Fullness is the number, and it is not a style choice
Fullness is the ratio of finished panel width to rod width. A 52 inch rod with 104 inches of panel across it is 2x. The same rod with 78 inches of panel is 1.5x. That single ratio decides whether the curtain reads as a curtain or as a flat sheet with a gathered top, and it is the thing that separates a made-to-measure pair from a bagged one.
Ready-made panels are usually sold 50 inches wide. Two of them across a 52 inch rod is 100 inches, which is 1.9x, which is fine. Two of them across an 84 inch rod is 1.2x, which is not. The panels do not change size when the window does, and the packaging never states a fullness, so the ratio quietly collapses as windows get wider. This is the most common reason curtains look wrong for reasons the owner cannot name.
| Fullness | What it looks like | Where it belongs |
|---|---|---|
| 1.2 - 1.5x | Shallow waves, flat between folds | Fixed side panels that never close, some sheers, blackout liners |
| 2x | Continuous soft folds when closed | The normal working figure for most rooms |
| 2.5x | Deep, even folds that hold their shape open or closed | What a workroom quotes unless told otherwise |
| 3x | Dense gather, reads as fabric rather than panel | Sheers, voile, heavy pleated drapery |
Fullness applies to the rod, not the window. That is worth saying twice, because it is where the arithmetic usually goes wrong. If you run a rod 8 inches past each side of a 36 inch window to give the open panels somewhere to sit, you are dressing 52 inches, and every fullness figure multiplies 52, not 36.
Where the top of the panel actually is
The finished length is measured from the top edge of the panel to the bottom of the hem, and the top edge is not the rod. Each header type puts it somewhere different, which is why the same rod height gives four different panel lengths.
A rod pocket has the rod threaded through a channel below the top edge, so the fabric above the channel stands up as a small ruffle and the panel top is above the rod. A grommet sits just under the top edge, so the panel top is about an inch above the rod. Pinch pleats hang from rings, and the ring drops the panel an inch or two below the rod. Tab tops drop it further still, which is why tab-top curtains bought to a stated length so often finish short. The calculator uses the ordinary allowances for each; if you are using deep rings or long tabs, measure from the rod to where the top of a hanging panel actually lands and adjust the rod height rather than the length.
The length target itself is a decision, not a measurement. Floor length means half an inch clear, which looks intentional and does not drag. Sill length stops just above the sill and suits a window over a counter or a radiator. Apron length runs a few inches below the sill and is the one that most often looks accidental, because it lands nowhere in particular. Puddling puts extra fabric on the floor deliberately and needs a fabric with enough weight to fall rather than crumple.
Yardage, and exactly what the number assumes
The yardage figure here is built one cut at a time, not from an area. Each full-length cut is the finished length plus the header allowance plus the bottom hem. If the fabric has a vertical repeat, every cut then rounds up to the next whole repeat so the motif sits at the same height on every panel. The number of cuts is the number of fabric widths across all panels, rounded up. Multiply, divide by 36, round up to the quarter yard.
What that number assumes, plainly:
- The fabric is the width you entered, and the pattern runs up the roll rather than across it. Wide sheers are often sold railroaded, meaning the pattern runs along the length, and then the whole calculation inverts — you cut across the width and the yardage collapses.
- All the cuts come off one continuous piece. Buying a remnant in two pieces does not work if the repeat has to align.
- There is no horizontal repeat to match across a seam. If there is, add roughly one horizontal repeat per seam.
- It is face fabric only. Lining is a separate calculation with no repeat and usually a different bolt width, and interlining another again.
- Nothing is set aside for tiebacks, a valance, or a mistake. Buying an extra half yard on a patterned fabric is cheap insurance, and dye lots on decorator fabric vary the same way wallpaper does.
Measure twice, and measure the rod first
Everything downstream depends on where the rod goes, so put the rod up, or at least mark and hold it, before cutting anything. Mounting the rod higher than the window is the cheapest way to make a room look taller, and mounting it wider is the cheapest way to make a window look bigger, but both change the drop and the width by inches that no amount of hemming recovers. Once the brackets are on the wall, measure from the rod to the floor at both ends and in the middle. Floors are not level and old ones are not close. If the three measurements differ by more than half an inch, cut to the shortest and let the hem take the rest, because a curtain that touches down at one end and floats at the other is more visible than one that clears the floor evenly.
Questions people ask
How wide should curtains be for a 36 inch window?
It depends on the rod, not the window. Run the rod 8 inches past each side, which is the usual allowance so the open panels sit off the glass, and you have a 52 inch rod. At 2x fullness that is 104 inches of finished panel, so two panels of about 52 inches each. Two ready-made 50 inch panels come to 100 inches, which is 1.9x, close enough. The mistake is to buy panels sized to the 36 inch window: two 20 inch panels would technically cover it and would look like tea towels.
How many yards of fabric for a pair of floor-length curtains?
For a 90 inch drop on 54 inch fabric with a doubled 4 inch hem and an 8 inch header allowance, each cut is 106 inches, which is just under 3 yards. A pair at one width per panel is two cuts, so about 6 yards. At one and a half widths per panel it is three cuts and about 9 yards. The width count is what moves the total, not the drop, so work out the widths first. Add a pattern repeat and every cut rounds up: a 24 inch repeat takes that 106 inch cut to 120 inches, which is nearly 4 extra yards across three cuts.
Should I add the pattern repeat if the fabric is plain?
No. Enter zero. The repeat only exists to make the motif land at the same height on every panel, and a plain or textured fabric with no directional pattern has nothing to align, so every cut is exactly the finished length plus the hems. Watch for two exceptions: a fabric with a nap or sheen, like velvet, has a direction even without a pattern and every cut has to run the same way, and a large-scale one-way print may have a repeat that the bolt label states in one direction only.
Does this work for a bay window or a corner?
Not directly. Work each straight run as its own calculation and add the results, because the rod width is what drives everything and a bay has three of them. The part the arithmetic will not give you is the returns and the joins: at each angle the panels have to meet or overlap, which costs a few inches of width per corner, and the stacking space has to go somewhere that is not the next window. For a bay, add roughly half a width per corner and expect to make a decision about which panels stack where before you cut.