Two ways to set the grid out, and they are not the same job
Equal coffers is the default here. You say how many you want each way, the perimeter band and the interior beams take their width off the room, and whatever is left divides evenly. With the defaults — a 204 by 156 in room, four coffers by three, 6 in beams and a 6 in band — every coffer comes out 43-1/2 in by 44 in.
Fixed field is the other approach: you set the size of the inner coffers and let the two against each pair of walls absorb the remainder. This is what you do when the coffer size is being driven by something else, a light fitting layout or a ceiling medallion, and it is also what happens by accident when somebody sets the grid out from one corner and stops thinking. The difference is that here you can see the border size before you cut.
When the border coffer stops looking deliberate
A border narrower than about two thirds of the field reads as a mistake, and the page flags it at that point with the number rather than an opinion. Nothing makes that threshold a rule — it is a place to stop and look. The two fixes are opposite: make the field smaller until everything matches, or make the perimeter band wider so the border coffer grows back. The second keeps your field size and costs you ceiling; the first keeps the ceiling and gives up the module.
What the lineal feet actually count
The perimeter band is four runs mitred at the corners, so it is simply twice the room length plus twice the width. The interior beams are counted at their full clear length between the bands, which is what you buy, and the page also tells you how many segments they become if you cut the long-direction beams at every crossing instead of running them through. With the defaults that is 2 continuous runs against 8 segments, and the difference matters at the lumber rack rather than in the total.
Board run is beam run times the boards-per-foot figure in the form. Three is a plain box: one face and two sides. Add a bottom and it is four, wrap a second layer or a build-up strip and it goes up again. That number is a field rather than a constant because nobody else can see your section.
Working back to the board you cut
The finished face width is what you see from below, and it is not the width of the face board when the two sides overlay it. Take twice the board thickness off. Six inch finished face out of 3/4 stock means a 4-1/2 in face board with two 3/4 sides wrapping it. Build it the other way, with the face lapping over the sides, and the face board is the full 6 in and the sides are hidden. Both get built; they differ in which edge grain shows and in what a mitre at the corner has to do.
The part this page has no view on
Everything here is decoration. It counts boxes and the wood they take. It says nothing about how those boxes attach to the ceiling, what the ceiling above them will carry, or whether a thing that looks like a beam is doing any structural work. If a member in that ceiling is holding anything up, or if a fixture is going to hang from a coffer, the question belongs to an engineer and to whoever inspects the job. No calculator can see inside a ceiling and this one does not pretend to.
The practical hazards are the ordinary ones and they get named, not explained: this is all overhead work on staging, the boxes go up in awkward long pieces that want two people, and a nail gun fired up into a ceiling has no idea what is running through the joist bays. Find the services first.
For packing the board run onto sticks, the cut list optimizer takes a piece list and shows the layout per stick. Board feet converts the run if the supplier prices by volume. Crown or cove where the beams meet the ceiling has its own geometry on the crown molding projection calculator.
Questions people ask
How big should a coffer be?
This page states no size and no proportion. It takes the counts you give and reports what the openings come out at, along with the two neighbouring counts so you can compare. With the defaults, four by three on a 17 by 13 ft room gives coffers 43-1/2 by 44 in. Whether that suits the ceiling height is a judgement about the room.
Should the coffers all be the same size?
That is the choice the page is built around. Equal coffers divide whatever is left after the beams, so the size comes out where it comes out. Fixed field lets you set the inner coffers and pushes the remainder into the border ones. The failure mode is a border coffer that ends up much narrower than the field, which reads as an error rather than a border, and the page flags it when it drops below about two thirds.
How many lineal feet of beam does a coffered ceiling take?
With the defaults — 17 by 13 ft, four coffers by three — 60 ft of perimeter band plus 68 ft of interior beam, so 128 ft in total. At three boards per beam foot and 15 percent waste that is 441.6 ft of board, or 37 sticks at 12 ft. Change the section and the multiplier changes with it.
What width board do I cut for a 6 inch beam face?
It depends which way the box is assembled. If the two side boards lap over the face, the face board is the finished width less twice the board thickness — 4-1/2 in out of 3/4 stock. If the face laps over the sides, the face board is the full 6 in. The page reports the first version because it is the more common one and the easier to get wrong.
Can a coffered ceiling carry a light or a fan?
That is not a question this page answers, and no calculator should answer it. What is printed here is lineal feet of hollow decorative boxwork. Whether anything in that ceiling can carry a load depends on the framing above it, on how the boxes are fixed, and on what the inspecting authority requires. That goes to an engineer.