Perimeter, not height
Return coil is bought by the linear foot and consumed by the perimeter, so the only measurement that matters is a tape run around the outside of the pattern. Letter height correlates with it and does not determine it: a 12 in O and a 12 in I have wildly different perimeters, and a script letter with an entry and exit stroke can have more perimeter than a block letter twice its cap height.
Take the perimeter off the full-size pattern, not off a scaled drawing, and average across the set. Measuring the widest letter and multiplying is how a set of coil arrives long; measuring the I is how it arrives short.
The counters are the part that gets missed
Every enclosed hole is a second closed return with its own bend, its own closure and its own lap. A, B, D, O, P, Q and R each carry one. B and a lowercase g carry two. In a twelve-letter sign it is entirely ordinary to have four or five of them, and they never appear in a take-off written as height times count.
Run the defaults and the arithmetic is plain. Twelve letters at 46 in of outline is 552 in. Four counters at 12 in is 48 in, and sixteen closures at a 1 in lap is another 16 in — 64 in in total, which is 11.6 percent on top of the outline figure. The waste allowance in the same form is set to 10 percent, and because it applies to the whole 616 in it adds 61.6 in, or 11.2 percent of the outline figure. On these numbers the parts that got left out are larger than the allowance meant to absorb them, and the page prints that comparison for whatever you actually enter rather than asserting it in general.
Coil width is not return depth
Coil arrives slit to a width and the letter has a depth, and they are not the same number. The difference is what the back flange and the trim cap have to live in, and it is what the brake setup depends on. With a 5 in return out of 5.5 in coil there is half an inch to work with, and that half inch is not spare — it is the flange.
Buy coil at exactly the return depth and there is nothing left to fold. The page reports the leftover as its own line for that reason, and if the coil is narrower than the depth it says so plainly rather than quietly producing a footage for a letter that cannot be built from it.
Trim cap runs the face edge, and sometimes both
Trim cap follows the same perimeter as the return, including the counters, so it scales with the same number. Capping both edges doubles it. The roll length matters more than it looks: a set that needs 105 ft of cap out of 100 ft rolls buys two rolls and puts 95 ft on the shelf, which is a real cost that belongs in this job rather than the next one.
What a linear figure cannot see
Notching. A tight radius on a script letter is formed by notching the flange, and the material that comes out of those notches is not in any perimeter measurement. Neither is a return that has to be seamed because the roll ran out mid-letter, which is a decision about where the seam goes rather than about footage. The number here is what to order; the person at the brake is who finds out whether it was enough.
The rest of the letter
Faces and backs come out of sheet, and parts-per-sheet with the offcut is on the sheet nesting calculator. What goes inside the can is on the channel letter LED module calculator. Anything about how the finished set attaches to a building, and who is allowed to attach it, is a question for the engineer and the jurisdiction rather than a material take-off.
Questions people ask
How do I calculate return coil for channel letters?
Add the outer perimeter of every letter, add the perimeter of every counter hole, then add one lap for each closed loop. Multiply by your waste allowance and divide by twelve for feet. The perimeters come off the full-size pattern with a flexible tape, not off letter heights, because two letters of the same height can differ by half again in perimeter.
Do I need to count the holes in letters like O and A?
Yes. Each enclosed hole is a separate closed return with its own bend and its own lap, and it is the single most commonly missed item in a letter take-off. On the default twelve-letter set the four counters and the sixteen laps together come to 64 in, which is 11.6 percent of the outline figure against the 11.2 percent that the ten percent waste allowance adds.
What coil width do I need for a 5 in deep letter?
More than 5 in, because the difference between the coil width and the return depth is the flange the back and the trim cap fold into. How much more is a property of your brake and your trim cap, not a general figure. The page reports the leftover so you can look at it against what your setup actually needs.
How much trim cap does a letter set take?
The same perimeter as the returns, counters included, once for the face edge and twice if you are capping the back as well, plus a lap at each closure. Then round up to whole rolls, because a set needing 105 ft out of 100 ft rolls buys two and the second one mostly goes on the shelf.
Why is my coil order always short on script letters?
Notching. A tight radius is formed by cutting notches out of the flange, and that material is not visible in any perimeter measurement. A perimeter take-off is right for block letters and optimistic for script, which is one reason the waste allowance on a script set is set higher by shops that build a lot of them.