Fareed Mohmed · Engineering
Weld Size & Type Selector by Plate Thickness
Choose · Size · Prepare
Pick a weld type and the plate thickness you're joining, and get the recommended weld size or groove preparation — fillet leg size, bevel angle, root gap and root face — with the reasoning and standard reference behind it.
Find Weld Size by Type & Thickness
Visual Reference
Schematic cross-sections of each joint prep, viewed along the weld line — plate thickness runs top to bottom.
Fillet Weld
A roughly triangular weld deposited in the inside corner of two plates set at an angle (usually 90°) to each other. No bevel or gap prep is needed on the base metal — it's the cheapest and most common weld in machine building, used wherever a full-penetration butt joint isn't required.
Square-Groove Weld
Two plates butted edge-to-edge with their original square-cut edges and a small root gap, welded straight through with no bevel prep at all. Simple and cheap, but only reliable for full penetration on thin material.
Single-V Groove Weld
Both plate edges are chamfered to form a V-shaped channel, welded from one side only. The standard full-penetration butt joint for moderate thickness plate when only one side of the joint is accessible.
Double-V Groove Weld
Both plate edges are chamfered from the top and the bottom, meeting at a root face roughly in the middle, and welded from both sides. Roughly halves the weld metal volume and angular distortion of a single-V for the same thickness — needs access to both sides.
Single-Bevel Groove Weld
Only one of the two members is chamfered; the other keeps its square edge. Common where only one member can be prepared — a T-joint web welded into a flange, or a butt joint where only one plate is accessible for beveling.
Double-Bevel Groove Weld
One member is chamfered on both its top and bottom face while the other member stays square throughout. Used on heavier T-joints and box-section stiffeners needing full penetration welded from both sides without beveling the member that can't be prepared.
Quick-Scan Tables
| Thickness of Thicker Part | Minimum Fillet Leg | Approx. (Imperial) |
|---|---|---|
| Up to 6 mm (thicker part) | 3 mm | 1/8 in |
| Over 6 mm up to 12 mm | 5 mm | 3/16 in |
| Over 12 mm up to 20 mm | 6 mm | 1/4 in |
| Over 20 mm | 8 mm | 5/16 in |
AWS D1.1 Table 5.8 minimum fillet weld size, based on the thicker of the two parts joined — a minimum to prevent cracking in thick base metal, not a strength design value.
| Groove Weld Type | Typical Thickness Range | Bevel Angle | Root Gap |
|---|---|---|---|
| Fillet Weld | Any — strength-driven, not thickness-driven | — | — |
| Square-Groove Weld | Up to ~6 mm | None | 0–3 mm |
| Single-V Groove Weld | ~6–20 mm (one-sided) | 60–70° | 1.5–3 mm |
| Double-V Groove Weld | Above ~16 mm (two-sided) | 60–70° per side | 1.5–3 mm |
| Single-Bevel Groove Weld | ~6–25 mm (one-sided) | 30–45° | 0–3 mm |
| Double-Bevel Groove Weld | Above ~20 mm (two-sided) | 30–45° per face | 0–3 mm |
Typical starting-point prep parameters — always confirm against the applicable code and your qualified WPS before it goes on a drawing.
Explanatory Notes
Fillet weld size is a minimum, not a strength calculation
The AWS D1.1 minimum fillet table exists to stop a weld cooling too fast and cracking in thick base metal — it is not how you size a weld for load. A properly engineered fillet size comes from the applied force and the allowable stress on the weld throat, and can end up larger than this minimum on a heavily loaded joint even in thin material.
Fillet or groove — pick by what the joint has to do
A fillet weld is the default choice whenever the joint doesn't need full penetration — it's cheaper, needs no edge prep, and covers the large majority of brackets, gussets and T-joints in machine building. Reach for a groove weld only when the design specifically calls for full (or partial) penetration through the joint — a pressure boundary, a fatigue-critical butt joint, or a structural splice.
Single vs. double prep is an access and distortion trade-off
A single-sided prep (single-V, single-bevel) only needs access from one face, but puts all the weld metal and heat input on that one side, which pulls the joint out of flat as it cools. A double-sided prep roughly halves the weld volume and balances the heat input from both sides, cutting distortion — but only works if the joint can actually be reached, or turned over, from both faces.
Root gap and root face control fusion at the back of the joint
Too little root gap (or too large a root face) and the arc can't reach through to fuse the very back of the joint, leaving lack-of-fusion at the root. Too much gap and the root pass burns through. The ranges here are a typical starting point — the qualified WPS for your process (SMAW, GMAW, FCAW, SAW) is what actually fixes these numbers for a real drawing.
Related tools
For what tolerance to call out on a machined or welded feature, see the Tolerance Selector by Application Scenario tool. For what a manufacturing process can realistically hold on that same feature, see Manufacturing Process vs. Achievable Tolerance.