What is a Fillet Weld?

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Fuller welds have a wide range of applicability in the welding industry because these are the cost-efficient types of welds. In addition, less preparation for making joints or junctions, in this type, increases the demand for fillet welds in the market.

So, what is a fillet weld? A fillet weld is a specific type of junction between two metal sheets that have a triangular shape. You can make fillet weld using different processes. The main methods are gas-influenced arc welding, metal inert gas welding, and tungsten metal fusion welding. These are the most overwhelming and top-notch welds in this industry.

Fillet welds also have different subtypes that include tee joints or lap joints. You can also consider some types of corner joints in the fillet welds. The plus point of fillet welds is they have less complexity as compared to groove welds. It is the reason that automotive industries prefer fillet weld for joining different parts.

Fillet Weld Symbols

Welding symbols are important to know if you want to understand technical drawings. These drawings tell the welder what to do with a joint. A fillet weld looks like a triangle that is above or below an arrow pointing at the joint. The arrow is called "reference line."

Fillet Weld Symbols

The direction of the triangle on the side is important. It tells you which side to weld. There are two ways to show this, called A-System and B-System. There are more drawings that use A-System, but it is OK if you want to use B-System instead.

In "A-System" there are two lines. One is a solid line and one is a dashed line.

In the "B-System", there is a single reference line. If a triangle is at the bottom of the line, then it will be on the arrow side.

If you see a B-System and the arrow is pointing to the opposite side of the line, then you need to weld on that side too. If there are two triangles, one below and one above the reference line that is even with each other, then there needs to be an extra weld on that side too.

You will see a small circle around the point where the flat line and arrow are connected if you want to weld something. And manufacturers tell you how strong the weld must be. This is shown by a letter and number just before the flat line.

In welding, there are symbols to tell you what needs to be welded. There is a gentle curve pointing away from the "top" of the triangle. This means it needs to be concave. If it is a straight line parallel with the top of the triangle, then it means that it needs to be flat-faced welded. And if there is a gentle curve going towards the "top" of the triangle, then that means that you need a convex weld for this part.

The surface of the weld can be changed by either the type of welding or by using a machine or a grinder. To read the manufacturer's drawings, you will see some numbers that show how big the weld is.

A weld can be sized in many different ways. For example, the length of a weld or how wide it is, and also the space between two welds. These sizes will usually be near a triangle. For example: 1⁄8" x 3⁄8".

This means that the vertical part of the weld should be 1⁄8” but the horizontal part needs to be 3⁄8”.

If you are using millimeters to measure something in a drawing, then the welds are measured in millimeters. For example, if it is 3mm by 10mm, then that means that mm is being understood automatically.

Characteristics of a Fillet Weld

Fillet welding is a way to join pieces of metal together. You can do this when they are at an angle or perpendicular. It will be triangular in shape, and the weld will depend on the way you do it. It will be concave, flat, or convex.

A fillet weld usually has five components: 

  1. Root.
  2. Toe.
  3. Face.
  4. Leg.
  5. Throat.

a. Root : The area of the deepest penetration on the fillet weld is called root.

b. Toes: A fillet weld has some edges. These are known as toes of the fillet weld on the angle of the hypotenuse.

c. Face: The outer surface of the fillet weld that is apparent from the outside is called the face.

d. Legs: The fillet weld has a triangular area, and the outer two sides of this triangle are called legs. The length of the leg represents the size of the fillet weld. 

e. Throat: The throat is the area between the middle of the face to the root. Therefore, the thickness of the throat matters a lot.

In all circumstances, the density of metal for welding must match with the depth of the throat. In other words, the thickness of metal must be equal to the throat depth of the weld to form a good fillet weld. 

The depth or thickness of the throat is not easy to measure. It is because the throat depth includes the volume of the super-heated zone. The extent or volume of this zone is quite hard to find. So, you cannot measure its approach with a high degree of precision. 

What is fillet size in welding?

In a fillet weld, the size of the weld is measured by how long the legs are of a triangle that can be drawn in it. The size of a weld makes a difference in how much space you have to work with.

The length is determined by the size of the metal that you are welding together. The thickness is what sets how big or small a fillet weld can be.

The minimum leg size of an acceptable fillet weld is 4 times the thickness of the metal sheet. A round-up to one inch, so if your sheet metal was .25 inches thick the legs must be at least 1 inch in length. This means that 90% of most metals will give you a good fillet weld with legs that are 2-3 inches long (4x.5). If you find that you need to make a smaller bead it is easier to use a small diameter welding rod (.030) with the same amount of heat.

This process can be tricky but can be less problematic than trying to weld on thinner metal. Keep in mind that if the finished piece will need to withstand significant force, you'll want fillets that are at least 3/8".

The length of legs cannot "overlap" and must fit within a circle with a radius equal to 1/2 the thickness of your sheet metal. So for example, your metal thickness is .25 so your max allowable leg length would be ~3 inches (1/2 x .25). The reason this formula works is that if you overlay the legs on top of one another, they fit perfectly within a circle that is 1/2 inch thick.

