Quick answer. A large knitted blanket does not fail on the stitch pattern, it fails on the row. Work it on a circular needle with a cable of at least 40 inches, knitting flat and turning at the end of every row, and rest the weight of the fabric on your lap or a table rather than hanging it off the needle. If you would rather knit in the evening in your hands, work it in strips on straight needles and seam them.
Everything about knitting a big blanket is the same as knitting a scarf until about the tenth inch. Then the fabric starts to have weight, the row starts to take twenty minutes, and the two decisions you made at the cast on turn out to have been the whole project.
The needle is a cable problem, not a needle problem
Straight needles hold a fixed number of stitches, and a blanket does not fit on them. That is the entire reason blankets are knitted on circulars, and it has nothing to do with knitting in the round. You cast on, knit to the end, and turn, exactly as you would on straights. The cable is doing the job of holding the stitches you are not currently working.
Cable length recommendations for blankets run from 32 to 60 inches, and for a very large afghan the usual advice is not to go below 40. Longer cable does three things at once: it lets the stitches sit without being crammed, it lets you see the full width of the work as you go, and it spreads the weight along its length rather than concentrating it where your hands are.
That last one is the reason wrists start complaining on the third evening rather than the first. The fabric is not getting heavier per row, it is getting heavier per hour you hold it up.
Buy the cable for the weight, not for the width. Stitches compress along a cable, so a 40 inch cable holds far more than 40 inches of finished blanket. What a short cable actually costs you is comfort: the work bunches, you cannot read your own fabric, and every row is fought rather than knitted.
The number that tells you what you signed up for
Work it out before you cast on, because it changes what you choose.
Say a 60 inch wide blanket in a yarn that gives 4 stitches to the inch. That is 240 stitches on the needle, and 240 stitches in every single row. At 6 rows to the inch, a 72 inch length is 432 rows. Multiply: 103,680 stitches.
Now the same blanket in a super bulky at 2 stitches and 3 rows to the inch. 120 stitches a row, 216 rows, 25,920 stitches. A quarter of the work for the same blanket, which is why nearly every fast blanket you have seen was worked in a thick yarn rather than by a fast knitter.
The thing that number does not tell you is how it feels, and this is where most people underestimate a large blanket. 240 stitches means you cannot put it down mid-row without losing your place in the pattern. If your knitting happens in twenty minute pieces, a 240 stitch row is the wrong shape for your life regardless of how many stitches it adds up to.
One piece or strips
Both are traditional, and the choice is about how you knit rather than how it looks.
One piece on a circular. No seaming at the end, and the fabric is continuous, so nothing can be mismatched. Each row is long, the work gets heavy in your hands, and it stops being portable somewhere around the halfway mark.
Strips or panels on straights. Each piece is a manageable width, it stays portable to the end, and straight needles are less intimidating if circulars are new to you. This is the traditional construction for heavily cabled Aran style blankets, where a strip is one or two motifs wide. The cost is seaming, and seaming long straight edges honestly is a skill of its own.
The stitch count is identical either way. Strips do not save you work, they break the same work into pieces you can carry.
Setting up a large blanket so it stays the width you cast on
1. Decide the finished width and length in inches
Write both down. Every other number here is derived from these two, and they are the only ones you cannot revise later.
2. Swatch and measure the centre four inches
Count stitches and rows inside the middle of the swatch, not at the edges. Edge stitches distort in every knitted fabric.
3. Multiply width by stitches per inch, and look at the result
This is the moment to change your mind. If the number is over 200 and you knit in short sittings, either go up a yarn weight or switch to strips before you cast on rather than after row forty.
4. Choose the cable to match, minimum 40 inches for a full size blanket
Shorter is workable and unpleasant. This is the cheapest part of the project to get right.
5. Add a border stitch you cannot forget
Three or four stitches of garter at each edge, worked on every row including the purl rows. It stops stocking stitch curling and it gives you a visible marker for where the row starts, which is what keeps the width honest over 400 rows.
6. Support the weight from below
Lap, table, or a cushion under the work. Nothing about knitting requires you to hold up five pounds of finished blanket with your forearms.
Where a large knitted blanket is the wrong project
If this is your first knitted thing, it is the wrong first thing. The stitches are not hard. The problem is that a mistake in row three is discovered in row three hundred, and the fix is to unpick 240 stitches at a time. Learn the edge control on something a foot wide first.
It is also the wrong project if you want it finished soon. The stitch counts above are the honest version. A worsted weight blanket at full size is measured in months of evenings, not weeks. For a fast finish, the answer is a thicker strand rather than more hours: the chunky yarn blanket guide covers the jumbo weights, and the hand methods take the needles out of it entirely.
And if you want the fabric without the row length, crochet is worth a look, because it works one live stitch at a time and can be put down anywhere. The beginner crochet blanket guide covers how that sizing works.
Questions people ask before casting on
What length circular needle do I need for a blanket?
32 to 60 inches covers most blankets, and 40 inches or longer is the usual recommendation for a full size afghan. Stitches compress on the cable, so the cable does not need to match the finished width.
Can I knit a blanket on straight needles?
Only in strips or panels, which are then seamed. A full width blanket will not fit on straights, and forcing it means stitches falling off the ends.
Does knitting on circulars mean knitting in the round?
No. For a blanket you knit flat: work to the end of the row, turn, work back. The circular is being used purely as a very long straight needle.
Is it faster to knit strips?
No. The stitch count is the same either way. Strips are more portable and use straight needles, but you pay for that in seaming at the end.
How do I stop the edges curling?
Work three or four stitches of garter at each edge on every row, purl rows included. Stocking stitch curls by nature and a garter border is what holds it flat.
