This free fillet weld calculator gives you the minimum and maximum fillet weld size, the effective throat, and the design strength of a fillet weld. It follows AISC 360 (LRFD or ASD), works in inches or millimetres, and covers E60 to E110 electrodes. Add a load and it also tells you the leg size you need and whether the weld passes.
Size limits per AISC 360 Table J2.4 and J2.2b. Strength per AISC 360 Section J2.4.
For estimating and learning only. Checks weld metal shear only. It does not include the directional strength increase for transverse welds, base metal rupture, eccentric loading, or fatigue. Final designs must be checked by a qualified engineer.
How to Use the Fillet Weld Calculator
- Pick your units and design method. Use LRFD with factored loads, or ASD with service (unfactored) loads.
- Enter the thinner plate thickness. AISC bases the minimum fillet size on the thinner of the two parts joined.
- Enter the weld leg size. Fractions like 5/16 work. Leave it blank to use the minimum allowed size.
- Enter the length of each weld and the number of welds. A lap joint welded on both sides is 2 welds.
- Choose the electrode. E70xx (like E7018 or ER70S-6) is the usual choice for mild steel.
- Optional: enter your load. The calculator then shows the leg size you need and whether your weld passes.
Fillet Weld Strength Formula
A fillet weld fails in shear across its throat, the shortest distance from the root to the face. For a weld with equal legs, the throat is 0.707 × leg size. AISC 360 gives the nominal strength as:
Rn = 0.60 × FEXX × (0.707 × w) × L
Here FEXX is the electrode tensile strength, w is the leg size, and L is the weld length. For LRFD, multiply by φ = 0.75. For ASD, divide by Ω = 2.00.
With E70 electrodes, this gives a handy rule of thumb: every 1/16 in of leg size carries 1.392 kips per inch of weld (LRFD), or 0.928 kips per inch (ASD).
Minimum Fillet Weld Size Chart (AISC Table J2.4)
The minimum fillet weld size depends on the thinner of the two parts being joined:
| Thinner Part Joined | Minimum Fillet Leg Size |
|---|---|
| Up to 1/4 in (6 mm) | 1/8 in (3 mm) |
| Over 1/4 in to 1/2 in (6–13 mm) | 3/16 in (5 mm) |
| Over 1/2 in to 3/4 in (13–19 mm) | 1/4 in (6 mm) |
| Over 3/4 in (19 mm) | 5/16 in (8 mm) |
These minimums are not about strength. Thick plates pull heat out of a small weld so fast that it can cool too quickly and crack. The minimum size makes sure there is enough heat input.
Fillet Weld Strength per Inch (E70 Electrode)
Design strength per inch of weld length for common leg sizes, using an E70 electrode:
| Leg Size | Throat | LRFD (φRn) | ASD (Rn/Ω) |
|---|---|---|---|
| 1/8 in | 0.088 in | 2.78 kips/in | 1.86 kips/in |
| 3/16 in | 0.133 in | 4.18 kips/in | 2.78 kips/in |
| 1/4 in | 0.177 in | 5.57 kips/in | 3.71 kips/in |
| 5/16 in | 0.221 in | 6.96 kips/in | 4.64 kips/in |
| 3/8 in | 0.265 in | 8.35 kips/in | 5.57 kips/in |
| 1/2 in | 0.354 in | 11.14 kips/in | 7.42 kips/in |
Frequently Asked Questions
How Strong Is a 1/4 Inch Fillet Weld?
With an E70 electrode, a 1/4 in fillet weld has a design strength of about 5.57 kips per inch of length using LRFD, or 3.71 kips per inch using ASD. So a 6 in long 1/4 in weld carries about 33.4 kips (LRFD), before checking the base metal.
What Is the Throat of a Fillet Weld?
The throat is the shortest distance from the root of the weld to its face. For a weld with equal legs, it is the leg size × 0.707 (that is, × sin 45°). Weld strength is calculated on the throat because that is where a fillet weld usually breaks.
Can a Fillet Weld Be Bigger Than the Plate Thickness?
Along the edge of a plate, no. AISC limits an edge weld to the plate thickness for plates under 1/4 in (6 mm), and to the thickness minus 1/16 in (2 mm) for thicker plates. This keeps the edge from melting away so the inspector can still measure the weld. Away from an edge, the weld can be larger, but oversizing only adds cost, heat and distortion.
What Does E70 Mean on a Welding Rod?
The "70" means the weld metal has a minimum tensile strength of 70 ksi (about 483 MPa). E7018 stick rod and ER70S-6 MIG wire both fall in this class. That is why E70 is the default for structural mild steel.
Is a Bigger Fillet Weld Always Stronger?
The weld gets stronger, but the joint may not. Once the weld is stronger than the plate next to it, the plate fails first. Strength also only grows in step with leg size, while the weld metal (and cost) grows with the square of the leg size. A 1/2 in weld uses four times the metal of a 1/4 in weld. Longer welds are often the cheaper way to get more strength.
Why Does the Calculator Say My Weld Is Below the Minimum Size?
For thicker plates, a small fillet weld cools too fast and can crack, even if it is strong enough on paper. Increase the leg size to at least the minimum shown, or check whether your welding procedure allows a smaller weld with preheat.
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