What this calculator does
Find out how much metal your welds will hold, and how much wire or rod to buy for the job. That weight drives the filler metal order, the arc time and much of the cost. It comes from simple geometry. You take the area of the weld's cross-section (its shape if you sliced across it), times the length of weld, times the density of the metal.
This calculator finds the cross-section of a fillet weld (the triangle of metal in a corner joint) or a butt weld between two plates. Butt welds can be single-V, double-V or square-groove. It adds an allowance for the weld's raised crown and bulge, then multiplies through to the weight deposited.
Then it divides by a deposition efficiency, because not all the wire or electrode you buy ends up in the joint. Stub ends, spatter and slag take a share. Weld sizes for structural work come from the drawings and the welding code that applies. This is an estimating tool, not a substitute for the welding procedure or the engineer.
Where it's used
Why this math matters outside the classroom:
- Job estimates. A shop estimating 10 ft of 1/4 in fillet in steel finds about 1.17 lb of weld metal and orders 1.23 lb of wire.
- Electrode orders. With stick electrodes at 55% efficiency, 1 kg of weld metal needs about 1 ÷ 0.55 ≈ 1.82 kg of electrodes.
- Joint design. Engineers compare a single V with a double V on 25 mm plate to cut the filler needed by beveling both sides.
- Shop class. Welding students find a 6 mm fillet's area as 6 × 6 ÷ 2 = 18 mm², then 18 cm³ for each meter of weld.
Grade level and standards: Grades 7–9; Common Core State Standards for Mathematics 7.G.B.6, HSG.SRT.C.8. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose inch or metric units and the weld type.
- Enter the fillet legs. For a butt weld, enter the plate thickness, root gap, root face and groove angle. Then enter the total weld length.
- Set the reinforcement allowance, the weld metal and the welding process.
- Read the weld metal deposited and the filler metal to buy.
What the inputs mean
- Leg 1
- The legs are the two sides of the fillet, one along each plate. A fillet is sized by its leg, so a 1/4 in fillet has 1/4 in legs. For unequal legs, enter each one.
- Leg 2
- Same as leg 1 for an equal-leg fillet.
- Root face (land)
- The root face, or land, is the flat strip left at the bottom of a beveled plate. The V doesn't fill it. So it's subtracted from the thickness before the V is worked out.
- Included groove angle
- The full angle of the V, twice the bevel on each plate: 60° for two 30° bevels.
- Allowance for reinforcement and convexity
- Real welds are fuller than the flat shape on paper. Groove welds have a crown and some extra metal at the root, and fillets often bulge outward slightly. A percentage allowance covers this without measuring each bead.
- Process (deposition efficiency)
- Deposition efficiency is the weight of weld metal deposited divided by the weight of filler used up. The defaults are typical figures from a filler-metal maker's consumption guide. Your consumable's data sheet may give its own.
- Deposition efficiency
- The share of filler weight bought that ends up in the weld; the rest is stubs, spatter and slag.
Formula and method
fillet area = leg 1 × leg 2 ÷ 2 single-V area = gap × t + (t − f)² × tan(α ÷ 2) double-V area = gap × t + 2 × ((t − f) ÷ 2)² × tan(α ÷ 2) square-groove area = gap × t weld metal = area × (1 + reinforcement ÷ 100) × length × density filler = weld metal ÷ (deposition efficiency ÷ 100)
t is the plate thickness, f is the root face and gap is the root gap between the plates. α (alpha) is the included groove angle, the full angle of the V. tan is the tangent, a standard function that turns an angle into a ratio of width to height.
A flat-faced fillet is a right triangle, with the legs as its two short sides. A V-groove is a rectangle as wide as the root gap and as tall as the full thickness t. On top of that sits a triangle on each side. Each triangle's height is the beveled depth, t − f, and its width is that depth times the tangent of half the angle α.
A double V splits the beveled depth into two equal Vs. That's why it needs less metal than a single V on the same plate.
Volume is area times length. In inch units, the density is converted using 1 in³ = 16.387064 cm³ and 1 lb = 453.59237 g. That turns 7.85 g/cm³ steel into about 0.2836 lb/in³. In metric units, mm² × mm gives mm³. Divide by 1,000 for cm³, and then by 1,000 again to go from grams to kilograms.
Worked example
A fabricator has 10 ft of 1/4 in equal-leg fillet weld in carbon steel. It's made with solid wire, allowing 10% for reinforcement.
- Cross-section: 0.25 × 0.25 ÷ 2 = 0.03125 in², and with 10% reinforcement 0.034375 in².
- Volume: 0.034375 × 120 in = 4.125 in³.
- Density: 7.85 × 16.387064 ÷ 453.59237 = 0.283599 lb/in³.
- Weld metal: 4.125 × 0.283599 = 1.170 lb.
- Filler at 95% efficiency: 1.170 ÷ 0.95 = 1.231 lb.
Result: The weld holds about 1.170 lb of metal, so plan on about 1.231 lb of wire.
Understanding the result
Weld metal weight grows with the square of the size. So doubling a fillet's leg makes four times the metal. Oversized fillets are one of the most common causes of wasted filler and arc time. That's why welding to the specified size matters.
The filler figure is the least you should buy. Add extra for tack welds, repairs, starts and stops, and so you don't run out partway through a spool or packet.
Assumptions and limitations
- The weld has the ideal shape, with the same size along its whole length.
- The reinforcement allowance covers the crown, any bulge and root penetration (metal that pushes through the back of the joint).
- The deposition efficiency is an average for the process and consumable.
- Bevel-and-fillet combinations, J and U grooves, backing bars and partial-penetration welds aren't modelled directly.
- Weld metal density varies slightly with the alloy. Use the custom density for other materials.
- How much stick electrode you use depends heavily on stub length and technique. So real use can differ noticeably from the estimate.
Frequently asked questions
How much does a fillet weld weigh per foot?
With no reinforcement, a 1/4 in steel fillet has 0.03125 in² of cross-section. That's 0.375 in³ per foot, or about 0.106 lb per foot. A 3/8 in fillet is 2.25 times that, about 0.239 lb per foot.
What is deposition efficiency?
It's the share of the filler metal you use up that ends up as weld metal. Wire processes such as solid-wire GMAW lose little to spatter. Stick electrodes lose their stub ends and coating. So you need much more electrode weight for the same weld.
Why does a double-V groove use less filler than a single V?
The V's area grows with the square of its depth. Splitting the beveled depth into two Vs, each half as deep, halves the total V area for the same angle. That's why thick plates are often beveled from both sides.
Does this tell me how long the weld will take?
Not directly. But you can divide the weld metal weight by your process's deposition rate, in pounds or kilograms per hour. That gives arc-on time. Then allow for setup, cleaning and inspection.
Sources
- Kobe Steel, The ABC's of Arc Welding: How to calculate welding consumption (checked 2026-10-19)
How this calculator is checked
Method: Cross-section areas from triangle and trapezoid geometry; weight = area × length × density; consumption with deposition efficiency per the cited manufacturer guide (SMAW 55%, GMAW 95%, FCAW 90%, SAW 100%; steel 7.85 g/cm³). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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