I dont always know what to buy either, for border. I always get 3 yds.

I bought some black and cream butterflies from Hobby Lobby about two yrs back not knowing what I would do with it. Turns out the fabric is created exclusively for them, and when my daughter was helping me pick out her backing for a king quilt, I went back and bought five more yds. I dont think even that is going to be enough.

Proplem: The cream color in the newer batch is not the same as the older fabric. I had to tea dye it yesterday. Not a problem since I am doing a "quilt as you go" and am cutting the fabric into blocks. Said all that to say this: try to get your fabric all at one time. The dye lots will be different.

There are several fabric calculators online that you can find through a search.
There is also a device that you can buy in some quilt shops. I believe it is called FabricCalc, but it is $48.

I like scrappy quilts so I use a bit of a LOT of different fabrics. I usually only need to worry about having enough background, border fab and backing, because the rest is all scraps.

Here's a list of approximately how many pieces of each size you can cut from an 18" x 21" fat quarter:
90 2" squares or

50 2 ½” squares or

42 3" squares or

30 3 ½” squares or

20 4" squares or

16 4 ½” squares or

12 5" squares or

12 5 ½” squares or

9 6" squares or

I use my Fabri-Calc all the time and usually find it accurate. Sometimes, though, I have to improvise because I didn't buy enough of something. That usually ends up to the good and ends up with a scrappy look I like.

Well, 180 (total squares) divided by 4 (different colors) is 45 squares of each color.

For fabric that is 40-44 inches wide (selvage to selvage) that is 40 (inches) divided by 5 (inch squares) = 8 squares per 5 inch strip.

45 (total squares) divided by 8 (squares per strip) = 5.6 strips, lets say 6 strips.

Then 6 strips x 5 inches each = 30 inches. Add enough for straightening the edges and to be safe get 1 yard of each color.

Does that make sense to you? It's easy if you just sit down and think about each step.

This little drill works for me:
1) Decide the size of the quilt & figure the backing. EX: a crib size (45x60) would require about 3 yards for backing & binding.
2) Gather the fabrics for the top and buy enough of each to total the yardage of the backing fabric. EX: for 6 different fabrics in the top, I'd buy about a half yard of each---a total of 3 yards.
Of course, I make mental adjustments according to the pattern, but the total yardage in the top should be AT LEAST as much fabric as the backing. As others have said, buying a quarter yard extra here and there is how scrap stashes are created!

That is a great idea for the backing the problem is I usually end up trying to use all of the fabric and I end up with a bigger quilt than what I had planned.

I say just buy the whole bolt...LOL

If I don't have a pattern yet and I like the fabric, I buy at least 2 yards!