Here is a question from a QuiltSandwich quilter who puzzled over the cutting diagram:
When defining a 6" square block with allowance of ½" on cut edge and 1" at selvage edge, the calculator states the need for ¼ yard. This seems to be more than I actually need. The cutting diagram shows the allowances in yellow, pieces cut on fold in pink. What is the gray area in reference to?
This is a great question! Once you fully understand how to read the cutting diagram you'll have all the information you need to put the frustrations of 'coming up short' after cutting fabric behind you!
Let's start with entering the settings:
Looking at the screenshot above, you can see that the squaring up and shrinkage allowances have been set to 1" on the selvage edges and 1/2" on the cut edges.
Scrolling the calculator back up, the piece has been set to a finished size of 6"x6", number of pieces to 4, and the seam allowance set to 1/4". For a 6" finished size block with these settings, QuiltSandwich calculates the cut size for you, adding 1/4" seam allowance and figuring the yardage needed for the block's cut size of 6 1/2".)
Above, the closeup of the drawing shows the strip fits nicely within the 9" wide fabric (1/4 yard).
The strip and the piece cut-lines are shown in white. The numbers '4' & '2' on the far right blocks indicates that the four blocks needed should be cut from the end of the folded strip. The numbers show that if you followed this cutting diagram — cutting the pieces from the folded fabric — you would have four 6 1/2" blocks. The block shown nearest the fold (white area without a number) plus the fabric in the gray area on the fold is not needed.
In yellow are the allowances — additional fabric that is added to the yardage calculation — that make sure you have enough fabric to cut out strips and blocks after shrinking and squaring up the fabric. (We all know fabric never comes off the bolt perfectly square — so it is important to compensate for the fabric lost due to trimming to square up.)
The diagram shows two gray areas: a rectangle at the fold of the fabric and another area running along the bottom of the strip. The gray areas show the unused or truly extra fabric — scrap — fabric not needed for either the piece or the allowances. The yardage calculator increments yardage in 1/8 increments. Therefore, because a 6 1/2" block (not even including the allowances!) is bigger than 1/8 yard (4 1/2"), the calculator must increment another 1/8 yard. With that increment, QuiltSandwich is able determine that 1/4 yard is the least fabric it to recommend for this piece and it creates the cutting diagram to show this.
Think of the cutting diagram as providing both guidance on  how to cut and  a way to see fabric utilization.
THIS EXAMPLE — HOW TO PROCEED?
The cutting diagram is useful because it shows you what you need to know before cutting —
A look at the cutting diagram shows just how the fabric is utilized —
Because the Cutting Diagram shows honestly and exactly how fabric will be utilized, you can confidently make a plan before cutting.
Of course, if you are purchasing fabric, you'll need to buy a full 1/4 yard at your local quilt shop. However, seeing that only an 8 1/2” length of fabric is needed, could be really helpful in the right situation!
Are you are pulling from your stash?
Trying to use scraps?
Working with a kit?
You might be able to take advantage of the insight provided by the cutting diagram and get by with less than the ideal yardage.
And — think about this — sometimes you're not sure if you have enough of the 'perfect fabric'.
The cutting diagram let's you adjust allowances to fit a particular piece of fabric so that you can confidently pre-plan your piece count before cutting! This can be especially helpful if you have a fabric scrap that is already squared up. If this is the case, set the squaring up allowance to zero and the cutting diagram show you exactly what you need!