Why the perimeter sum is the wrong number for came
Draw a cartoon, measure round every piece, add it up. On the default panel here that comes to 531.1 inches — a shade over 44 feet. Buy 44 feet of came and you will finish the panel with most of a box left, because the sum counts every interior line twice.
Look at any line inside the panel. It has glass on both sides of it, and a single length of H came holds both of those pieces at once. So each interior line appears twice in the perimeter sum and once in the panel. The border is different: it has glass on one side only, and it appears once in the sum and once in the panel. The arithmetic falls out of that. Take the border off the perimeter sum, halve what is left, and add the border back:
came length = (sum of piece perimeters − border) ÷ 2 + border
On the default list that is (531.1 − 84) ÷ 2 + 84, which is 307.5 inches, or 25.6 feet of line to fill. Against the naive 44 feet, you were about to buy 73 percent more came than the panel takes. Both of those are before any waste allowance; the page adds yours on top, which on the default 10 percent makes it 28.2 feet to order.
Foil does not get the halving
Copper foil is applied to each piece separately before anything is assembled. Every edge of every piece gets wrapped, including both sides of every interior line. So for foil the perimeter sum is the answer, and the halving does not apply at all. The same design that needs 25.6 feet of came needs 44.3 feet of foil, both before waste.
The soldering is the other way round. A foiled panel gets a bead run along the entire seam, on both faces, which is why it drinks solder. A leaded panel is soldered only where came meets came, so the solder goes on in dabs at the joints. Two panels, identical design, and the foil one can take ten times the solder. That is worked through on the solder page.
Grouping pieces without drawing a schedule
Five groups gets a mixed panel within a few percent, which is all you need to buy material. Put the pieces into rough size bands and use a representative width and height for each band — the errors cancel because you are summing, not sizing. What does not cancel is systematically underestimating irregular pieces, which is why there is a separate allowance for them.
A wandering cut line is genuinely longer than the rectangle around it. A leaf shape in a 3 by 4 box has a perimeter longer than 14 inches, sometimes by a fifth. The allowance field defaults to 12 percent, which is a guess and is labelled as one. The way to replace it is to lay a piece of string round one of your own pattern pieces and measure it against twice the width plus the height.
The panel there is a check on you, not on the calculator. The glass face area of the piece list should land close to the panel area. If the list totals half the panel, a group is missing; if it totals twice, a quantity is wrong. The page prints both figures side by side so the mistake shows before you order.
Stretching came, and why bars are counted by profile
Came arrives coiled or in soft bars and is pulled straight in a vise or a came stretcher before use. It comes out longer than it went in. How much longer depends on the alloy, the profile and how hard you pull, so the field defaults to 3 percent and asks you to measure your own — put a mark on a bar, stretch it, measure again, and use that number from then on. Setting it to zero is the conservative choice.
The bar count is split between interior and border because they are different profiles and you cannot cut one out of the other. Within a profile, though, came is unusually forgiving of offcuts: a two inch scrap fills a two inch line, and on a busy panel there are plenty of two inch lines. That is why a 10 percent allowance is generous here where it would be thin on trim or flooring.
Foil width is set by the glass, not by the design
The foil has to cover the edge of the glass and fold down onto both faces, so the width you need is the glass thickness plus twice whatever overlap you want. At an eighth of an inch of glass and a sixteenth of overlap each side, that is a quarter inch of foil. The visible line is twice the overlap — an eighth of an inch — before the solder bead crowns over it.
Two things trip people up. Foil is sold in stepped widths, so the number you calculate is unlikely to be one you can buy, and you have to decide whether a slightly wider line or a slightly short fold is the lesser evil. And sheet glass is not one thickness across its own area: the same sheet can measure noticeably different at opposite ends, which means a foil width that wraps beautifully at one end runs short at the other. Measure the actual sheet at several points before committing to a roll.
Questions people ask
Why is the came length so much less than the perimeter of all my pieces?
Because every line inside the panel belongs to two pieces at once and one length of came serves both. Your perimeter sum counts each of those lines twice. The border is the exception — it appears once in the sum and once in the panel — so the arithmetic is to subtract the border, halve the remainder, and add the border back. On the default panel here that turns a 531 inch perimeter sum into 307.5 inches of came. Ordering the full sum is the single most common first-panel mistake.
Does the same halving apply to copper foil?
No, and that is the useful contrast. Foil is wrapped onto each piece individually before assembly, so both sides of every interior line get foiled and the perimeter sum is exactly the length you need. The default design takes 25.6 feet of came or 44.3 feet of foil before any waste allowance. The ratio between the two moves with how many pieces the design is cut into rather than with the panel size, so a small busy panel uses more foil than a large plain one.
What should I put in for the irregular piece allowance?
Measure one. Lay a piece of string round a pattern piece of the kind you actually cut, then measure the string against twice the width plus the height of the box it sits in. The ratio between those two is your number. The 12 percent sitting in the field is a placeholder for a mixed design with some curves in it, and a page of tight scrollwork will beat it badly while a geometric panel with a few softened corners will not get near it.
How wide does my copper foil need to be?
Glass thickness plus twice the overlap you want on each face. An eighth inch sheet with a sixteenth of overlap each side wants a quarter inch foil, and the line you end up seeing is an eighth wide before the solder crowns it. Two practical complications: foil is sold in stepped widths so your calculated figure probably is not buyable, and sheet glass varies in thickness across its own area, so measure the actual sheet at several points rather than trusting a nominal number.
Do I need to allow for stretching the came?
Only if you want the bar count to be right rather than conservative. Came is pulled straight before use and gets longer doing it, so a bar covers more line than its nominal length. How much more depends on the alloy, the profile and your arm, so the field asks for your own measurement and defaults to 3 percent. Setting it to zero simply means you buy slightly more than you need, which is the safe direction to be wrong in.