How the fillet weld calculator works
Tell it what you know about the weld: equal legs, a throat, or two unequal legs. It builds the right triangle the legs make at the root and takes the throat as the shortest line from the root to a flat face. Add a length and the number of identical welds, and it carries the triangle's area through to volume and weight, row by row, so a whole joint list totals itself. Use it to check a size on a drawing, to turn a throat callout into the leg you set on a gauge, or to estimate filler for a job. New to the terms? Start with the fillet weld leg and the theoretical throat.
Fillet weld calculation formulas
Let a and b be the two leg lengths, t the theoretical throat, L the effective length you enter for one weld line, N the number of identical lines, and ρ the selected density. All dimensions must use the same length unit.
- Equal-leg, 90° flat fillet: t = a / √2 ≈ 0.707 × a. Reverse: a = t × √2. Calculations use the exact √2 relation.
- Unequal legs at 90°: t = a × b / √(a² + b²).
- Triangular cross-section: A = a × b / 2.
- Geometric effective area per line: Aₑ = t × L.
- Triangular deposited-metal volume per line: V = A × L. Row volume = N × V.
- Mass in kg: m = V(mm³) × ρ(kg/m³) / 1,000,000,000.
- Volume per meter = A(mm²) × 1,000; mass per meter = A(mm²) × ρ / 1,000,000 kg/m.
- Volume per foot = A(in²) × 12 in³/ft; mass per foot = mass(kg/m) × 0.3048 / 0.45359237 lb/ft.
The throat and triangle-area model follows TWI, Design, Part 2. Our unequal-leg derivation equates the triangle area a × b / 2 with t × √(a² + b²) / 2. Volume is area times length; mass is density times volume. Unit conversions use 1 in = 25.4 mm and 1 lb = 0.45359237 kg.
Convexity, concavity and actual throat
TWI distinguishes the shortest root-to-face distance from the straight-face theoretical throat. A curved face needs its measured profile; maximum bulge alone does not locate the shortest distance.
This tool accepts a user-measured change Δ in that minimum distance: adjusted actual throat = t + Δ for convexity, or t − Δ for concavity. Adjusted actual-throat area = adjusted throat × L. The toes remain on the face, so the adjusted throat cannot exceed the shorter leg. No penetration is assumed.
For example, if a 6 mm equal-leg weld has an independently measured minimum throat 0.2 mm below its flat-face value, select concave and enter 0.2 mm. The estimate is 4.2426 − 0.2 = 4.0426 mm. Entering a 0.2 mm maximum depression without establishing the minimum throat is not sufficient.
Profile adjustment does not change the geometric effective area or triangular volume in this calculator. Actual reinforcement or concavity volume needs a measured cross-section. Convexity is not credited as extra design strength. See TWI's discussion of fillet profiles.
Worked example: 6 mm fillet, 1 m long
For equal legs a = b = 6 mm, flat face, L = 1,000 mm and carbon-steel density 7,850 kg/m³:
- Theoretical throat = 6 / √2 = 4.2426 mm.
- Geometric effective area = 4.242640687… × 1,000 = 4,242.6407 mm².
- Cross-section = 6 × 6 / 2 = 18 mm².
- Volume = 18 × 1,000 = 18,000 mm³ (18 cm³).
- Weight (mass) = 18,000 × 7,850 / 1,000,000,000 = 0.1413 kg, also 0.1413 kg/m.
For two identical weld lines, use quantity 2: total volume is 36,000 mm³ and mass is 0.2826 kg. For unequal legs 6 mm and 8 mm, t = 48 / √100 = 4.8 mm and A = 24 mm². These are original geometry examples, not exam questions.
Worked example in inches: 1/4 in fillet, two 12 in welds
Invented inputs: equal 1/4 in legs, a flat face, and two identical 12 in welds in carbon steel at 7,850 kg/m³.
- Theoretical throat = 0.25 / √2 = 0.1768 in.
- Cross-section = 0.25 × 0.25 / 2 = 0.03125 in².
- Volume = 0.03125 × 12 × 2 = 0.75 in³ for both welds.
