When looking for end caps and inserts for your outdoor furniture, first determine the size of the tube. Measure the outside diameter (O.D.) of the tube, though a few parts are measured by the tube’s inside diameter (I.D.) Click on a part, and the description will explain which diameter the part fits. Measuring the part itself will yield a size that won't fit. You'll find multi-gauge, round, square, domed and flat parts.