Roughly 1/8" to 3/16" between each leg is fine. That's just enough space to be able to get your filler rod into the joint and move it around enough to efficiently melt into the piece you're working on.

Once you've got your legs in place and melted in well, give them a little tug and see how easily they come apart. If they separate very quickly, add more heat or try making two smaller beads instead of one big one next time. If they don't budge at all when you tug on them you're probably going to have to grind it out and start over.

What are the types of fillet joints?

The types of fillet joints are:

  • Butt Joint.
  • Lap Joint.
  • Corner Joint.
  • Edge Joint.
  • Tee Joint.

Now let's have a brief discussion on each type of fillet joints that are mentioned above.

  • Butt Joint:

The butt joint is the most common form of joining two pieces of wood together at 90° to each other. At the end of one board, you simply cut or saw off a square section to produce a flat surface. Then just apply some wood glue on the mating surface, bring the two pieces together, clamp them and let it set.

Simple butt joint where boards are joined at 90° angle with one over lapping the other.

  • Lap Joint:

The lap joint used for joining wide boards edge-to-edge is made slightly differently than the butt joint. When you cut off the square end of one board, make sure that the new cut is longer than half of that board's thickness. This extra length will allow an extra amount of wood along both sides into which your adjoining boards can be inserted after first applying some glue between them first. After gluing up, re-clamp your boards together and let them dry.

Lap joint where one board is split down the middle and half laps the other board.

  • Corner Joint:

The corner joint is used for joining boards at a right angle (90°). One of the boards has a cut at 45° on each end and it fits into two cuts made in this same way in the adjacent board. The resulting corner is very strong and rigid because all four wood pieces are locked together by their mutually perpendicular cut surfaces, as well as glued or pinned if you use metal fasteners.

  • Edge Joint:

A good edge joint needs to be as perfect as possible because any gaps will weaken your workpiece. Like with any carpentry project, precision is key here too, so take your time and make sure everything is lined up correctly.

  • Tee Joint:

This type of joint may not be very strong because it has only one point of solid contact between the boards, but for some light-duty applications, it's good enough. Just cut a 1"-thick board down to two equal pieces at 45° on each end, then reassemble them by butting them together in the same orientation as when they were cut apart. You can use screws or nails to fasten your tee joints, though glue also works well.

What are common fillet welds shapes?

Fillet welds exist in different shapes and forms:

  • Miter Fillet weld: It is a type of fillet weld in which the area of the cross-section of these welds is like a right-angle triangle.
  • Concave fillet weld: This type of fillet weld also has a right-angled size cross-sectional area. The difference between concave and meter fillet weld is that the former fillet weld has a concave-shaped face. The outer surface of these fillet welds has bulged inwards. It is just like a concave lens in which the outer surface bends inward. The thickness of the throat for this fillet weld is always less than a miter weld. The density decreases due to the concave-shaped surface of the weld.
  • Convex fillet weld: The convex fillet weld, as the name represents, has a convex outer face. The outer surface is bulged out or bends out in this type of weld. Due to the outward bending of the convex fillet weld, the throat penetration or thickness is more than all other types, including miter and convex fillet weld.

What is a fillet weld used for?

When two pipes are joined together, it is sometimes necessary to join them with a weld. The weld may be flat, concave, or convex shaped. Welders use this welding technique when connecting flanges to pipes and welding cross-sections of infrastructure where the bolts are not strong enough.

A fillet weld is a kind of weld that you use to join two pieces of metal edge-to-edge. It is also sometimes called a “fish mouth” or an “L-bead.” The pieces are not completely flush, but instead, have a concave area between them that creates what looks like a "U." Fillet welding requires a filler rod and the angles must be precise. The base metal should be clean, as any impurities can affect the quality of your work.

There are several types of welds: butt joint, corner joint, tee joint, and lap joint. In this blog, I will discuss how to make a fillet weld.

When you fit two pieces of metal together without any gap, you would use the butt joint type of welding. When there is a gap between the pieces, you will need to first start with making a corner joint and then a tee joint before using a fillet weld. But first, I will teach how to make a basic lap joint which requires some preparation work before joining them together.

Why do we avoid concave shape fillet weld?

The welds might be concave. That means the toes on the weld will have sharp edges. The poor profile of the weld results from these defects, but it can also make it easier for cracks to form in your joints. This makes them weaker and more likely to break over time.

Concave-shaped fillet welds cause accelerated fatigue in high yield areas. The weld line stress concentrations and the residual stresses lead to premature fracture of the component. The sharp edge of the fillet welds can cause stress concentrations.

The main reasons for this are:

Cracks in the toe area; Cracks at the root; Transverse crack in the throat; Uneven distribution of filler metal; Poor fusion between the base material and filler metal; Poor penetration.

