The first thing to know is that there is an industry standard formular that is 1 cu-in (volume) = 1 pound (weight of loved one at passing). This means that a 225 cu-in urn will hold the cremains of a loved one that weighed up to 225 pounds. Most if not all urn manufacturers will use this formula and clearly disclose the volume of the urn in cu-in in the description of the urn.
