Weld Size Calculator

Inputs

Example data table

Formula used

This calculator sizes welds using an average shear-stress approach:

• Design stress: σ_d = σ_allow / SF
• Effective length (continuous): L_e = L
• Effective length (intermittent, approximate): L_e = L × (l_s/p)
• Required throat: t = F / (σ_d × L_e × n)
• Fillet weld relation: t ≈ 0.707 × a where a is the leg size.

The tool rounds the required weld size up to the nearest 0.5 mm for practical detailing.

How to use this calculator

1. Select the weld type (fillet or groove).
2. Enter the applied load in kN and the allowable weld stress in MPa.
3. Set a safety factor appropriate for your design method.
4. Provide the total weld length per line and the number of weld lines.
5. If the weld is intermittent, enter segment length and pitch.
6. Optionally add a provided weld size to check adequacy and utilization.
7. Press Calculate, then download CSV or PDF if needed.

Professional guide to weld sizing

1) Why weld size matters in construction

Welds transfer forces between plates, angles, and stiffeners, so sizing directly influences safety and serviceability. Undersized welds can crack or tear under repeated loading, while oversized welds waste filler metal and add distortion. For many shop connections, common fillet legs fall in the 3–10 mm range, but demand depends on load, length, and stress limits.

2) Loads and simple weld-group assumptions

This calculator treats the applied load as shear distributed across the effective weld area. Convert load to Newtons, then divide by the resisting area built from effective throat, weld length, and the number of weld lines. The approach is suitable for quick checks on straight weld runs where eccentricity and bending are small.

3) Allowable stress and safety factor selection

Allowable weld stress is often derived from the electrode strength and the chosen design format. Using a safety factor reduces the working stress to a design stress, improving reliability against variability in fit-up, welding quality, and loading uncertainty. As a practical guide, higher safety factors are used for dynamic or uncertain loads.

4) Effective throat and fillet geometry

The effective throat is the shortest distance from the weld root to the weld face. For a standard 45° fillet, throat is approximately 0.707 times the leg size. That means a 6 mm fillet provides about 4.24 mm of throat. Groove welds are handled by entering the effective throat directly.

5) Continuous versus intermittent welds

Intermittent welds reduce heat input and cost, but they also reduce effective length. This tool approximates that reduction by multiplying total length by the ratio segment/pitch. For example, 40 mm segments at 80 mm pitch yield 50% effective length. Always check detailing rules for minimum segment length and maximum spacing.

6) Rounding, minimums, and constructability

Weld sizes are typically specified in practical increments, so the calculator rounds up to the nearest 0.5 mm. Even when calculations suggest a tiny weld, minimum size requirements may apply due to fusion and toughness limits. Oversized welds can cause warping, so balancing strength and fabrication quality is important.

7) Using utilization to validate a provided weld

When you enter a provided weld size, the calculator reports capacity and utilization (load divided by capacity). A utilization under 1.0 indicates the weld is adequate for the simplified check. Values near 1.0 leave little reserve, so consider workmanship tolerances, fatigue, and any load eccentricity before finalizing.

8) Documenting results for review and QA

Clear documentation helps reviewers confirm assumptions and spot missing checks. Exporting results to CSV supports quantity takeoffs, and PDF output is useful for submittals and inspection records. Store input values, calculated throat, required size, and adequacy decisions together so future changes to load or layout can be rechecked quickly.

FAQs

1) What weld size does the calculator output for fillet welds?

It outputs the required fillet leg size in millimeters, rounded up to the nearest 0.5 mm. The effective throat is also shown for clarity.

2) How does the tool treat groove welds?

For groove welds, the calculator sizes the effective throat directly. If you provide a weld size, enter the effective throat value in millimeters.

3) What does “design weld stress” mean here?

Design weld stress equals allowable weld stress divided by the safety factor. It represents the working stress level used to compute required throat.

4) How is intermittent weld “effective length” estimated?

Effective length is approximated as total length multiplied by segment length divided by pitch. Example: 40/80 gives 50% effective length for capacity.

5) Why can two weld lines reduce the required size?

Two lines share the same load, doubling the resisting area for the same throat and length. That reduces required throat and therefore weld size.

6) What does utilization tell me?

Utilization is load divided by calculated capacity using your provided weld size. Values below 1.0 indicate adequacy for the simplified shear check.

7) Does this replace structural design codes?

No. It is a sizing aid for quick checks and documentation. Confirm code minimum weld sizes, eccentric loading, fatigue, and detailing requirements before issuing drawings.