Walking the sequence
The page does not divide the rows by the number of colours. It counts full repeats, then walks the leftover rows through the sequence in order, giving each colour what it takes until the rows run out. That is what actually happens on the needles, and it is why a 420 row piece with a 16 row repeat gives colour A one more band than colour D.
On the defaults — 60 inches at 28 rows per 4 inches, so 420 rows, against an A6 B2 C6 D2 repeat of 16 — there are 26 full repeats and 4 rows left. Those 4 go to A, which finishes with 160 rows against C at 156 and the two accent colours at 52 each. Four rows sounds trivial and it is, in yardage; it is not trivial in where the piece stops, because the last band is the one anybody looking at the finished edge sees.
Four roundings cost more than one
Split 900 yards across four colours and each one rounds up to a whole skein by itself. A colour that needs 52 rows worth of yarn — around 111 yards on these numbers — still buys a 220 yard skein, and more than half of it stays in the basket. The page prints what the same total yardage would cost in a single colour so the gap is visible rather than a surprise at the till.
The gap is never negative. Rounding each part up and adding can only ever be more than adding first and rounding once, and it grows with the number of colours and with skein size. A design with two wide colours and two narrow accents pays for the accents twice over: they need little yarn and cost a full skein each.
Ends are a real cost
Every colour change is two ends unless the yarn is carried up the side, and a 16 row repeat over 420 rows produces well over a hundred bands. That is a couple of hundred ends to weave, which for most people is a longer evening than they expected and is the reason narrow stripes get carried rather than cut. Carrying keeps the edge firmer and the end count down to two per colour, at the price of a slightly thicker selvedge that wants to be inside a seam.
The page prints both counts so the decision is made before the first stripe rather than after the hundredth.
Ending where you meant to
A piece that stops four rows into a repeat has an unbalanced last band, and the fix is usually to add or drop a few rows rather than to redesign the stripe. The block at the bottom prints the length at the last full repeat and at the next one, so you can see what landing cleanly costs in inches. On a blanket half an inch either way is nothing. On a sleeve it is the difference between two sleeves matching and not.
One caution on the yardage split. It divides the total by rows, which assumes every row uses the same yarn. That is true of a rectangle at a constant stitch count and false of anything shaped, anything worked in the round on a changing circumference, and anything where one colour carries a denser stitch pattern than another. Where the rows are not equal, work the split by stitches and feed the result in as the total.
Questions people ask
How many skeins of each colour do I need for a striped blanket?
Split the total yardage in proportion to the rows each colour works, then round each colour up to a whole skein separately. That last step is the one people skip, and it is why a striped blanket buys more yarn than a plain one of the same size. The table on this page does both steps and shows how much of each skein is left over.
Why does striping need more yarn than a single colour?
It does not need more yarn; it needs more skeins. Each colour rounds up on its own, so four colours means four part-used skeins instead of one. On the defaults that is a whole extra skein of yarn sitting in the basket. The effect gets worse with more colours and with larger skeins relative to the piece.
How do I make a stripe sequence end on a whole repeat?
Add or remove rows until the row count divides by the repeat. The page prints the row count and the length at the last full repeat and at the next one, so you can see what each costs in inches at your row gauge. On a blanket either is fine; on anything that has to match a second piece, land on the same one both times.
Should I cut the yarn at every stripe or carry it up the side?
That depends on how wide the stripes are, whether the edge will be seamed or exposed, and how much weaving in you are willing to do. The page counts both — ends if you cut at every change, and ends if you carry — so the trade is visible. Carrying leaves a firmer selvedge, which matters more on an exposed edge than inside a seam.
Does the yardage split work for a shaped garment?
Only roughly. It divides yarn by rows, which assumes every row is the same length. A shaped piece, a yoke worked in the round, or a colour carrying a denser stitch pattern all break that. Where the rows differ, split by stitch count instead and enter the result as the total yardage.