Letters are not a width, they are a list
The single most common way a letter layout goes wrong on paper is treating the set as a rectangle. It is not. The widths in the default field — 9.5, 13, 12.5, 12.5, 6.5, 13.5, 11.5 — add to 79 in, and the narrowest is less than half the widest. There is no average that places anything, and there is no way to work out where the fifth letter starts without adding up the first four.
So the output here is a list of coordinates: where each letter starts, where its centre falls, and where the two mounting studs sit in from its edges. That is what goes on the pattern, and it is what an installer with a tape and a laser can actually use.
Two ways to set the spacing
Spacing mode takes the gap you want and centres the result, which is what you do when the letterspacing came out of the design and the fascia is bigger than the word. At the defaults, seven letters at 2.5 in spacing come to 94 in overall on a 144 in fascia, leaving 25 in clear at each end.
Fit mode does the opposite: you set the end margins and it solves the gap that fills what is left. With 6 in margins, the same seven letters get 132 in to live in, 79 of it letters, so the six gaps come out at just over 8.8 in each. That is a completely different sign, and the page says so — a gap wider than the narrowest letter in the set reads as letters near each other rather than as a word, and a wide fascia is usually better solved with a bigger cap height than with more air.
The drift is the actual content of this page
Everything above is arithmetic anyone can do. This part is the one that costs money. If every letter is marked from a single datum — one end of a tape stretched the whole run, or a printed pattern hung once and left up — then each letter carries one measurement error and no more. At a sixteenth of an inch, the worst any letter is out is a sixteenth.
If instead each letter is marked off the one before it, the errors accumulate. Seven letters is six hops. Six hops at a sixteenth is three eighths of an inch if they all lean the same way, and the ordinary result, with errors of random sign, is a little over an eighth. On a fascia where the last letter has to land a fixed distance from a corner, three eighths is visible from the parking lot and is the difference between a set that reads right and a set that comes back down.
A sixteenth is also an optimistic figure for a mark made from a boom lift with a breeze on. Put your own number in and look at what six hops does to it.
Level, and the thing that is never level
Fascias are not level. Shoot both ends and enter the difference, and the page tells you how much of it falls across the letter run rather than across the whole wall. Then there is a decision, and it is a judgement rather than a calculation: set the letters level and the gap to the fascia edge changes end to end, or follow the fascia and the word itself sits crooked. Which one the eye catches depends on how close the letters are to the edge and whether there is anything else nearby that is genuinely level.
Mechanical spacing is not optical spacing
The gap here is between bounding boxes, which is the only thing arithmetic can do. Real letterspacing is not that and never has been. A round letter beside a straight one needs less air than two straight ones. A capital A beside a capital V needs less again, because the diagonals already close the gap. Anyone who has set the word AVAILABLE from a fixed mechanical gap has seen the result.
Treat the coordinates as the layout to start from and then move things on the pattern by eye. The arithmetic gets the set centred, gets the studs in the right place, and tells you what will happen if the marks are chained. It does not have taste.
Nearby
How big the letters need to be for the distance people read them from is on the sign letter height calculator. How this same layout gets turned into a square-footage figure for a permit application — and why three defensible methods give three different answers — is on the sign area measurement calculator.
Questions people ask
How do I lay out individual letters on a wall evenly?
Add the individual letter widths, add the gaps between them, and you have the overall run. Subtract that from the available width and half of what is left is the margin at each end. Then work out the start coordinate of every letter by running along the list. The one thing not to do is treat the letters as equal widths, because they are not close.
Why does my letter set drift by the end of the word?
Because each letter was measured from the one before it. Errors chain: seven letters is six hops, and six hops at a sixteenth of an inch is three eighths if they all lean the same way. Marking every letter from one datum — a tape stretched the whole run or a pattern hung once — keeps the worst error at a single sixteenth no matter how many letters there are.
What letterspacing should I use between dimensional letters?
There is no number, because mechanical spacing between bounding boxes is not what the eye reads. A round letter next to a straight one wants less air than two straight ones, and a diagonal pair like A and V wants less still. Use the calculated coordinates to get the set centred and the studs placed, then adjust by eye on the full-size pattern.
Should letters follow an out-of-level fascia or sit level?
That is a judgement, not a calculation. Setting them level makes the gap to the fascia edge change from one end to the other; following the fascia makes the word itself sit crooked. Which one gets noticed depends on how close the letters are to the edge and whether anything genuinely level is visible nearby. Measure the out-of-level first, because half an inch over twelve feet is ordinary.
How many mounting studs does each letter need?
This page assumes two per letter, inset from each edge, because that is what the coordinates are for. How many a given letter actually needs, and what will hold them in the wall behind the fascia, depends on the letter, the substrate and the installation method, and it is a question for the letter maker and whoever built the wall rather than for a layout calculation.