- Weight = 0.0965 kg, about 0.2127 lb, for both welds.
Common fillet weld calculation mistakes
- Converting the throat a second time. A 1/4 in size on an AWS symbol is the leg. Multiply its 0.1768 in throat by 0.707 again and you get 0.125 in, a number that describes no part of the weld.
- Using 0.707 on unequal legs. The shortcut holds only when the legs match. A 6 by 8 mm fillet has a 4.8 mm throat, not 0.707 × 6 = 4.24 mm.
- Counting the crown as throat. Convexity makes the actual throat bigger, but design credit stops at the flat face. The effective throat page explains why.
- Measuring across the face. The face runs toe to toe and is longer than either leg: about 8.49 mm on a 6 mm equal-leg fillet. Measure each leg along its own member.
- Counting a double fillet once. A T-joint welded on both sides is two weld lines. Set the quantity to 2 before you read volume or weight.
- Mixing units. Every dimension in a row uses the unit you pick, so convert the drawing first rather than typing inches into a millimeter row.
Density choices and quantity assumptions
- Carbon steel: 7,850 kg/m³. Source: SSAB, Structural hollow sections, precision tubes, line pipes, steel sections and piles: Environmental product declaration (2014), p. 4 (density).
- Stainless steel 304: 7,900 kg/m³. Source: Atlas Steels, Technical Handbook of Stainless Steels, Appendix 3, p. 53 (12 August 2021).
- Aluminum: 2,700 kg/m³. Source: Hydro, Hygroove aluminium tube matches performance – at lower cost (21 April 2010).
These are approximate bulk-material references, not certified weld-deposit densities. Use custom density for the actual filler alloy. The model assumes a uniform right-angle fillet, no root gap or penetration, and a flat triangular deposited cross-section. It excludes face-curvature volume, weld shrinkage, dilution, spatter, stubs and process deposition losses. It does not calculate the consumable purchase weight.
Enter the effective weld length from your approved method; this tool does not deduct ends or gaps for you. It does not determine AWS D1.1 or AISC minimum weld sizes, weld acceptance, capacity, fatigue or allowable shear. A calculated throat is not evidence that a joint meets a code or WPS.
Fillet weld calculator for Excel
Use “Download spreadsheet (CSV)” to open the current inputs, row results, totals, assumptions and citations in Excel or another spreadsheet application. This is a snapshot of values; it does not contain live Excel formulas. Change the inputs here and export again to recalculate.
Fillet weld size FAQ
How do you calculate fillet weld throat from leg size?
For an equal-leg fillet with a flat face on a 90° joint, multiply the leg by 0.707, or divide it by √2 for the exact figure. A 1/4 in leg gives a 0.1768 in theoretical throat and a 6 mm leg gives 4.2426 mm. For unequal legs a and b, the throat is a × b ÷ √(a² + b²), so 0.707 times one leg is the wrong shortcut there.
Is fillet weld size the leg or the throat?
On an AWS welding symbol, the size of an equal-leg fillet is its leg length. ISO 2553 drawings can give either one: a marks the throat and z marks the leg. Check the letter before you convert. This calculator runs both ways: enter a leg to get the throat, or a throat to get equal legs.
How do I find the minimum fillet weld size?
Start with the drawing or WPS. Where AWS D1.1 governs, Clause 7 (subclause 7.13) points to a minimum fillet weld size table read against base metal thickness, and its notes say which part's thickness counts for your process. Open that table in your own edition. This calculator does not pick a minimum size; it converts the size you have.
Does this calculator check fillet weld strength?
No. Strength comes from the effective throat, the effective length and an allowable stress the governing code sets, which for AWS D1.1 is Clause 4 on the design of welded connections. This tool gives you the geometry those checks start from: throat, area, volume and weight.
Is a bigger fillet weld always better?
No. Weld metal grows with the square of the leg, so moving from a 1/4 in to a 5/16 in fillet puts about 56% more metal into every inch of weld, with more heat and distortion to match. Weld to the size the drawing asks for.