All these potential causes should be carefully considered when designing, fabricating, and welding any concave (or convex) fillet specimens. We should also consider that not only the geometry affects the failure modes but also other factors like residual stresses, type of loading, or thermal effects that may affect on final behavior of components after testing with concave-shaped fillets.

As you can see there are several issues with feet profiles; the most common one is that it makes no difference for them to be concave or convex. The problem still occurs with both profiles, however, even if the concavity is very slight.

To prevent these defects you can follow some recommendations:

Using short arc welding; Using hot pass filler metal; Introducing stay welds (stitch weld); Allowing minimum root opening (for example, using fillers with larger diameters); Adding an extra pass at feet location.

The best way to avoid this problem is carefully to design your components before fabricating them, also great attention should be paid during the fabrication process to produce high-quality fillets at toes locations.

Potential problem faced in T-joint Fillet weld

If you want to make a strong fillet weld, you must adjust the dimensions of the weld accurately. The reliable and premium quality fillet weld demands a specific length of leg and throat thickness.

Designers usually work to select a proper range of all the dimensions. You can consider different safety factors for the determination of dimensions. The welding society of the UK denotes the length of the leg by using a symbol such as z. The size is usually measurable in millimeters.

The designer prepares a design and sends it to the welding shop for further verification. The welder inspects the blueprint from different perspectives and makes some alterations. For example, some people say that the fillet weld becomes stronger by increasing the leg length.

However, increasing length also magnifies the cost of welding. For example, if you determine to increases leg length by 2 mm. And the original size of the weld leg was 6 mm. This 2 mm increase will participate as 80 % of the total volume of the weld. So, it will increase the cost of the welding as well as the electrode arc consumption.

If you fabricate the leg length according to the schedule, you will not face too many problems. It is because a smaller weld will have a higher travel speed. If the travel speed is more, it will result in greater penetration of the weld. It will ultimately give rise to a greater throat thickness.

Welds having greater length entail less penetration that will result in weaker welds. As a result, the weak welds undergo an immediate deterioration. In addition, they may fall prey to different distractions. In short, a fillet weld with a longer leg length is not as sturdy as a weld with a shorter leg length.

Problems faced in a Lap fillet welding

In these types of welding, welders face the same problems as explained in the T-joint Fillet weld. In addition, such welds must have a thickness of the metal to be fuse comparable with the length of the leg. Different problems faced in a fillet weld of laps are as follows:

  • The upper end of the plate fuses many times during welding in the Lap-Joint weld. The ultimate result of this is the reduction in the length of the leg.
  • Another problem, in this case, may be the creation of an undersized weld. This problem arises because the throat penetration decreases due to the fusion of the upper corner of the weld.

To solve these problems, you must follow these steps.

The welder must care about the corner of the Lap-Joint. The upper plate must not melt due to heat during the whole process of welding. Moreover, you can consider another factor for the safety and sturdiness of the weld. You can lower the upper corner of the weld from the top plate. The distance between them must b 0.5 mm. The designers make some adjustments in the leg length to sort out this issue.

The less thickness of the throat is compensable by employing a fillet weld with deep penetration. Some other problems also appear during the process of fillet welding. In this type of fabrication, the pool size of the weld is large as compared to others. So, it can give rise to some potential issues.

For example, the overlapping of the weld toe can occur due to the larger pool size. Similarly, the weld toe exhibits sharp and sleek notches that can transpire trouble for you. The face of the fillet weld becomes a little bit concave that decreases the leg length resultantly. 


A fillet weld usually provides extra penetration and durability due to its triangular cross-sectional area. The automotive industries depend on this type of welding to weld metallic sheets. A fillet weld with an approximate leg length of 6 mm is enough to provide good grip and penetration.

Similarly, the fillet weld becomes sturdier by making a deep throat of the weld. To make a deep throat, try to make the length of the leg shorter. Fillet welders use different processes for designing these welds according to the circumstances and types. For example, conditions will be variant for a T-shaped fillet weld, corner fillet weld, and lap joint filled weld.


What is a fillet weld symbol?

The welders usually represent a fillet weld with a symbol of the triangle. The dimension representing the leg is present on the left side of the fillet weld symbol. However, nearly all the fillet welds have an equal length approximately. But this may not be true for all fillet welds as some have different volumes.

How is fillet weld measured?

The first step in the measurement of the fillet weld is to estimate the density of the metal used in the fillet weld. For determining the thickness of the metal, place the gauge against every corner of the metal. Thus, the throat of the weld represents the front of the fillet weld.

Which processes are useable for making a fillet weld?

You can fabricate a fillet weld using different methods available in the market. The most useable methods by the welders include: Gas-influenced arc welding. Metal inert gas welding. Tungsten metal fusion welding.

What is the effective length for a fillet weld?

A fillet weld has a length or size less than 4 s or 40 nm is not strong enough to bear the load of heavy-duty machinery. Therefore, to measure the effective length for a fillet, you must know the size of the fillet weld that could be less than one length of leg.

Last Updated on October 25, 2021 by weldinghubs

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