Mechanical

Weld Strength Calculator

Estimate equal-leg fillet weld throat area, nominal strength, LRFD design strength, ASD allowable strength, utilization, or required weld leg size.

Formula Rn = 0.60 × FEXX × Awe × (1 + 0.50 sin(θ)^1.5)Reviewed Aug 29, 2026

Weld strength checks are usually controlled by the effective throat, not the visible fillet leg. This calculator handles a simple equal-leg fillet weld under direct loading: it converts leg size to throat size, multiplies by total effective weld length, applies the weld-metal shear stress model, and reports nominal, LRFD, or ASD capacity. It is useful for screening and learning, not for final weld procedure or structural connection approval.

Calculation Bench

Solve for

Design method

01Fillet Weld Leg Size

s · Equal-leg fillet size. The effective throat is 0.707 × s.

02Total Weld Length

Lw · Total effective weld length. Include both sides if both carry load.

03Electrode Strength

FEXX · Filler-metal classification strength. E70 is about 70 ksi or 490 MPa.

04Applied Load

P · Optional for capacity mode; required when solving required leg size.

05Load Angle to Weld Axis

θ · 0° is longitudinal loading; 90° is transverse loading.

Solution

Enter weld geometry and electrode strength, then calculate the fillet-weld capacity or required leg size.

te = 0.707 × s

Formula Sheet

t_e=0.707s

A_{we}=t_eL_w

R_n=0.60F_{EXX}A_{we}\left(1+0.50\sin^{1.5}\theta\right)

s=\dfrac{P}{factor\cdot0.707L_w\cdot0.60F_{EXX}\cdot C_\theta}

sFillet Weld Leg Size
teEffective Throat
LwTotal Effective Weld Length
AweEffective Weld Area
FEXXElectrode Strength
θLoad Angle
PApplied Load or Strength

Variables & Units

Symbol	Variable	Description	Common Units
s	Fillet Weld Leg Size	Visible equal-leg fillet weld size called out on the drawing.	mm, in
te	Effective Throat	Shortest theoretical throat through the fillet weld; for a 45° equal-leg fillet, te = 0.707s.	mm, in
Lw	Total Effective Weld Length	Combined effective length of the weld lines resisting the direct load.	mm, in, ft
Awe	Effective Weld Area	Effective throat multiplied by total effective weld length.	mm², in²
FEXX	Electrode Strength	Filler-metal classification strength, such as 70 ksi for E70 electrodes.	MPa, ksi
θ	Load Angle	Angle between load direction and weld axis used for the optional directional factor.	deg, rad
P	Applied Load or Strength	Direct load on the weld group or calculated weld resistance.	N, kN, lbf

How to Use This Calculator

01Choose Capacity Check when you know the weld leg size and want the available weld strength.
02Choose Required Leg Size when you know the applied load and need the approximate fillet leg size.
03Enter total effective weld length. If two weld lines share the load, enter their combined effective length.
04Enter electrode strength. E70 electrodes are commonly represented as 70 ksi, approximately 490 MPa.
05Set the load angle only if your method permits the directional strength increase. Use 0° for the conservative longitudinal direct-shear case.
06Compare utilization only as a screening result; final weld design must follow the governing welding or structural standard.

How the Formula Works

For a standard 45° equal-leg fillet weld, the effective throat is te = 0.707s, where s is the leg size. The effective weld area is Aweb = te × Lw, using the total effective weld length.

The weld-metal nominal shear stress is taken as Fnw = 0.60FEXX. AISC provisions can permit an increase with load angle, often expressed as 1 + 0.50 sin(θ)^1.5, up to 1.50 at transverse loading. This calculator exposes that factor, but the default 0° case avoids claiming an increase unless the user intentionally enters one.

Nominal strength is Rn = Fnw Awe. LRFD design strength is φRn with φ = 0.75, while ASD allowable strength is Rn/Ω with Ω = 2.0. Required leg size reverses the same equation for the selected method.

