Pattern repeat is the distance before a fabric's pattern repeats itself. You will want to know the repeat to determine how much fabric to buy for large projects. When making a pair of curtain panels, for example, buy extra fabric equal to the length of the pattern repeat so you will have enough fabric to cut both the panels from the same portion of the pattern. Most patterned fabrics have a repeat on the width and length. With the exception of true plaids, the length and width repeats are usually different.
Lay the fabric out flat.
Pull a thread out along the cut edge of the fabric to find the straight grain along the width of the fabric.
Measure along the line of the pulled thread from the inside edge of the selvage to the point where the fabric's pattern begins to repeat. This distance is the pattern repeat along the width of the fabric.
Measure along the selvage edge from the point where you pulled the thread to the point where the fabric's pattern begins to repeat. This distance is the pattern repeat along the length of the fabric.
The selvage is the woven band along the edge of the fabric.
The pattern repeat is sometimes given on the label on the bolt of fabric. Sometimes this information is wrong, so it is good to check the repeat yourself.