The fold depth is an output, not an input
A Roman shade folds where the rings are. The rings sit in horizontal rows, the rows have to be evenly spaced or the folds come out different sizes, and the spacing has to divide the available drop into a whole number of steps. So the fold depth is whatever the division produces, and asking for a round number is asking for a coincidence.
The available drop is not the whole shade. The bottom section, from the hem up to the first ring row, is usually made about half a fold deep so that when the shade is raised the bottom fold matches the ones above it. The top section, from the last ring row to the headrail, is whatever the headrail assembly needs. What is left in the middle is the span, and the span is what gets divided.
Because a fold is half the ring spacing — the fabric between two ring rows folds in half when it is drawn up — the number of steps is the span divided by twice the target fold depth, rounded to a whole number. On a 60 inch shade with a 3.5 inch bottom and a 2 inch top, the span is 54.5 inches. At a 7 inch target that is 3.89 steps, which rounds to 4, giving 13.6 inch spacing and 6.8 inch folds. Ask for 7.5 and it still rounds to 4. Ask for 6 and it goes to 5 steps, 10.9 inch spacing and 5.45 inch folds. There is nothing in between.
Ring columns and why the number is odd
The cords run up vertical columns of rings. Two columns is the minimum, one near each edge. Any more are there to stop the fabric sagging in the middle between the cords when the shade is partly raised, and the wider the shade the more it needs.
The arithmetic is: take the finished width, subtract the inset at each side, and divide what is left by the maximum spacing you are willing to accept. Round that up and add one, because a run of gaps needs one more line than it has gaps. A 36 inch shade with 1.5 inch insets has 33 inches of face; at an 11 inch maximum that is three gaps, so four columns at exactly 11 inches.
The inset matters more than it looks. Set the outer columns too close to the edge and the side hems pull; set them too far in and the outer few inches of the shade hang loose and flare. An inch to two inches is the usual compromise, and the fabric weight decides where in that range you land.
Cord lengths, which are all different
Every column needs a cord that runs from its bottom ring, up the shade, across the headrail to the control side, and then down far enough to be pulled. So the column nearest the control side gets the shortest cord and the far column gets the longest, and the difference is nearly the full width of the shade.
Cutting them all to the longest length is the usual shortcut and it works, at the cost of cord and of a messier gather at the cleat. Cutting each to its own length is tidier and means the ends finish level. Either way, buy the total this page reports and then some, because knots, the run through the cord lock, and the trimming at the end all take more than the geometry suggests.
The rise figure used here runs from the lowest ring row rather than from the hem, since that is where the cord actually starts. On a shade with a deep bottom section that is several inches less than the finished drop.
Cut sizes, and the seam nobody wants
The face fabric is cut wider and longer than the finished shade: side hems on both edges, a bottom hem deep enough to take a weight bar or a doubled turn, and enough at the top to wrap the headrail and staple. Those four allowances are all fields on this page because every workroom does them differently and none of the figures are laws.
The problem case is width. A 36 inch shade with 4 inches of side hem needs a 40 inch cut, which fits comfortably on 54 inch decorator goods. A 70 inch shade needs 74, which does not, and then you are choosing between a vertical seam down the face of the shade or railroading the fabric so its width runs down the drop. A seam shows every time the shade is lowered unless it can be hidden in a dowel pocket. Railroading avoids the seam but only works if the fabric has no direction to it, since the pattern and any nap will be running sideways. The fabric yardage calculator handles that trade-off across a set of pieces.
Cords and children
Loose or looped cords on window coverings are a strangulation hazard to small children. This is a live area of product safety work, with both federal attention and voluntary industry standards, and the rules have changed more than once. This page will not tell you what is required, because what is required depends on the product, the year and the jurisdiction. Ask the manufacturer of the hardware you are using, and check with the Consumer Product Safety Commission, before you decide how a shade in a child occupied room will be controlled.
For the window itself rather than the covering, the window film calculator and the curtain size calculator handle the other two common treatments.
Questions people ask
Why can I not get the fold depth I asked for?
Because the ring rows have to be evenly spaced across the span, and an even division only produces certain depths. On a 54.5 inch span, four steps gives 6.81 inch folds and five gives 5.45; nothing exists between them. If you need a specific depth, adjust the span instead: changing the bottom section by an inch or the top section by half an inch shifts the whole division. That is why both are separate fields here rather than being buried in the formula.
How deep should the bottom section be?
About half the ring spacing is the common approach, so that the fabric below the first ring row folds up to the same visible depth as the folds above it. This page takes your figure and shows what spacing results, so you can run it once, look at the spacing, set the bottom to half of that, and run it again. A deeper bottom section is sometimes used deliberately to give the shade a visible base, and a shallower one when the shade will mostly sit raised.
How many cord columns does a wide shade need?
Enough that the fabric does not sag between them, which is a fabric weight question as much as a width one. Ten to twelve inches maximum spacing is a common working limit and that is the default here, but a heavy lined fabric wants them closer and a light one tolerates more. The count is forced to be at least two, and it goes up in steps because you cannot have a fractional column. Widening the inset slightly is sometimes enough to drop a whole column off a borderline width.
Do I need a dowel in every ring row?
This page counts one per row, which is the version that gives the crispest fold, because the rod holds the fold line straight across the width. Some workrooms skip the top row, where the fabric is close to the headrail and does not need help, and some make an unstructured or hobbled shade with no rods at all, which drapes softly instead of folding sharply. If you skip rows, take that many dowels off the count; the ring rows themselves do not change.
How accurate is the stack height?
It is an estimate built from a number you supply, which is why the stacked thickness per fold is a field. Fold your actual face fabric and lining together the way the shade will fold, press a rule against the stack, and divide by the number of folds you made. A lined cotton is often around three quarters of an inch per fold; a heavy interlined fabric can be twice that. The estimate matters when the shade has to clear the top of the glass when raised, and in that case measuring beats assuming.