Worked Example 01

6 mm E70 fillet weld, 200 mm total length

Known

Leg size: 6 mm
Total weld length: 200 mm
Electrode strength: 490 MPa
Method: LRFD, φ = 0.75

Formula

Rn = 0.60 × FEXX × Awe × (1 + 0.50 sin(θ)^1.5)

Substitution

φRn = 0.75 × 0.60 × 490 MPa × (0.707 × 6 mm × 200 mm)

Result

φRn ≈ 187 kN

The throat is about 4.24 mm and the effective weld area is about 849 mm². LRFD design strength is lower than nominal strength because φ is applied.

Worked Example 02

Required leg size for a 120 kN LRFD load

Known

Applied load: 120 kN
Total weld length: 200 mm
Electrode strength: 490 MPa
Method: LRFD, θ = 0°

Formula

s = P / [factor × 0.707 × Lw × 0.60FEXX × direction factor]

Substitution

s = 120,000 / (0.75 × 0.707 × 0.200 × 0.60 × 490,000,000)

Result

s ≈ 3.85 mm before rounding to an available/code-allowed weld size

The mathematical requirement is only the first step. Real drawings must still satisfy minimum weld sizes, connected-part limits, inspection requirements, and the governing standard.

Applications

01Estimating direct fillet-weld capacity for early mechanical or structural screening
02Back-solving an approximate weld leg size from a direct load and total weld length
03Teaching why weld throat area, not visible leg size alone, controls fillet-weld strength
04Comparing LRFD, ASD, and nominal weld strength for the same geometry

Common electrode strength starting points. Use the actual welding consumable and governing standard.

Electrode class	Nominal strength	Typical entry
E60	60 ksi	414 MPa
E70	70 ksi	490 MPa
E80	80 ksi	552 MPa
E90	90 ksi	621 MPa

Design method factors used by this simplified weld-metal check.

Method	Capacity used	Factor
Nominal	Rn	1.00
LRFD	φRn	φ = 0.75
ASD	Rn/Ω	Ω = 2.0

Assumptions

01Equal-leg, 45° fillet weld with theoretical throat te = 0.707s.
02Direct load distributed over the entered total effective weld length.
03Weld metal strength is based on 0.60FEXX and the selected LRFD, ASD, or nominal method.
04Load-angle strength increase is used only when the user enters a nonzero angle.

Where This Model Stops

01Not a final AWS D1.1, AISC 360, pressure-vessel, lifting, seismic, fatigue, or code-submittal weld design.
02Does not check base-metal yielding/rupture, block shear, connected-part thickness limits, minimum/maximum weld size, weld access, fit-up, inspection class, or weld procedure qualification.
03Does not analyze eccentric weld groups, torsion, bending, out-of-plane loading, intermittent welds, plug/slot welds, groove welds, fatigue, or impact loading.
04The directional strength increase has standard-specific restrictions; use the conservative 0° setting unless your governing method permits the increase.

References

[1]
Engineering FAQs: Welding
AISC
Reference for fillet-weld throat behavior and directional strength caveats in AISC practice.
[2]
Fillet Weld Size Chart - Minimum & Maximum per AISC & AWS
SteelCalculator.app
Reference for AISC-style fillet weld design strength equation, 0.707 throat factor, φ = 0.75, and E70 capacity examples.
[3]
Welding Calculator - Strength of Weld Joints
Omni Calculator
Reference for the basic throat-area times allowable-stress relationship used in simple weld strength checks.

Frequently Asked Questions

Why does the calculator use 0.707 times the weld leg size?

For an equal-leg 45° fillet weld, the shortest theoretical throat is the leg size times sin 45°, which is about 0.707. Fillet weld strength is checked on that throat area, not the visible leg area.

Should I use LRFD or ASD?

Use whichever method your governing design basis requires. LRFD reports φRn with φ = 0.75 in this simplified weld-metal check. ASD reports Rn/Ω with Ω = 2.0.

Can this calculate eccentric weld groups?

No. Eccentric weld groups need weld-line geometry, centroid, polar properties, and often instantaneous-center or elastic vector analysis. This page is only a direct-load fillet weld strength calculator.

Does this prove the weld is safe?

No. It checks only simplified weld-metal strength. Real weld design also needs base-metal checks, minimum/maximum weld sizing, access, procedure qualification, inspection, fatigue if applicable, and local code requirements.

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