Strip the bed to remove all sheets and blankets.
Measure the mattress from the top center to the bottom center and record the measurement. This is measurement A.
Measure from the top edge of the mattress to the point you want the comforter to reach where it hangs off the end of the bed. Add that to measurement A.
Measure from the right side of the mattress in the middle to the same point on the left side and record that measurement. This is measurement B.
Decide how far you want each side to hang and measure from that point to the top of the mattress. Add these measurements to measurement B.
Add a 1-inch seam allowance to both measurements.
Divide measurement B by the width of the fabric chosen minus 1 inch for seams. This will tell you how many pieces of fabric wide your comforter will be.
For example, your bed is 72 inches wide and you want the comforter to hang 10 inches down each side. That is 92 inches plus 1 inch for seam allowances. That is 93 inches. If your fabric measures 54 inches wide, divide 93 by 53 (subtracting one inch from the width for the seam allowance). Since the result is 1.75, you would need to start with two pieces of fabric each the length of measurement A.