Sewing circles into tubes requires different size circles depending if the circle is a cap on the end of a tube or a hold the tube is inserting into.

The circle sewn onto the end of the fabric tube is not cut the same size as the circle for the hole the fabric tube inserts into. The finished sewn diameter is the same, but the cut edge is different because seam allowance is added in opposite directions.

A tube cap needs seam allowance added outside the finished circle.

A receiving hole in a larger fabric piece needs seam allowance accounted for inside the opening, so the cut hole must be smaller than the finished diameter.

• Cap circle cut size = finished diameter + 1"
• Hole cut size = finished diameter - 1"
• This assumes a .5" seam allowance

Why the cap and the hole are different sizes

Suppose you want a tube with a finished diameter of 8". The seam line is what must match, not the raw cut edge.

For a cap:

• The seam is sewn .5" in from the cut edge
• So the pattern must extend .5" beyond the finished circle all the way around
• That adds 1" total to the diameter
• Cut diameter = 9"

For a hole:

• The seam is also sewn .5" in from the cut edge
• But here the cut edge is inside the opening
• So the opening must start 1" smaller than the finished diameter
• Cut diameter = 7"

Both pieces sew to the same tube because their seam-line diameter is 8".