Single vs double fold hems and how much fabric each needs
How single and double fold hems differ, why the double fold uses twice the allowance, and how to pick the right depth for your fabric.
What the two hem styles look like
A single fold hem turns the raw edge under once and stitches it down, which is quick but leaves the cut edge sitting inside the fold where it can fray over time. A double fold hem turns the edge under twice, so the raw edge is completely wrapped inside the second fold and never touches the surface of the garment. The double fold is neater and more durable, which is why it is the default choice on shirts, trousers and anything that will be washed often.
Why the double fold uses twice the fabric
With a single fold, the hem allowance you set is the depth of the finished hem, and that is all the fabric it consumes. With a double fold, the same depth is folded up twice, so the fabric travels the hem depth once to make the first fold and again to make the second. That is why ticking Double fold hem doubles the hem used in the calculator. A 2 inch double fold hem needs 4 inches of fabric length, then the seam allowance is added on top to give the total allowance.
Choosing a hem depth for your fabric
Lightweight fabrics like cotton lawn or chiffon take a narrow hem of half an inch to one inch, which keeps them from looking bulky. Medium weights such as shirting and quilting cotton sit well with a one to two inch hem. Heavy fabrics and home items like curtains benefit from a deep hem of three to four inches, because the extra weight helps them hang straight and resist flapping. Always pair a deeper hem with a double fold when the fabric frays easily.
From finished length to cut length
The calculator keeps the finished length fixed and adds allowances so the piece comes out the size you want after sewing. It counts the hem used at one end and a seam allowance at the other, which is the typical layout for a panel that joins to another piece. If your piece is hemmed at both ends, put your second hem depth in the seam allowance field so both folds are included before you cut.