I'll walk you through the process of calculating one quilt, using a 12" Starflower Block as the Example.     The Supplies List for that pattern tells you that you need the following amounts for one block:

If we are making a FULL sized quilt, and doing Block-to-Block construction (No Sashings) we will plan on making 56 blocks, so will need 56 of each of the fabric units needed. Our job is to figure out how many yards that means! There are just FIVE STEPS:

Step One: First, multiply the strips/units you know you need( shown above in the Supplies List) times the number of blocks you plan to make ( 56):

For Color 1,  two strips 3-7/8" x 7-3/4" times 56 = we will need 112 strips 3-7/8" x 7-3/4" 

For Color 2,  two strips 3-7/8" x 7-3/4" times 56 = we will need 112 strips 3-7/8" x 7-3/4" 

For Color 3, one strip 3-1/2" x 14" times 56 = we will need 56 strips 3-1/2" x 14"              AND two strips 3-7/8" x 7-3/4" times 56 = we will need 112 strips 3-7/8" x 7-3/4" 

Step Two: Next, working with one color at a time, find out how many full length strips/units you can get from each 42" width of fabric you cut:

For Color 1,  42" divided by 7-3/4" = 42divided by 7.75 = 5.42...so call it FIVE (as .42 of a unit won't be long enough to use) (Always round DOWN on this part!)

For Color 2,  42" divided by 7-3/4" = 42/7.75 = 5.42...so call it FIVE (as .42 of a unit won't be long enough to use) (Always round DOWN on this part!)

For Color 3,  42" divided by 14" = 3-THREE ( this came out exactly-we love that!) for the first unit  AND   for the second Color 3 strip, 42" divided by 7-3/4" = 42/7.75 = 5.42...so call it FIVE (as .42 of a unit won't be long enough to use) (Always round DOWN on this part!)

Step Three: Next, still working with one color at a time, divide the number of units you NEED, by the number of units you can get from each strip you cut:

For Color 1, You NEED 112  (3-7/8" x 7-3/4") strips . You are going to get FIVE from each 3-7/8" wide strip you cut... so 112 divided by 5 = 22.4  Call it 23 strips of that width you will need to cut.( Always round UP on this part!)

For Color 2, You NEED 112  (3-7/8" x 7-3/4") strips . You are going to get FIVE from each 3-7/8" wide strip you cut... so 112 divided by 5 = 22.4  Call it 23 strips of that width you will need to cut.( Always round UP on this part!)

For Color 3, You NEED 56 (3-1/2" x 14") strips. You are going to get THREE from each 3-1/2" wide strip you cut. 56 divided by 3 = 18.6 Call it 19 strips of that width you will need to cut AND you NEED  112  (3-7/8" x 7-3/4") strips . You are going to get FIVE from each 3-7/8" wide strip you cut... so 112 divided by 5 = 22.4  Call it 23 strips of that width you will need to cut.( Always round UP on this part!)

Step Four: Now, still working with one Color at a  time, MULTIPLY the number of Strips you are going to cut times the width of each strip to find out how many inches of fabric that is:

For Color 1, 23 strips x 3.875 = 89.125", or 89 1/8". Find the nearest real measurement = 90 inches of Color 1 you need to get your units

For Color 2, 23 strips x 3.875 = 89.125", or 89 1/8". Find the nearest real measurement = 90 inches of Color 2 you need to get your units

For Color 3, 19 strips x 3.5 = 66.5"  Find the nearest real measurement = 67 inches of Color 3 you need to get your first units, AND 23 strips x 3.875 = 89.125", or 89 1/8". Find the nearest real measurement = 90 inches of Color 1 you need to get your second units. 67 plus 90 = 157 inches to get all units needed.

Step Five: And now, still working with one color at a time, DIVIDE the number of inches you need by 36" ( one Yard) to find out how many YARDS you need:

For Color 1, 90" divided by 36" = 2.5 yards

For Color 2, 90" divided by 36" = 2.5 yards

For Color 3, 157" divided by 36" = 4.361  or 4.5 yards

Then when you go to buy your fabric you may want to bump each figure UP a little to plan for errors, or whatever. Add more if you want to make BINDING. Add more if you want to add BACKING. Add more if you absolutely LOVE IT :o)