A flounce is a ruffle cut as a circle ie a circular ruffle with a lot of fullness at the bottom edge without the same fullness at the top edge. A peplum is a flounce; many sleeves are cut as a flounce; flounces make the best of trims. need I say more about the use of flounce in sewing. It is a trimmer cousin of the gathered ruffles.
There is more than one way of cutting the flounce. For the fullest of flounces you can cut it out as a full circle. A half circle makes a shorter flounce or one with lesser fullness. You can also cut out several circles to get the length of your edge to join the flounce. Another option is to layer different width flounces for a beautiful effect.
However you make it, a flounce is always better made in a thin lightweight flowy fabric.
How to cut out a full circle flounce
Measure the edge you have to attach the flounce to. And decide on the width you want for the flounce.
The length of the edge to which the flounce will be attached to, is taken as X. This is going to be the circumference of the inner circle you should draw. To calculate the radius from this measure; Divide X by 6.28
For example for an edge of 10 inches, calculating 9/6.28 you get 1.43. This is the radius of the circle you should cut first.
To mark this circle you can use a piece of string, measuring the radius of your circle, attached to a marking pen. Hold one end on the middle and then rotate the marking pen.
Now take the width of the ruffle you want.
With this measure mark the outer circle circumference. Add seam allowance.
Start cutting it out along the white lines.
Cut it out.
One important step in sewing the flounces is to stay stitch the top edge and then clip the seam allowance. This will help the flounce to lay flat without trouble.
Sew the hem of the flounce with a rolled hemmer foot before attaching.
If you have a similar circle and you want to cut a longer but less wider flounce, mark the circle as in the picture below.
The resultant flounce is less fuller; but you get a longer flounce this way, from the same sized circle – very useful when you want full flounces as frills along frocks, curtains, pillowcases etc.
You can find a similar flounce in this post: How to sew a flounce hem skirt