A bead is a volume, not a length
The instinct is to think in wire: so many feet of seam, so many feet of solder. That only works if the wire and the bead happen to have the same cross-section, and they usually do not. What the seam actually consumes is a volume of metal, and the volume is the shape of the bead multiplied by how far it runs.
A finished bead is roughly half an ellipse sitting on the glass — as wide as the line, as high as the crown. Its area is π × width × height ÷ 4. At the default 3/16 inch wide and 1/16 inch high that is 0.0092 square inches. Run that along 25.6 feet of seam on both faces, which is 614 inches of run, and you get 5.65 cubic inches of solder before any waste.
Turning that into weight needs a density, and density is where alloys differ enough to matter. That field is deliberately a plain number with no alloy named beside it, because the right figure is the one on the data sheet for what you actually bought. At the placeholder 0.30 pounds per cubic inch, 5.65 cubic inches is 1.70 pounds bare, or 1.87 with the ten percent allowance.
Why wire length is not seam length
Once you have a volume, the wire length is that volume divided by the wire cross-section. Eighth inch wire has a section of 0.0123 square inches, which is a third more than the default bead. So one inch of wire lays down about a third more than an inch of seam, and 614 inches of run needs about 461 inches of wire for the bare volume, or 507 once the ten percent allowance is on.
Change to a thinner wire and it flips: at a sixteenth of an inch the wire section is 0.0031 square inches, a third of the bead, and now you feed three inches of wire for every inch of seam. Neither is better. It changes how often you stop to reload and how much heat the wire pulls out of the joint as it melts, and those are bench preferences, not arithmetic.
Came is a completely different quantity
On a foiled panel the metal runs everywhere. On a leaded panel the came is what holds the glass, and the solder only has to lock the joints where came meets came. So the input changes from a length to a count: how many joints, and how big a dab at each.
Treating each dab as a dome of a given diameter, a quarter inch dome is 0.0041 cubic inches. Sixty joints on both faces is 120 dabs, which is 0.49 cubic inches — under a tenth of what the same design took as a foiled panel. At the placeholder density that is 0.15 pounds bare, 0.16 with the allowance, against 1.87. Same drawing, same glass, an order of magnitude apart in solder.
Counting joints sounds tedious and is a one-off. Do it on the cartoon with a pencil before you cut anything and the number belongs to that design permanently, however many times you build it.
Flux and patina: the page has no coverage rate
There is no published figure here for how far a bottle of flux or patina goes, because it depends on the product, the applicator, the temperature and how heavy-handed you are — and because a calculator asserting a coverage rate for a chemical is exactly the kind of confident wrong number that is worth avoiding. Both fields arrive with a placeholder and a request to replace it.
Getting your own number takes one panel. Mark the bottle level, do the job, mark it again, and divide the seam footage by what went. That figure will be right for your bench and nobody else, which is what you want. Note also that on a came panel the flux goes onto joints while the patina goes over every line, so the two will not scale together.
What this page will not tell you
It will not name an alloy, recommend one, or tell you what is in the one you have. Lead-bearing solders and lead came are a cumulative exposure that leaves the studio on your hands and your clothes, and the handling, ventilation, hygiene and disposal side of that is governed by published occupational guidance and by the safety data sheet for the specific product. None of that belongs in a calculator and none of it is here. Read the data sheet, and read the guidance from your own occupational health authority, before the box is open.
Questions people ask
How much solder does a stained glass panel take?
It depends far more on the construction than on the size. A foiled panel gets a continuous bead along every seam on both faces, so the answer scales with seam length: the default here is 25.6 feet of seam at a 3/16 by 1/16 bead, which works out at 5.65 cubic inches, or 1.87 pounds at the placeholder density with waste. The same design in came needs solder only at the joints — 60 joints on both faces at a quarter inch dab is 0.16 pounds. Under a tenth.
Why does the page ask for a bead width and height instead of just the seam length?
Because the seam length tells you how far the metal runs and nothing about how much of it there is. A bead is a volume: its cross-section, which is close to half an ellipse of the width and crown you actually run, multiplied by the run. Two people soldering the same panel to different bead profiles will use noticeably different amounts of solder, and only the profile explains it. Measure a seam you were happy with and use those numbers.
What density should I enter for the solder?
Whatever the data sheet for your alloy says. This page deliberately names no alloys and publishes no densities, because solder alloys differ enough that a wrong assumption throws the weight out by a useful fraction. The 0.30 pounds per cubic inch sitting in the field is a placeholder to keep the arithmetic visible, not a claim about anything you might buy. If you have the spool in front of you, the number is on the sheet that came with it.
How far does a bottle of flux go?
This page does not say, and you should be suspicious of anything that does. It depends on the product, how you apply it, the temperature and your own habits. What it does instead is take a coverage figure from you and apply it. To get one, mark the level on a bottle, solder a panel whose seam length you know, mark it again, and divide. One job gives you a number that is right for your bench, which beats any published figure.
Is the leftover on the last spool worth worrying about?
Only for ordering. The page prints it because solder is bought in whole spools and the difference between needing 1.87 pounds and buying two one-pound spools is 0.13 pounds sitting on the shelf, which is fine, whereas needing 2.05 pounds means a third spool for a fifth of a pound. Seeing that before you order occasionally changes whether you trim the bead a little or just buy the extra spool.