TDW is an acronym that stands for "total diamond weight" and describes the total size of all diamonds in a piece of jewelry such as a bracelet, ring or necklace. When purchasing an engagement ring, an anniversary necklace or even a tennis bracelet, it is not only important to ask your jeweler about the size of the center, or largest, diamond, but also the total diamond weight.

The total diamond weight refers to the total combined weight of all diamonds in a piece of jewelry. For example, if you own a ring containing a large center diamond and two smaller diamonds on each side, the TDW includes all three diamonds. The center diamond may be 1/2 carat and each smaller diamond may be 1/8 carat, resulting in a TDW of 3/4 carat. A solitaire ring does not have a total diamond weight because there is only one diamond. It is referred to by the size of the one single stone, which is measured in carats.

Total diamond weight may also be referred to as total weight, carat total weight or carat weight total. These acronyms are used directly after a numerical value. You may see advertisements using anyone of these labels. For instance, a jeweler may state that a necklace is 0.80 CTW which is the same value as another at 0.80 TDW. When you see the acronym "CT," this means "carat" and often describes a solitaire ring containing one diamond.

The TDW of a ring doesn't mean that it is a highly valued piece of jewelry. For example, if a jeweler labels a diamond ring as 1.25 TDW, it may mean that the ring has a 1 carat stone in the center and a 1/8 carat diamond on each side. Alternatively, it may mean that the ring contains ten 1/8 carat diamond chips across the band, making it much less valuable because each stone is smaller and costs less individually. The Federal Trade Commission requires jewelers to remain within a 0.02 range of accuracy regarding the total diamond weight on a piece of jewelry.

Various formulas exist for calculating the weight of diamonds according to their shape. First, add the length and the width of the stone and divide by two to find the average diameter. Multiply the average diameter by the average diameter again, times the depth of the diamond. Multiply that value by 0.0061 for a round diamond and 0.0062 for an oval-shaped diamond. Mathematical formulas for other shapes include length times width times depth, multiplied by 0.0059 for a heart and 0.0057 for a triangular diamond. Add the values of all diamonds together to find TDW.