Let's say you're making a sampler quilt, with 12 inch finished squares. You may ask, "How many blocks do I need?" Well if you will answer a few questions, then and only then can I tell you how many blocks you will need.
Standard mattress sizes are - crib 27 X 52, youth 33 X 66, twin 39 X 74, full 54 X 74, queen 60 X 80 and king 78 X 80. This is what size the top of the mattress measures, period!
A comforter usually covers the top, sides and foot of the mattress. You use it with a dust ruffle to pretty up your bed. Sizes for those are - twin 66 X 86, full 76 X 86, queen 86 X 88 and king 102 X 88.
Bedspreads usually fall almost to or all the way to the floor and allow for a pillow tuck. Measurements for those are - twin 80 X 108, full 96 X 108, queen 102 X 118 and king 120 X 118. However, how many pillow that allows for, or what thickness, I can't say. To be safe, you probably should get out the old tape measure and see what measurements you come up with yourself.
Now, what does this all mean to you, and how can you apply this to your sampler quilt? Remember that each of your blocks should be 12 inches when finished (in the quilt), and figure from there. A quilt with 5 rows across and 6 down would measure 60 X 72 (5 x 12 = 60 and 6 x 12 = 72) What if you want sashing between each block, lets say three inches, three inches of sashing, four times, adds twelve inches to the width, and three inches five times would add 15 inches to the length of the quilt. Sashed borders are also nice, so add another six inches to the length and width, our quilt is now up to 78 X 93. If you were making a twin quilt, you would be just a little long for a comforter size, which would be perfect.
If you are trying for a queen size comforter, you want about 86 X 88 inches. Divide 86 by 12, and you get seven, and 88 by 12 is also seven. A quilt seven blocks wide and seven blocks long would be 84 X 84, and you could add a two inch border to the top and bottom to lengthen it, and would then probably add the same to the sides to make it look even. However, that is 49 blocks, and maybe you don't want to make that many. Six rows by six, or 36 squares, with three inch sashes and a three inch sashed border would give you more than enough for your queen size comforter, 93 X 93. Still too many blocks? How about five rows by five, with eight inch sashing and border, you are up to 124 X 124 now. Three blocks by four with 12 inch sashing and borders, 108 X 132. But, I don't think you want all it sashes, do you???
What if you want a bedspread type comforter, say just enough blocks to cover the top of your bed, but a little hanging off the sides to cover the mattress and box spring. The top of a queen is 60 X 80, or five blocks by six blocks, almost, (5 x 12 = 60 and 6 x 12 = 72). If you wanted sashing, you would need fewer blocks, 4 by 5 with three inch sashing, (4 x 12 = 48 5 x 3 = 15 48 + 15 = 63 inches wide 5 x 12 = 60 6 x 3 = 18 60 + 18 = 78 inches long.) But that just covers the top of the mattress, so you need more. You can add borders, one two three, as many as you want. One six inch border all the way around makes your quilt 75 X 90, add a second six inch border 87 X 102.
To choose the correct size batting, add 4" to the size of your finished quilt top and use the next batting size.