Four numbers and two of them are invisible
A wall of applied moulding boxes has a run, a box count, a gap and an end margin. The run is given and the count is a choice, so the layout is really settled by the two spacings — and once the wall is painted a single colour, nobody can see them as dimensions. They are read as rhythm instead, which is why getting them wrong feels like something is off without anyone being able to say what.
With the defaults — a 168 in run, three boxes, 6 in gaps and a 9 in end margin — the boxes come out 46 in wide and 65 in tall. Drop the end margin to match the gap and the same three boxes go to 50 in.
Equal margins or a wider one
Setting the end margin equal to the gap makes the run read as a repeat that happens to stop where the wall does. It is the right answer when the boxes continue round a corner or past a door, because the eye then reads a continuous field. Making the margin wider than the gap frames the group and gives it a beginning and an end, which suits a single wall between two corners.
The page prints what the equal version would give on your run, so the comparison is a number rather than a feeling. Neither is a rule and nothing here says one is correct.
Which lever to pull first
On the default run, going from three boxes to four takes the box from 46 in to 33 in wide — a thirteen inch move, and the box goes from 0.71 of its own height to 0.51. Moving the gap from 6 in to 8 in, by comparison, takes it from 46 to 44-11/16, which is an inch and five sixteenths.
That ordering is not a law, and the page works it out for your numbers rather than asserting it. Swept across runs from 5 to 33 ft, gaps from 2 to 12 in and counts from two to ten boxes, the count is the bigger lever about 89 percent of the time. Where it reverses is the crowded end: ten narrow boxes on a 5 ft wall, where one more box barely changes a width that is already small but two inches on nine gaps moves it a lot. On anything that looks like a normal wall, change the count first.
Where the boxes actually go wrong
Outlets. Switches. A thermostat, a return air grille, the place the old phone jack was. Equal boxes take no view on any of them, and a box edge that lands an inch to one side of a switch plate is the single most common way this treatment fails, because it looks like a mistake rather than a decision. Find them with a tape before anything is cut. The fixes are all cheap at that stage and all expensive afterwards: move the band up or down, change the count so a gap falls where the obstruction is, or put the outlet inside a box and centre it deliberately.
Cut lengths and the mitres
Each box is four pieces and eight mitred ends. The lengths printed are long point to long point, which is what a mitre saw with a stop block cuts to, and the offcut allowance charges the full moulding width for every end. That is the pessimistic reading — opposing mitres cut from one stick sometimes share — but on a 1-1/4 in profile the whole allowance is a few feet across a wall, so buying it is cheaper than counting it precisely.
Two stick counts get printed and they disagree. Packing the actual pieces knows that a 65 in vertical out of an 8 ft stick leaves 31 in that is never another vertical; dividing the ordered footage carries the mitre and waste allowances the pack leaves out. On the defaults that is 9 packed against 8 by footage. Set the end margins equal to the gap and the boxes go to 48 in wide, at which point two of them plus a kerf come to 96-1/8 in on a 96 in stick and the pack jumps to 12. That is not a rounding artefact, it is the eighth of an inch the blade takes, and it is the sort of thing only a piece-by-piece count sees.
What this is not
These boxes are moulding stuck to flat drywall. They are not frame-and-panel wainscot, where the panel sits in a real frame of rails and stiles and the wall behind it is not the panel — that layout is on the wainscot panel layout calculator and the arithmetic is different, because the stiles have width and the panels live between them. Board treatments run in courses are on the wall panel calculator. And if what you are really spacing is framed pictures, the gallery wall layout calculator does centres and hanging heights.
Questions people ask
How far apart should picture frame moulding boxes be?
This page states no spacing. What it does is show you what your spacing produces and what the alternatives give. With the defaults — a 14 ft run, three boxes, 6 in gaps and a 9 in end margin — the boxes are 46 by 65 in. The consequential decision is whether the end margin matches the gap or exceeds it, because that is what makes the run read as a repeat or as a framed group.
Should the end margin be the same as the gap between boxes?
Both are done and neither is wrong. Equal reads as a continuous field, which suits boxes that carry on round a corner or past a door. A wider margin frames the group, which suits one wall between two corners. The page prints the equal-spacing figure for your run alongside whatever you set, so you can compare the two as numbers.
How do I make the boxes come out even on an odd wall length?
Fix the gap and the margin and let the box width take the remainder, which is what this calculator does — every box comes out identical and the odd inches disappear into a width nobody measures. The alternative, fixing the box width and letting the gaps float, gives boxes that match a drawing and spacing that does not repeat, which is the more visible error.
What do I do about an outlet in the middle of a box?
Decide before you cut, because all three fixes are cheap then. Shift the whole band up or down so the outlet falls in the margin, change the box count so a gap lands on it, or leave it inside a box and centre it on purpose. What reads as a mistake is an outlet sitting an inch off a box edge, which is what happens when the layout was set without looking at the wall.
How much moulding does a wall of boxes take?
With the defaults, 2 times 46 plus 2 times 65 is 222 in per box, so 55.5 ft across three boxes, plus 2.5 ft of mitre offcut and 10 percent on top. That orders at 63.8 ft. Packed onto 8 ft sticks piece by piece it takes more sticks than footage alone suggests, because a 65 in side leaves 31 in that is only ever another short side.