Enter the measurements in the units you use
You don't need to rewrite a metric quilt plan in inches before using QuiltMargins. Choose cm or mm first, then enter the quilt dimensions and measured usable fabric width. The unit buttons convert values already in the form, so selecting cm after typing 60 inches will show 152.4 cm. It won't reinterpret that entry as 60 cm.
Measure the fabric instead of its label
Even when a fabric shop lists width in centimeters, measure the part you'll actually cut. Exclude both selvages and any damaged edge. If you've washed the fabric, use its width after washing and pressing. A small width difference can change how many squares fit across or how many backing panels are needed, regardless of the units you use.
Convert the cutting length to yards
One yard is 91.44 cm, or 914.4 mm. Divide a millimeter cutting length by 914.4 to get yards. For example, 500 mm of fabric is about 0.547 yard. That figure describes the raw cutting requirement, before shop rounding. The calculator's fabric-to-buy result is already in yards, so don't convert that yardage a second time.
Round once, at the purchase step
If the shop sells in ⅛-yard increments, round the 500 mm example up to 0.625 yard, or ⅝ yard. A ½-yard cut would be too short. Add any separately planned extra fabric before you round. Keeping raw length, extra and rounded purchase amount on separate lines makes it much easier to see what you're paying for.
Keep the units on the shopping list
Label each amount with the fabric name and unit: for example, “Binding fabric — ⅝ yd.” Keep the cutting dimensions beside the plan rather than mixing them into the purchase total. The SVG, CSV and PDF exports use labeled inch and yard measurements, even when you've entered the project in metric. Save the editable project file as well if you want to revisit the measurements later.