Fillet weld strength and size calculator
Enter the leg size, length and filler and see what the weld holds in kips and pounds. Or start from the load and get the length or leg size you need, checked against the AISC minimum and edge maximum. Built on AISC 360-22 for fabricators and students checking their numbers.
Updated November 2026
Quick answer
A 1/4" fillet in E70XX filler carries 5.57 kips per inch of weld (5,568 lb/in) by LRFD, or 3.71 kips per inch by ASD, pulled along its length. Every 1/16" of leg is worth 1.392 kips/in LRFD. That is weld metal only, per AISC 360-22, so check the plate too.
How fillet weld strength is calculated
AISC 360-22 §J2.4 rates a fillet weld by its throat, the shortest line from the root to the face. On an equal-leg fillet that is the leg times 0.707. The weld metal is good for 0.60 times the filler's tensile strength in shear, and a load across the weld gets an increase over a load along it.
Worked example: 10" of 1/4" E70XX fillet
- Throat: 0.25 × 0.70711 = 0.17678 in.
- Weld metal stress: Fnw = 0.60 × 70 = 42 ksi.
- Nominal strength: Rn = 42 × 0.17678 × 10 × 1.0 = 74.25 kips.
- LRFD: φRn = 0.75 × Rn = 55.68 kips, or 5.57 kips per inch.
- ASD: Rn ÷ 2.00 = 37.12 kips.
Purdue's CE 470 course notes (Example 3b.1) publish 55.68 kips LRFD for the same weld. Load it straight across instead (θ = 90°) and kds is 1.5, so it carries 83.53 kips. That increase only counts for a straight weld loaded through its center. It never counts for welds on the end of a rectangular tube in tension, or for a single fillet loaded so its root pulls open, like a one-sided T in tension. The calculator lists the conditions as soon as you enter an angle.
A shortcut for E70XX: every 1/16" of leg is worth 1.392 kips per inch LRFD and 0.928 ASD. The AISC Manual publishes the same rule as 1.392D and 0.928D, with D the leg in sixteenths.
Fillet weld strength per inch (E70XX)
Load along the weld (θ = 0°), welds at least 4 leg sizes long and not long end-loaded runs. For other fillers, scale by the filler strength: E60XX is 60 ksi, E70XX is 70 ksi, E80XX is 80 ksi.
Minimum and maximum fillet weld size
AISC 360-22 Table J2.4 sets the smallest fillet by the thinner part joined, whatever the load.
Along the edge of a part there is a maximum too (§J2.2b). Edge under 1/4": not more than the thickness. Edge 1/4" or thicker: not more than thickness − 1/16". So a 3/8" edge takes a 5/16" fillet at most. AISC allows a bigger weld along an edge when the drawings call for it to be built out to full throat thickness.
Once the size is set, the filler metal calculator tells you how much wire it takes, and the welding cost calculator prices it per inch.
Questions welders keep asking about fillet welds
How much weight can a 1/4" fillet weld hold?
Per AISC 360, a 1/4" fillet in E70XX filler (7018, ER70S-6 and the like) is good for 5.57 kips per inch of weld by LRFD, or 3.71 kips per inch by ASD, pulled along its length. In pounds that is 5,568 and 3,712 lb per inch. A 10" weld carries 55.68 kips LRFD (55,685 lb) or 37.12 kips ASD. That is the weld metal only. The plate it is welded to can give out first, so check the base metal too.
What is the minimum fillet weld size?
AISC 360 Table J2.4 sets it by the thinner of the two parts joined. To 1/4" inclusive: 1/8"; Over 1/4" to 1/2": 3/16"; Over 1/2" to 3/4": 1/4"; Over 3/4": 5/16". The minimum holds even when a smaller weld would carry the load: a little bead on thick plate cools too fast and can crack. These sizes are meant to go in one pass.
How big can a fillet weld be along a plate edge?
Edge under 1/4": not more than the thickness. Edge 1/4" or thicker: not more than thickness − 1/16". On a 3/8" edge that is 5/16" max. Holding back from the corner keeps the edge from melting off and leaves something to measure the leg against. AISC allows a bigger weld along an edge when the drawings call for it to be built out to full throat thickness.
How do you size a fillet weld for a load?
Work out what one inch of weld holds at a leg size, then divide the load by it. A 3/8" E70XX fillet is good for 8.35 kips per inch by LRFD, so a 250 kip factored load needs 29.93 inches of weld. Purdue's CE 470 notes work the same example and publish 29.94 in, because they divide by the rounded 8.35. Then check the size against the minimum for the plate, the maximum along an edge, and a length of at least 4 leg sizes. The calculator's "Length for a load" and "Size for a load" modes run all of it.
LRFD or ASD: which one do I use?
Use the method the drawings or your engineer use, and don't mix them. LRFD bumps the loads up with load factors and takes φ = 0.75 of the weld's nominal strength. ASD uses the loads as they will actually sit and divides the nominal strength by Ω = 2.00. That is why the LRFD number for the same weld is 1.5 times the ASD number: a 1/4" E70XX fillet is 5.57 kips/in LRFD and 3.71 kips/in ASD. The weld is the same. The load you compare it to is different.
Does a bigger fillet weld always make a stronger joint?
No. Strength goes up with the leg, but weld metal goes up with the leg squared. Going from 1/4" to 3/8" adds 50% strength and takes 2.25 times the filler. A short weld gets no credit for a big leg either: a 1.5" weld with a 1/2" leg is rated as a 3/8" fillet, because AISC rates any fillet shorter than 4 leg sizes at 1/4 of its length. Past a point the plate is the weak link, not the weld. Size it for the load and the minimum and leave it there.
Is a 7018 weld stronger than a 6011 weld?
On paper, yes. 6011 is an E60XX rod (60 ksi) and 7018 is E70XX (70 ksi), and fillet strength goes straight with that number. A 1/4" fillet is 4.77 kips/in in E60XX against 5.57 in E70XX (LRFD), about 17% more. Most mild steel structural work is designed around E70XX, so 6011 on that work comes up short of what the engineer counted on. E80XX is 6.36 kips/in and is meant for higher strength steels like chromoly. On A36 the base metal usually gives out before E80 weld metal does. This calculator checks weld metal only.
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