Bender routinely claims to be made of thirty to forty percent of various metals, typically following his claims up by knocking on his own chassis. Unfortunately, his credibility on this matter is thrown into question by the fact that his various claims now add up to well over one hundred percent. Bender has alternately claimed to be:

40% Zinc (1ACV13).
40% Titanium (2ACV03).
30% Iron (3ACV22), presumably mixed with osmium, (3ACV17).
40% Dolomite (4ACV07).
40% Lead (FVG). Canon status of video game uncertain.
40% Luck/horseshoes (ITWGY). Composition of the horseshoes unknown.
40% Chromium (6ACV03). Bender was not the one to make this claim. It was an Antarian.
60% Storage space (6ACV10).
40% Scrap metal (6ACV23). 

Total: 150% - 330% or greater. The number is dependent on whether or not one counts various claims as true. First the "40% Horseshoes" claim, as the supposed horseshoes could have been made of one or more of the other metals previously stated (Titanium, Iron, or Lead), meaning that one could have been a restatement rather than a new figure. Second, it depends on whether or not he considers his storage space to be part of the total percentage of himself, in which case the remaining 40% that he often claims to be would constitute the rest of his body. Third, the claim of "40% Lead" is dependent on whether the video game can be considered canon. Fourth and finally, whether the claim of "40% Chromium" is accepted at all. This claim was not made by Bender but rather one of the Antarians. It is plausible that the claim is false and that it was just made to justify the in-progress cannibalization of Bender's body. Even if all previous points made are taken into account and only the first four claims were used, Bender's composition would be 150%, far more than what is possible. This error can be explained as due to Bender's being a poor calculator, making it plausible that he has given the wrong percentages (3ACV09).

Additionally, he has a 0.04% nickel impurity, which doesn't increase total percentage, but slightly decreases the percentages of everything else. This is apparently not a standard impurity, and is cited as making him unique (3ACV17).

There's also a mention that robots are made from recycled beer cans and beer cans are made from recycled robots (1ACV08), so depending on the year 3000 composition of beer cans, he may have a certain percentage of aluminum in his alloy.

As a side note, Dolemite is also the title of a 1974 film. The film's main character, Dolemite, is a pimp who leads an army of hookers skilled in kung-fu.[1] Bender has been a pimp many times throughout the series.

Bender's composition will possibly be referenced in the episode "Forty Percent Leadbelly".

Source: The Infosphere