, trailer or bracket and want a sense of how much a weld holds before you trust it with a load.
- Start with a 1/4″ fillet
- Add length for more capacity
- Keep a safety margin
Size a fillet weld right
Seven things that keep a weld estimate sound and buildable.
It’s the throat that carries
A fillet fails across its throat, 0.707 of the leg — not the leg itself. All the capacity comes from that diagonal plane.
Weld both sides first
Two sides double the capacity for the same leg. Often cheaper and less distorting than one oversized fillet.
Add length, not just leg
Capacity scales directly with length. A longer weld is often easier and stronger than a bigger, multi-pass one.
Mind the minimum size
Thicker base metal needs a minimum fillet so the weld cools without cracking — use the larger of that and the load size.
Check the base metal too
This sizes the weld metal only. The connected steel can govern through shear rupture or block shear — check it separately.
Use the right electrode
E70 is standard for mild steel; match the filler to the base metal. A higher FEXX raises capacity proportionally.
Eccentric loads need more
This is a direct-shear check. An off-center or combined load needs a full weld-group analysis, not this simple capacity.
Frequently asked questions
Common questions about fillet weld strength, throat, electrodes and when a weld check needs more.
A fillet weld’s strength is based on shear across its effective throat. First find the throat, which is 0.707 times the leg size. The allowable shear stress is 0.30 times the electrode strength (FEXX). The weld capacity is then that stress times the throat times the length — and times two if you weld both sides.
Per inch of weld, the unit strength is the allowable stress times the throat; multiply by the length for the total capacity.
For example, a 1/4 inch fillet, 6 inches long, with an E70 electrode has a throat of about 0.177 inch, an allowable stress of 21,000 psi, and a capacity of about 22,300 pounds on one side. This calculator works out the throat, unit strength and capacity, and checks any applied load.
The effective throat is the shortest distance from the root of the weld to its face — the plane where a fillet weld actually fails in shear. For an equal-leg fillet it is 0.707 times the leg size, because the throat is the leg times the cosine of 45 degrees.
It is the throat, not the leg, that carries the load, so all the strength formulas use it. A larger leg gives a proportionally larger throat and more capacity.
For example, a 1/4 inch leg has an effective throat of about 0.177 inch. This calculator reports the throat for your leg size and uses it to work out the capacity.
It depends on the length, the electrode and whether it is welded on one or both sides, but the unit strength is fixed per inch. A 1/4 inch fillet with an E70 electrode carries about 3,710 pounds per inch of weld on one side, from an effective throat of 0.177 inch and an allowable stress of 21,000 psi.
Multiply by the length for the total: a 6 inch weld carries about 22,300 pounds on one side, or twice that on two sides.
A higher electrode or a bigger leg raises the figure. This calculator gives the exact unit strength and total capacity for your 1/4 inch weld, or any other size.
The number in an electrode classification is the filler metal’s minimum tensile strength in thousands of pounds per square inch (ksi). An E70 electrode has a 70 ksi tensile strength, E60 is 60 ksi, and so on up through E80 and E110 for higher-strength steels.
For weld strength, the allowable shear stress is taken as 0.30 times that number, so an E70 weld has an allowable stress of 0.30 times 70, which is 21 ksi, or 21,000 psi.
E70 is the most common choice for mild structural steel. This calculator includes E60, E70, E80 and E110 so you can see how the capacity changes with the electrode.
Yes. A fillet weld on both sides of the joint has twice the throat area of a single-sided weld of the same leg and length, so it carries twice the load. It is often the easiest way to gain capacity without increasing the leg size.
Two smaller welds on both sides can also be better than one large weld on one side, because a very large single fillet needs more passes and can distort the part.
For example, a 1/4 inch, 6 inch E70 weld carries about 22,300 pounds on one side and about 44,500 pounds on two. This calculator lets you pick one or two sides and doubles the capacity accordingly.
The minimum fillet weld size is set by the thickness of the thicker part being joined, not by the load. AWS and AISC give a table: thin material down to 1/4 inch needs at least a 1/8 inch fillet, up to 3/4 inch needs 1/4 inch, and thicker material needs more.
The minimum exists so the weld cools slowly enough not to crack against the heavier part — a weld that is strong enough for the load can still be too small for the base metal.
So always check both: the size the load needs and the minimum the thickness requires, and use the larger. This calculator sizes the weld for the load; confirm the minimum size against the code for your material thickness.
No. This calculator checks the weld metal — the shear capacity of the fillet across its throat. It does not check the base metal, which can govern through shear rupture of the connected part, block shear, or the limits where the weld meets the steel.
A complete connection design also checks the base material at the weld, the member itself, and any eccentricity in the load path. The weld is only one of several limit states.
Use this to size and check the fillet itself, then confirm the base-metal checks and the overall connection against AISC and AWS or with a qualified engineer.
Use the service (unfactored) load. This calculator works in allowable stress design (ASD), where the allowable weld stress is 0.30 times the electrode strength and the load is the actual, unfactored service load the connection carries.
If you are designing in LRFD, the factored loads and resistance factors are different, so the numbers will not match — do not mix the two methods. For eccentric or combined loading, a full weld-group analysis is needed instead of this direct check.
This calculator compares a service load against the ASD capacity and reports the utilization and the minimum leg size. Confirm the method and loads against your governing code.
Disclaimer: US units, fillet welds, allowable stress design (ASD). Effective throat = 0.707 × leg; allowable weld shear stress = 0.30 × FEXX; weld capacity = allowable stress × throat × length × sides. It checks the weld metal in direct shear only — it does not check the base metal (shear rupture, block shear), minimum weld size, eccentric or combined loading, or weld-group effects, and it is not LRFD. A reference aid only — confirm the connection, base-metal limits, minimum sizes and load path against AISC and AWS D1.1 or with a licensed engineer.