The grid steps, it does not slide
Set a block size and a sashing width and you have fixed the step. Twelve inch blocks with two inch sashing move in fourteen inch jumps: five across gives 72 inches of grid, six gives 86, and there is nothing at 78. A target size is therefore a request, not an instruction, and the border is the only part of the quilt that can take any width you like.
This is worth knowing before you cut, because the usual failure is the other order: someone cuts sixty blocks, sews them up, and then finds the quilt is 74 inches on a bed that wanted 90. At that point the only lever left is a very wide border, which looks like what it is.
Two ways round it. Change the block size — an eleven inch block with two inch sashing steps in thirteens and lands differently. Or decide the border first: pick the number you actually want, subtract twice the border you want to look at, and see which grid fits inside what is left.
What sashing costs you, in pieces rather than inches
Sashing looks cheap. Two inches of sashing between the blocks and round the outside of a five by seven grid adds twelve inches across and sixteen down, which is a modest amount of cloth. The cost is not cloth.
On that grid with sashing all round and cornerstones, you cut 35 blocks, 42 short sashing pieces down the rows, 40 more across the cross rows, and 48 cornerstones. That is 165 pieces where the plain grid had 35. Every one of them is a cut, a placement and at least one seam, and every seam is a chance for a quarter inch to become three sixteenths.
Cornerstones are the trade. Skip them and the cross sashing runs in one strip the width of the grid, which is far fewer pieces — but that one strip has to carry every block seam into alignment across the whole quilt, and any accumulated error shows up as blocks that do not line up column to column. Cornerstones give each intersection a fixed reference point. Neither is right; they fail in different directions.
Cut size, finished size, and the quarter inch
Everything on this page is stated twice: the finished size, which is what the piece measures once it is sewn in, and the cut size, which is finished plus a seam allowance on every edge. A twelve inch finished block is cut at twelve and a half. Two inch finished sashing is cut at two and a half. A cornerstone that finishes two inches square is cut two and a half square, the same as the sashing width, which is why the two are usually cut from the same strip.
The convention is a quarter inch, and the convention is also that a lot of quilters sew a scant quarter — a thread or two under — because the fold of the pressed seam takes up a fraction of the fabric. If your blocks come out consistently small, that is what is happening, and the fix is at the machine rather than in the cutting. The seam allowance and cut size calculator works the same arithmetic for a single piece across a run of sizes.
Borders, and why they get pieced
A border strip for a 74 inch quilt has to be 74 inches long. Fabric off a 44 inch bolt is 43 inches of usable width. So a crosswise-cut border is always joined, and this page counts the widths of fabric it takes rather than pretending one strip does it.
The alternative is cutting borders lengthwise, parallel to the selvage, where you can get any length the piece of fabric is long with no join at all. That costs more fabric — you are buying length you will not use across the width — and it gives you a border on the straight lengthwise grain, which stretches less than the crosswise. On a large quilt that stability is worth something. On a small one it rarely is.
Whichever you choose, measure the quilt through the middle rather than along the edge before cutting borders. The edge of a pieced top has bias in it and will happily stretch two inches to meet a border that is too long, and then the quilt will not lie flat.
What this page does not know
It does not know what is inside your blocks. A block made of sixteen small squares uses far more cloth than its finished area because each of the 24 internal seams consumes two seam allowances; the half square triangle calculator handles the most common case of that. It does not price fabric shrinkage, which the fabric shrinkage test calculator measures from a swatch. And it stops at the quilt top: backing, batting and binding are a separate purchase, worked out by the quilt backing and binding calculator.
Rotary cutters are the reason quilters have scars. The blade is exposed whenever the guard is not closed, it is sharper than it looks, and it cuts skin more easily than it cuts cotton.
Questions people ask
How wide should sashing be?
There is no correct width, and the calculator takes yours rather than suggesting one. What is worth knowing is the proportion: sashing reads as a frame when it is roughly a sixth to a tenth of the block, and as a design element in its own right when it goes wider than that. Two inch sashing on a twelve inch block is a thin line between blocks. Four inch sashing on the same block starts to look like a grid of windows, which may be exactly what you want. Cut a strip of the actual fabric and lay it between two finished blocks before you commit to 160 pieces.
Why does the calculator give a different quilt size than I asked for?
Because a grid of whole blocks can only make certain sizes. With twelve inch blocks and two inch sashing the grid steps in fourteen inch increments, so a 64 inch target lands on 58 or 72 depending on which way it rounds, before borders. The border is the adjustable part: it is the only element whose width is not tied to a repeating unit. If you need an exact finished size, decide the grid first, then set the border to whatever closes the gap, even if that is an odd number like 3.75 inches.
Do I need cornerstones?
No. They are one of two ways to handle the cross sashing. With cornerstones, the sashing that runs between block rows is pieced from short segments and small squares, and each intersection becomes a fixed alignment point. Without them, that sashing is one continuous strip and the block columns have to line up on their own. The continuous version is much faster to cut and sew. The cornerstone version is more forgiving of accumulated error across a wide quilt, and it gives you somewhere to put a third fabric.
Should I cut borders crosswise or lengthwise?
Crosswise is the default because it wastes less: you cut strips across the width and join them to length. Lengthwise cutting, parallel to the selvage, gives you a border with no joins and on the more stable grain, at the cost of buying a piece of fabric as long as the quilt. For a 90 inch quilt that is two and a half yards for a border, most of which becomes scrap. A common compromise is lengthwise for the outer border, where a join would be most visible and the stability matters most, and crosswise for anything inside it.
How much does the plain block yardage figure actually mean?
It means the fabric for blocks that are single whole squares — a wholecloth block, an alternating plain block, or a panel print cut to size. If your blocks are pieced, that figure is a floor and not an estimate, because internal seams add two seam allowances each and the pieces are cut oversize before trimming. Work the block out separately, and treat this page as the layout and the framing rather than the block itself.