Find the gather ratio between flat and finished lengths.
A ratio of 2:1 means twice the flat fabric is gathered into the finished length.
The gather ratio compares the length of flat fabric with the length after it is gathered. Divide the flat length by the finished length, so 60 inches gathered down to 30 inches gives a ratio of 2 to 1. The ratio controls how full the gathers look.
A ratio of about 1.5 to 1 gives a gentle gather, 2 to 1 is a standard fullness for skirts and sleeves, and 3 to 1 is very full and works well with sheer or lightweight fabrics. Heavy or stiff fabrics bunch at high ratios, so use less fullness for them.
You can use the ratio in either direction. To plan a project, multiply the finished length by the ratio to find how much flat fabric you need, and to check a pattern, divide the cut length by the finished length to see how full it will be.
Sew two rows of long stitches just inside the seam allowance and pull the bobbin threads to gather evenly. Test on a scrap of the same fabric, since the gather ratio that looks right depends on how the fabric drapes.