Girth is not width
Lay a tape across the top of anything with a rise in it and the tape reads longer than a straight measurement between the two edges. That is the whole idea, and it is why a cover cut to the plan dimension of a crowned object comes up short across the middle.
The page models the crown as a circular arc through the two edges and the high point. For the default 96 in width with a 14 in rise, the arc comes out at 101.4 in — 5.4 in more than the chord. Then the cloth carries on down each side by the skirt drop, and gets a hem on each edge, and the whole lot is eased.
The surprise is which number is big
Everybody worries about the crown and almost nobody measures the skirt carefully. On the defaults the crown is worth 5.4 in and the two skirts are worth 36. The crown is a five per cent correction; the skirt is a third of the cut width.
That ordering holds over a wide range of shapes, because an arc is a remarkably efficient way to cross a gap. A rise of a seventh of the span buys you five per cent of extra length. You have to get to a rise of about a quarter of the span before the arc is fifteen per cent longer than the chord. So if a measurement is going to be rough, let it be the crown — and take the skirt drop with a tape from the edge to wherever the cover actually has to reach.
Which way to run the panels
Sixty inch goods with an inch of selvage off gives 59 in of usable width, and joining panels costs half an inch on each side of every seam. To make up the 144.6 in cut width takes three panels; to make up the 250.3 in cut length takes five.
Three panels running the length of the cover is 3 times 250.3, which is 20.9 yd, and two seams totalling 41.7 ft. Five panels running across is 5 times 144.6, which is 20.1 yd, and four seams totalling 48.2 ft. So the crosswise layout saves about three quarters of a yard of cloth and costs six and a half extra feet of seam to sew.
Whether that is a good trade depends on what a foot of double-lapped seam costs you and what a yard of the goods costs. What it does not depend on is any rule about the shape. Both layouts put the same area of cloth into the cover; the only difference is the strip left over in the last panel, and that strip gets multiplied by the length of a panel. So the direction with the shorter panels usually comes out ahead, because its leftover is short — across nearly three thousand cover shapes that held about three times in four, and the exceptions are covers where one span happens to divide the goods width almost exactly. The page prints both because one time in four is not rare enough to ignore.
The seam side of it is close to a coin toss. Over the same set, the layout that saved cloth was the one with more seam to sew slightly under half the time. So there is no shortcut there either: read the two seam figures the page prints and price them against your own yardage.
What a flat pattern cannot do
Everything here assumes the cloth only has to bend one way at a time. It can wrap over a crown running across the cover, or over a peak running along it, and lie flat either way. It cannot lie flat over a surface curving in two directions at once, and that is exactly what the bow of a boat, a rounded corner and a domed top all are.
Compound curvature needs darts, shaped panels or a pattern taken off the object itself with staple tape and marker. There is no yardage formula for that, and a cover made to the flat figure over a shape like that will pull into diagonal wrinkles running from the corners. Treat the number here as the starting quantity for the flat areas and expect to buy more for the shaped end.
The things this deliberately says nothing about
What holds a cover down, and what happens when the wind gets under it. A cover that ponds water is carrying a load nobody planned for, and one that fills like a sail is putting all of it into the hems and the tie-off points. How much any given fabric, seam construction or fixing will take is a supplier and specifier question. This calculator counts cloth.
Questions people ask
How do I measure for a boat or patio cover?
Over the top, not across the plan. Run a tape from one edge, over the crown, to the other edge — that is the girth, and it is what the cut width is built from. Then measure how far below each edge the cover has to hang, add the hem on each edge, and add whatever ease you want so the cover is not stretched tight. The page does the arc arithmetic if you would rather give it the width and the rise.
How much extra fabric does a crowned top need?
Less than most people assume. A 14 in rise across a 96 in width makes the arc 101.4 in — 5.4 in, or about five and a half per cent, more than the straight measurement. You have to get to a rise of roughly a quarter of the span before the arc runs fifteen per cent long. The skirt drop is the number that actually dominates the cut width.
Should cover panels run along the length or across?
The page prints both, because there is no reliable shape rule. On the defaults, panels running across use about three quarters of a yard less cloth and add six and a half feet of seam to sew. The mechanism is that both layouts hold the same area and differ only in the leftover in the last panel, which is multiplied by the panel length — so the direction with the shorter panels wins about three times in four. Seam is close to a coin toss: over nearly three thousand shapes, the layout that saved cloth was the one with more seam slightly under half the time, so the yardage line alone does not tell you which job is cheaper.
Can this calculate a cover for a curved bow?
No, and it says so. A flat panel bends over a crown running one way and lies flat doing it, but nothing flat lies over a surface that curves in two directions at once. A bow, a rounded corner or a dome needs darts, shaped panels or a pattern taken directly off the object. Use the figure here for the flat run and expect to buy extra for the shaped end.
How much ease should a cover have?
That is a field rather than a number this page supplies, because it depends on the fabric, on how the cover is held down and on whether it has to go on over anything. The default sits at three per cent as a placeholder. Too little and the cover is drum tight over the frame with all the load in the seams; too much and it flogs in the wind.