bolt of fabric
Sizes and methods of sale vary wildly. Maybe around 90% of the time, fabric is sold by the yard, and just stored on bolts, rather than sold by the bolt. Unless it is some kind of staple fabric that many professionals are likely to buy in quantity–like muslin fabric or white broadcloth; that sort of thing.
Bolts vary in size, though can sometimes have fairly expected lengths depending on vendor and fabric type. fashion fabrics sold at chain retail stores like Hancock fabrics and JoAnn’s often come on bolts of approximately 10 or 12 yards. However, it’s not rare for them to get bolts of 15 or 20ish. They’ll often come with fractions of yards. And it’s not a terrible shock for a bolt to be off by a quarter yard or even more. fashion fabrics tend to have smaller bolts than many other fabric types, perhaps because many of the expected uses for them only require 1 to 3 yards, so many customers can buy enough for one garment from one small bolt.
The cheapest muslin fabric often comes on 50-yard bolts, and is around 38 to 40ish inches wide. The usual widths for fashion fabrics are 45 and 60 inches wide. Drapery is usually 54 inches. However, 50 is not a terribly uncommon width.
Drapery and upholstery fabrics vary depending on how thick the fabric is. A very plush one might come on a smaller bolt, but it can be upwards of 50ish.
Many discount stores will sell fabrics by the bolt. Or a regular fabric store might if it got a job lot of a bunch of random stuff they want to get rid of without having to deal with measuring. Or if something has been sitting around a while, they’ll try a by-the-bolt tactic. In this case, it may have been cut from. If it has not been cut, most bolts will come with markings indicating how many yards they have. Either a stamp on one end or a sticker. Perhaps a tag on the edge of the fabric.
As for the price of a bolt…when there have been crazy-good sales, I’ve been able to get about $10 for a bolt of fashion fabric. $1/yard is a ridiculously good price (and there is probably something wrong with this fabric). $3-4 is about average/decent. The price is going to vary by type of fabric. Muslin is usually about a dollar or a couple dollars. I’ve seen it go for a lot less than a dollar. Cottons/quilting type fabrics will go for about $2 at the cheapest end, though they are getting up to about $9 and more, at JoaAnn’s. Panne velvet should be between $3 and 8, Plain polyester velvet should be around $8 to $12/yd. Upholstery and drapery fabrics can range around $15 and $20 at the relatively low end. $5/yd is a great price for these.
There are two main types of bolt: a rectangle of cardboard that is about an inch thick. fabric is folded in half and wrapped around it. If you count the folds, every two folds will be approximately a yard, so that can give you an idea. The other type of bolt is a long tube. This is harder to count without having experience. As far as I can tell, with these bolts, you just kinda have to work in a fabric store for a couple years until you can eyeball it.